STP identifies the network topology by transmitting BPDUs between STP compliant network devices. BPDUs contain sufficient information for the network devices to complete the spanning tree computing.

In STP, BPDUs come in two types:

l Configuration BPDUs, used to maintain the spanning tree topology.

l Topology change notification (TCN) BPDUs, used to notify concerned devices of network topology changes, if any.

III. Basic concepts in STP

1) Root bridge

A tree network must have a root; hence the concept of “root bridge” has been introduced in STP.

There is one and only one root bridge in the entire network, and the root bridge can change alone with changes of the network topology. Therefore, the root bridge is not fixed.

Upon network convergence, the root bridge generates and sends out at a certain interval a BPDU and other devices just forward this BPDU. This mechanism ensures the topological stability.

2) Root port

On a non-root bridge device, the root port is the port with the lowest path cost to the root bridge. The root port is responsible for forwarding data to the root bridge. A non-root-bridge device has one and only one root port. The root bridge has no root port.

3) Designated bridge and designated port

Refer to the following table for the description of designated bridge and designated port.

Table 1-1 Description of designated bridge and designated port

Classification	Designated bridge	Designated port
For a device	The device directly connected with this device and responsible for forwarding BPDUs	The port through which the designated bridge forwards BPDUs to this device
For a LAN	The device responsible for forwarding BPDUs to this LAN segment	The port through which the designated forwards BPDUs to this LAN segment

Figure 1-1 shows designated bridges and designated ports. In the figure, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Switch A, Switch B, and Switch C respectively.

l If Switch A forwards BPDUs to Switch B through AP1, the designated bridge for Switch B is Switch A, and the designated port is the port AP1 on Switch A.

l Two devices are connected to the LAN: Switch B and Switch C. If Switch B forwards BPDUs to the LAN, the designated bridge for the LAN is Switch B, and the designated port is the port BP2 on Switch B.

Figure 1-1 A schematic diagram of designated bridges and designated ports

& Note:

All the ports on the root bridge are designated ports or backup ports.

IV. How STP works

STP identifies the network topology by transmitting bridge protocol data units (BPDUs, also known as configuration BPDUs in IEEE 802.1D) between network devices. Configuration BPDUs contain sufficient information for network devices to complete the spanning tree computing. Important fields in a configuration BPDU include:

l Root bridge ID: consisting of root bridge priority and MAC address.

l Root path cost: the cost of the shortest path to the root bridge.

l Designated bridge ID: designated bridge priority plus MAC address.

l Designated port ID, designated port priority plus port name.

l Message age: age of the configuration BPDU

l Max age: maximum age of the configuration BPDU.

l Hello time: configuration BPDU interval.

l Forward delay: forward delay of the port.

& Note:

For the convenience of description, the description and examples below involve only four parts of a configuration BPDU:

l Root bridge ID (in the form of device priority)

l Root path cost

l Designated bridge ID (in the form of device priority)

l Designated port ID (in the form of port ID)

1) Specific computing process of the STP algorithm

l Initial state

Upon initialization of each port of each device, each port generates a BPDU with itself as the root, in which the root path cost is 0, and designated bridge ID is the local device ID. In such a BPDU, the root bridge ID is the local device ID, the root path cost is 0, the designated bridge ID is the local device ID, and the designated port is the local port.

l Selection of the optimum configuration BPDU

Each device sends out its configuration BPDU and receives configuration BPDUs from other devices.

The process of selecting the optimum configuration BPDU is as follows:

Table 1-2 Selection of the optimum configuration BPDU

Step	Description
1	Upon receiving a configuration BPDU on a port, the device performs the following processing: l If the received configuration BPDU has a lower priority than that of the configuration BPDU generated by the port, the device will discard the received configuration BPDU without doing any processing on the configuration BPDU of this port. l If the received configuration BPDU has a higher priority than that of the configuration BPDU generated by the port, the device will replace the content of the configuration BPDU generated by the port with the content of the received configuration BPDU.
2	The device compares the configuration BPDUs of all the ports and chooses the optimum configuration BPDU.

Step

Description

Upon receiving a configuration BPDU on a port, the device performs the following processing:

l If the received configuration BPDU has a lower priority than that of the configuration BPDU generated by the port, the device will discard the received configuration BPDU without doing any processing on the configuration BPDU of this port.

l If the received configuration BPDU has a higher priority than that of the configuration BPDU generated by the port, the device will replace the content of the configuration BPDU generated by the port with the content of the received configuration BPDU.

The device compares the configuration BPDUs of all the ports and chooses the optimum configuration BPDU.

& Note:

Principle for configuration BPDU comparison:

l The configuration BPDU that has the lowest root bridge ID has the highest priority.

l If all the configuration BPDUs have the same root bridge ID, they will be compared for their root path costs. If the root path cost in a configuration BPDU plus the path cost corresponding to this port is S, the configuration BPDU with the smallest S value has the highest priority.

l If all configuration BPDU have the same root path cost, they will be compared for their designated bridge IDs, then their designated port IDs, and then the IDs of the ports on which they are received. The smaller the ID, the higher message priority.

l Selection of the root bridge

At network initialization, each STP-compliant device on the network assumes itself to be the root bridge, with the root bridge ID being their own device ID. By exchanging configuration BPDUs, the devices compare one another’s root bridge ID. The device with the smallest root bridge ID is elected as the root bridge.

l Selection of the root port and designated ports

The process of selecting the root port and designated ports is as follows:

Table 1-3 Selection of the root port and designated ports

Step	Description
1	The root port is the port on which the optimum configuration BPDU was received.
2	Based on the configuration BPDU and the path cost of the root port, the device calculates a designated port configuration BPDU for the rest ports. l The root bridge ID is replaced with that of the configuration BPDU of the root port. l The root path cost is replaced with that of the configuration BPDU of the root port plus the path cost corresponding to the root port. l The designated bridge ID is replaced with the ID of this device. l The designated port ID is replaced with the ID of this port.
3	The device compares the computed configuration BPDU with the configuration BPDUs of other ports, and performs processing accordingly based on the comparison result: l If the existing configuration BPDU on the port is superior, the device will block this port without changing its configuration BPDU, so that the port will only receive BPDUs, but not send any, and will not forward data. l If the computed configuration BPDU is superior, this port will serve as the designated port, and the configuration BPDU on the port will be replaced with the computed configuration BPDU, which will be sent out periodically.

& Note:

When the network topology is stable, only the root port and designated ports forward traffic, while other ports are all in the blocked state – they only receive STP packets but do not forward user traffic.

Once the root bridge, the root port on each non-root bridge and designated ports have been successfully elected, the entire tree-shaped topology has been constructed.

The following is an example of how the STP algorithm works. The specific network diagram is shown in Figure 1-2. In the feature, the priority of Switch A is 0, the priority of Switch B is 1, the priority of Switch C is 2, and the path costs of these links are 5, 10 and 4 respectively.

Figure 1-2 Network diagram for STP algorithm

l Initial state of each device

The following table shows the initial state of each device.

Table 1-4 Initial state of each device

Device	Port name	BPDU of port
Switch A	AP1	{0, 0, 0, AP1}
Switch A	AP2	{0, 0, 0, AP2}
Switch B	BP1	{1, 0, 1, BP1}
Switch B	BP2	{1, 0, 1, BP2}
Switch C	CP1	{2, 0, 2, CP1}
Switch C	CP2	{2, 0, 2, CP2}

l Comparison process and result on each device

The following table shows the comparison process and result on each device.

Table 1-5 Comparison process and result on each device

Device	Comparison process	BPDU of port after comparison
Switch A	l Port AP1 receives the configuration BPDU of Switch B {1, 0, 1, BP1}. Switch A finds that the configuration BPDU of the local port {0, 0, 0, AP1} is superior to the configuration received message, and discards the received configuration BPDU. l Port AP2 receives the configuration BPDU of Switch C {2, 0, 2, CP1}. Switch A finds that the BPDU of the local port {0, 0, 0, AP2} is superior to the received configuration BPDU, and discards the received configuration BPDU. l Switch A finds that both the root bridge and designated bridge in the configuration BPDUs of all its ports are Switch A itself, so it assumes itself to be the root bridge. In this case, it does not make any change to the configuration BPDU of each port, and starts sending out configuration BPDUs periodically.	AP1: {0, 0, 0, AP1} AP2: {0, 0, 0, AP2}
Switch B	l Port BP1 receives the configuration BPDU of Switch A {0, 0, 0, AP1}. Switch B finds that the received configuration BPDU is superior to the configuration BPDU of the local port {1, 0,1, BP1}, and updates the configuration BPDU of BP1. l Port BP2 receives the configuration BPDU of Switch C {2, 0, 2, CP2}. Switch B finds that the configuration BPDU of the local port {1, 0, 1, BP2} is superior to the received configuration BPDU, and discards the received configuration BPDU.	BP1: {0, 0, 0, AP1} BP2: {1, 0, 1, BP2}
l Switch B compares the configuration BPDUs of all its ports, and determines that the configuration BPDU of BP1 is the optimum configuration BPDU. Then, it uses BP1 as the root port, the configuration BPDUs of which will not be changed. l Based on the configuration BPDU of BP1 and the path cost of the root port (5), Switch B calculates a designated port configuration BPDU for BP2 {0, 5, 1, BP2}. l Switch B compares the computed configuration BPDU {0, 5, 1, BP2} with the configuration BPDU of BP2. If the computed BPDU is superior, BP2 will act as the designated port, and the configuration BPDU on this port will be replaced with the computed configuration BPDU, which will be sent out periodically.	Root port BP1: {0, 0, 0, AP1} Designated port BP2: {0, 5, 1, BP2}
Switch C	l Port CP1 receives the configuration BPDU of Switch A {0, 0, 0, AP2}. Switch C finds that the received configuration BPDU is superior to the configuration BPDU of the local port {2, 0, 2, CP1}, and updates the configuration BPDU of CP1. l Port CP2 receives the configuration BPDU of port BP2 of Switch B {1, 0, 1, BP2} before the message was updated. Switch C finds that the received configuration BPDU is superior to the configuration BPDU of the local port {2, 0, 2, CP2}, and updates the configuration BPDU of CP2.	CP1: {0, 0, 0, AP2} CP2: {1, 0, 1, BP2}
By comparison: l The configuration BPDUs of CP1 is elected as the optimum configuration BPDU, so CP1 is identified as the root port, the configuration BPDUs of which will not be changed. l Switch C compares the computed designated port configuration BPDU {0, 10, 2, CP2} with the configuration BPDU of CP2, and CP2 becomes the designated port, and the configuration BPDU of this port will be replaced with the computed configuration BPDU.	Root port CP1: {0, 0, 0, AP2} Designated port CP2: {0, 10, 2, CP2}
l Next, port CP2 receives the updated configuration BPDU of Switch B {0, 5, 1, BP2}. Because the received configuration BPDU is superior to its old one, Switch C launches a BPDU update process. l At the same time, port CP1 receives configuration BPDUs periodically from Switch A. Switch C does not launch an update process after comparison.	CP1: {0, 0, 0, AP2} CP2: {0, 5, 1, BP2}
By comparison: l Because the root path cost of CP2 (9) (root path cost of the BPDU (5) + path cost corresponding to CP2 (4)) is smaller than the root path cost of CP1 (10) (root path cost of the BPDU (0) + path cost corresponding to CP2 (10)), the BPDU of CP2 is elected as the optimum BPDU, and CP2 is elected as the root port, the messages of which will not be changed. l After comparison between the configuration BPDU of CP1 and the computed designated port configuration BPDU, port CP1 is blocked, with the configuration BPDU of the port remaining unchanged, and the port will not receive data from Switch A until a spanning tree computing process is triggered by a new condition, for example, the link from Switch B to Switch C becomes down.	Blocked port CP2: {0, 0, 0, AP2} Root port CP2: {0, 5, 1, BP2}

Device

Comparison process

BPDU of port after comparison

Switch A

l Port AP1 receives the configuration BPDU of Switch B {1, 0, 1, BP1}. Switch A finds that the configuration BPDU of the local port {0, 0, 0, AP1} is superior to the configuration received message, and discards the received configuration BPDU.

l Port AP2 receives the configuration BPDU of Switch C {2, 0, 2, CP1}. Switch A finds that the BPDU of the local port {0, 0, 0, AP2} is superior to the received configuration BPDU, and discards the received configuration BPDU.

l Switch A finds that both the root bridge and designated bridge in the configuration BPDUs of all its ports are Switch A itself, so it assumes itself to be the root bridge. In this case, it does not make any change to the configuration BPDU of each port, and starts sending out configuration BPDUs periodically.

AP1: {0, 0, 0, AP1}

AP2: {0, 0, 0, AP2}

Switch B

l Port BP1 receives the configuration BPDU of Switch A {0, 0, 0, AP1}. Switch B finds that the received configuration BPDU is superior to the configuration BPDU of the local port {1, 0,1, BP1}, and updates the configuration BPDU of BP1.

l Port BP2 receives the configuration BPDU of Switch C {2, 0, 2, CP2}. Switch B finds that the configuration BPDU of the local port {1, 0, 1, BP2} is superior to the received configuration BPDU, and discards the received configuration BPDU.

BP1: {0, 0, 0, AP1}

BP2: {1, 0, 1, BP2}

l Switch B compares the configuration BPDUs of all its ports, and determines that the configuration BPDU of BP1 is the optimum configuration BPDU. Then, it uses BP1 as the root port, the configuration BPDUs of which will not be changed.

l Based on the configuration BPDU of BP1 and the path cost of the root port (5), Switch B calculates a designated port configuration BPDU for BP2 {0, 5, 1, BP2}.

l Switch B compares the computed configuration BPDU {0, 5, 1, BP2} with the configuration BPDU of BP2. If the computed BPDU is superior, BP2 will act as the designated port, and the configuration BPDU on this port will be replaced with the computed configuration BPDU, which will be sent out periodically.

Root port BP1:

{0, 0, 0, AP1}

Designated port BP2:

{0, 5, 1, BP2}

Switch C

l Port CP1 receives the configuration BPDU of Switch A {0, 0, 0, AP2}. Switch C finds that the received configuration BPDU is superior to the configuration BPDU of the local port {2, 0, 2, CP1}, and updates the configuration BPDU of CP1.

l Port CP2 receives the configuration BPDU of port BP2 of Switch B {1, 0, 1, BP2} before the message was updated. Switch C finds that the received configuration BPDU is superior to the configuration BPDU of the local port {2, 0, 2, CP2}, and updates the configuration BPDU of CP2.

CP1: {0, 0, 0, AP2}

CP2: {1, 0, 1, BP2}

By comparison:

l The configuration BPDUs of CP1 is elected as the optimum configuration BPDU, so CP1 is identified as the root port, the configuration BPDUs of which will not be changed.

l Switch C compares the computed designated port configuration BPDU {0, 10, 2, CP2} with the configuration BPDU of CP2, and CP2 becomes the designated port, and the configuration BPDU of this port will be replaced with the computed configuration BPDU.

Root port CP1:

{0, 0, 0, AP2}

Designated port CP2:

{0, 10, 2, CP2}

l Next, port CP2 receives the updated configuration BPDU of Switch B {0, 5, 1, BP2}. Because the received configuration BPDU is superior to its old one, Switch C launches a BPDU update process.

l At the same time, port CP1 receives configuration BPDUs periodically from Switch A. Switch C does not launch an update process after comparison.

CP1: {0, 0, 0, AP2}

CP2: {0, 5, 1, BP2}

By comparison:

l Because the root path cost of CP2 (9) (root path cost of the BPDU (5) + path cost corresponding to CP2 (4)) is smaller than the root path cost of CP1 (10) (root path cost of the BPDU (0) + path cost corresponding to CP2 (10)), the BPDU of CP2 is elected as the optimum BPDU, and CP2 is elected as the root port, the messages of which will not be changed.

l After comparison between the configuration BPDU of CP1 and the computed designated port configuration BPDU, port CP1 is blocked, with the configuration BPDU of the port remaining unchanged, and the port will not receive data from Switch A until a spanning tree computing process is triggered by a new condition, for example, the link from Switch B to Switch C becomes down.

Blocked port CP2:

{0, 0, 0, AP2}

Root port CP2:

{0, 5, 1, BP2}

After the comparison processes described in the table above, a spanning tree with Switch A as the root bridge is determined, as shown in Figure 1-3.

Figure 1-3 The final computed spanning tree

& Note:

To facilitate description, the spanning tree computing process in this example is simplified, while the actual process is more complicated.

2) The BPDU forwarding mechanism in STP

l Upon network initiation, every switch regards itself as the root bridge, generates configuration BPDUs with itself as the root, and sends the configuration BPDUs at a regular interval of hello time.

l If it is the root port that received the configuration BPDU and the received configuration BPDU is superior to the configuration BPDU of the port, the device will increase message age carried in the configuration BPDU by a certain rule and start a timer to time the configuration BPDU while it sends out this configuration BPDU through the designated port.

l If the configuration BPDU received on the designated port has a lower priority than the configuration BPDU of the local port, the port will immediately sends out its better configuration BPDU in response.

l If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs and the old configuration BPDUs will be discarded due to timeout. In this case, the device will generate a configuration BPDU with itself as the root and sends out the BPDU. This triggers a new spanning tree computing process so that a new path is established to restore the network connectivity.

However, the newly computed configuration BPDU will not be propagated throughout the network immediately, so the old root ports and designated ports that have not detected the topology change continue forwarding data through the old path. If the new root port and designated port begin to forward data as soon as they are elected, a temporary loop may occur. For this reason, STP uses a state transition mechanism. Namely, a newly elected root port or designated port requires twice the forward delay time before transitioning to the forwarding state, when the new configuration BPDU has been propagated throughout the network.

1.1.2 Introduction to MSTP

I. Why MSTP

1) Disadvantages of STP and RSTP

STP does not support rapid state transition of ports. A newly elected root port or designated port must wait twice the forward delay time before transitioning to the forwarding state, even if it is a port on a point-to-point link or it is an edge port, which directly connects to a user terminal rather than to another device or a shared LAN segment.

The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows a newly elected root port or designated port to enter the forwarding state much quicker under certain conditions than in STP. As a result, it takes a shorter time for the network to reach the final topology stability.

& Note:

l In RSTP, a newly elected root port can enter the forwarding state rapidly if this condition is met: The old root port on the device has stopped forwarding data and the upstream designated port has started forwarding data.

l In RSTP, a newly elected designated port can enter the forwarding state rapidly if this condition is met: The designated port is an edge port or a port connected with a point-to-point link. If the designated port is an edge port, it can enter the forwarding state directly; if the designated port is connected with a point-to-point link, it can enter the forwarding state immediately after the device undergoes handshake with the downstream device and gets a response.

Although RSTP support rapid network convergence, it has the same drawback as STP does: All bridges within a LAN share the same spanning tree, so redundant links cannot be blocked based on VLANs, and the packets of all VLANs are forwarded along the same spanning tree.

2) Features of MSTP

The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP and RSTP. In addition to support for rapid network convergence, it also allows data flows of different VLANs to be forwarded along their own paths, thus providing a better load sharing mechanism for redundant links.

MSTP features the following:

l MSTP supports mapping VLANs to MST instances by means of a VLAN-to-instance mapping table.

l MSTP divides a switched network into multiple regions, each containing multiple spanning trees that are independent of one another.

l MSTP prunes loop networks into a loop-free tree, thus avoiding proliferation and endless recycling of packets in a loop network. In addition, it provides multiple redundant paths for data forwarding, thus supporting load balancing of VLAN data in the data forwarding process.

l MSTP is compatible with STP and RSTP.

II. Some concepts in MSTP

As shown in Figure 1-4, there are four multiple spanning tree (MST) regions, each made up of four switches running MSTP. In light with the diagram, the following paragraphs will present some concepts of MSTP.

Figure 1-4 Basic concepts in MSTP

1) MST region

An MST region is composed of multiple devices in a switched network and network segments among them. These devices have the following characteristics:

l All are MSTP-enabled,

l They have the same region name,

l They have the same VLAN-to-instance mapping configuration,

l They have the same MSTP revision level configuration, and

l They are physically linked with one another.

In area A0 in Figure 1-4, for example, all the device have the same MST region configuration: the same region name, the same VLAN-to-instance mapping (VLAN1 is mapped to MST instance 1, VLAN2 to MST instance 2, and the rest to the command and internal spanning tree (CIST). CIST refers to MST instance 0), and the same MSTP revision level (not shown in the figure).

Multiple MST regions can exist in a switched network. You can use an MSTP command to group multiple devices to the same MST region.

2) VLAN-to-instance mapping table

As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping relationships between VLANs and MST instances. In Figure 1-4, for example, the VLAN-to-instance mapping table of region A0 describes that the same region name, the same VLAN-to-instance mapping (VLAN1 is mapped to MST instance 1, VLAN2 to MST instance 2, and the rest to CIST.

3) CST

The CST is a single spanning tree that connects all MST regions in a switched network. If you regard each MST region as a “device”, the CST is a spanning tree computed by these devices through MSTP. For example, the red lines in Figure 1-4 describe the CST.

4) IST

Internal spanning tree (IST) is a spanning tree that runs in an MSTP region, with the instance number of 0. ISTs in all MST regions the common spanning tree (CST) jointly constitute the common and internal spanning tree (CIST) of the entire network. An IST is a section of the CIST in an MST region. In Figure 1-4, for example, the CIST has a section is each MST region, and this section is the IST in each MST region.

5) CIST

Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that connects all devices in a switched network. In Figure 1-4, for example, the ISTs in all MST regions plus the inter-region CST constitute the CIST of the entire network.

6) MSTI

Multiple spanning trees can be generated in an MST region through MSTP, one spanning tree being independent of another. Each spanning tree is referred to as a multiple spanning tree instance (MSTI). In Figure 1-4, for example, multiple spanning tree can exist in each MST region, each spanning tree corresponding to a VLAN. These spanning trees are called MSTIs.

7) Regional root bridge

The root bridge of the IST or an MSTI within an MST region is the regional root bridge of the MST or that MSTI. Based on the topology, different spanning trees in an MST region may have different regional roots. For example, in region D0 in Figure 1-4, the regional root of instance 1 is device B, while that of instance 2 is device C.

8) Common root bridge

The root bridge of the CIST is the common root bridge. In Figure 1-4, for example, the common root bridge is a device in region A0.

9) Region edge port

An region edge port is a port that connects an MST region to another MST configuration, or to a single spanning-tree region running STP, or to a single spanning-tree region running RSTP.

During MSTP computing, an region edge port assumes the same role on the CIST and on MST instances. Namely, if an region edge port is master port on the CIST, it is also the master port on all MST instances within this region. In Figure 1-4, for example, if a device in region A0 is interconnected with the first port of a device in region D0 and the common root bridge of the entire switched network is located in region A0, the first port of that device in region D0 is the region edge port of region D0.

10) Roles of ports

In the MSTP computing process, port roles include designated port, root port, master port, alternate port, backup port, and so on.

l Root port: a port responsible for forwarding data to the root bridge.

l Designated port: a port responsible for forwarding data to the downstream network segment or device.

l Master port: A port on the shortest path from the entire region to the common root bridge, connect the MST region to the common root bridge.

l Alternate port: The standby port for a root port or master port. If a root port or master port is blocked, the alternate port becomes the new root port or master port.

l Backup port: If a loop occurs when two ports of the same device are interconnected, the device will block either of the two ports, and the backup port is that port to be blocked.

A port can assume different roles in different MST instances.

Figure 1-5 Port roles

Figure 1-5 helps understand these concepts. Where,

l Devices A, B, C, and D constitute an MST region.

l Port 1 and port 2 of device A connect to the common root bridge.

l Port 5 and port 6 of device C form a loop.

l Port 3 and port 4 of device D connect downstream to other MST regions.

11) Port state

In MSTP, according to the status of MAC address learning and user traffic forwarding, the port state can be:

l Forwarding state, where the port learns MAC addresses and forwards user traffic.

l Learning state, where the port learns MAC addresses but does not forward user traffic.

l Discarding state, where the port neither learns MAC addresses nor forwards user traffic.

& Note:

A port can be in different states in different MSTIs.

The state and the role of a port are not closely correlated. Table 1-6 lists the possible combinations of port states and port roles:

Table 1-6 Combinations of port states and port roles

	Root/ port/Master port	Designated port	Region edge port	Alternate port	Backup port
Forwarding	√	√	√	—	—
Learning	√	√	√	—	—
Discarding	√	√	√	√	√

III. How MSTP works

MSTP divides an entire Layer 2 network into multiple MST regions, which are interconnected by a computed CST. Inside an MST region, multiple spanning trees are generated through computing, each spanning tree called an MST instance. Among these MST instances, instance 0 is the IST, while all the others are MSTIs. Similar to RSTP, MSTP uses configuration BPDUs to compute spanning trees. The only difference between the two protocols being in that what is carried in an MSTP BPDU is the MSTP configuration on the device from which this BPDU is sent.

1) CIST computing

By comparison of “configuration BPDUs”, one device with the highest priority is elected as the root bridge of the CIST. MSTP generates an IST within each MST region through computing, and, at the same time, MSTP regards each MST region as a single device and generates a CST among these MST regions through computing. The CST and ISTs constitute the CIST of the entire network.

2) MSTI computing

Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to-instance mappings.

MSTP performs a separate computing process, which is similar to spanning tree computing in STP, for each spanning tree. For details, refer to “How STP works” in section 1.1.1.

In MSTP, a VLAN packet is forwarded along the following paths:

l Within an MST region, the packet is forwarded along the corresponding MSTI.

l Between two MST regions, the packet is forwarded along the CST.

IV. Implementation of MSTP on devices

MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be recognized by devices running MSTP and used for spanning tree computing.

In addition to basic MSTP functions, many management-facilitating special functions are provided, as follows:

l BPDU guard

l Root guard

l Loop guard

l TC-BPDU attack guard

1.2 Configuration Task List

Before configuration, you need to know the position of each device in each MST instance: root bridge or leave node. In each instance, one, and only one device acts as the root bridge, while all others as leaf nodes.

Task		Remarks
Configuring the Root Bridge	Configuring an MST Region	Required
	Specifying the Root Bridge or a Secondary Root Bridge	Optional
	Configuring the Work Mode of MSTP	Optional
	Configuring the Priority of the Current Device	Optional
	Configuring the Maximum Hops of an MST Region	Optional
	Configuring the Network Diameter of a Switched Network	Optional
	Configuring Timers of MSTP	Optional
	Configuring the Timeout Factor	Optional
	Configuring the Maximum Transmission Rate of Ports	Optional
	Configuring Ports as Edge Ports	Optional
	Configuring Whether Ports Connect to Point-to-Point Links	Optional
	Configuring the MSTP Packet Format for Ports	Optional
	Enabling the MSTP Feature	Required
Configuring Leaf Nodes	Configuring an MST Region	Required
	Configuring the Work Mode of MSTP	Optional
	Configuring the Timeout Factor	Optional
	Configuring the Maximum Transmission Rate of Ports	Optional
	Configuring Ports as Edge Ports	Optional
	Configuring Path Costs of Ports	Optional
	Configuring Port Priority	Optional
	Configuring Whether Ports Connect to Point-to-Point Links	Optional
	Configuring the MSTP Packet Format for Ports	Optional
	Enabling the MSTP Feature	Required
Performing mCheck		Optional
Configuring Digest Snooping		Optional
Configuring No Agreement Check		Optional
Configuring Protection Functions		Optional

& Note:

If both GVRP and MSTP are enabled on a device at the same time, GVRP packets will be forwarded along the CIST. Therefore, if both GVRP and MSTP are running on the same device and you wish to advertise a certain VLAN within the network through GVRP, make sure that this VLAN is mapped to the CIST (instance 0) when configuring the VLAN-to-instance mapping table.

1.3 Configuring the Root Bridge

1.3.1 Configuring an MST Region

I. Configuration procedure

Follow these steps to configure an MST region:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter MST region view	stp region-configuration	—
Configure the MST region name	region-name name	Required The MST region name is the MAC address by default
Configure the VLAN-to-instance mapping table	instance instance-id vlan vlan-list	Use either command All VLANs in an MST region are mapped to MST instance 0 by default
Configure the VLAN-to-instance mapping table	vlan-mapping modulo modulo
Configure the MSTP revision level of the MST region	revision-level level	Optional 0 by default
Activate MST region configuration manually	active region-configuration	Required
Display all the configuration information of the MST region	check region-configuration	Optional
Display the currently effective MST region configuration information	display stp region-configuration	The display command can be executed in any view

Caution:

Two device belong to the same MST region only if they are configure to have the same MST region name, the same VLAN-to-instance mapping entries in the MST region and the same MST region revision level, and they are interconnected via a physical link.

Your configuration of MST region–related parameters, especially the VLAN-to-instance mapping table, will cause MSTP to launch a new spanning tree computing process, which may result in network topology instability. To reduce the possibility of topology instability caused by configuration, MSTP will not immediately launch a new spanning tree computing process when processing MST region–related configurations; instead, such configurations will take effect only if you:

l activate the MST region–related parameters suing the active region-configuration command, or

l enable MSTP using the stp enable command.

II. Configuration example

# Configure the MST region name to be “info”, the MSTP revision level to be 1, and VLAN 2 through VLAN 10 to be mapped to instance 1 and VLAN 20 through VLAN 30 to instance 2.

<Sysname> system-view

[Sysname] stp region-configuration

[Sysname-mst-region] region-name info

[Sysname-mst-region] instance 1 vlan 2 to 10

[Sysname-mst-region] instance 2 vlan 20 to 30

[Sysname-mst-region] revision-level 1

[Sysname-mst-region] active region-configuration

1.3.2 Specifying the Root Bridge or a Secondary Root Bridge

MSTP can determine the root bridge of a spanning tree through MSTP computing. Alternatively, you can specify the current device as the root bridge using the commands provided by the system.

I. Specifying the current device as the root bridge of a specific spanning tree

Follow these steps to specify the current device as the root bridge of a specific spanning tree:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Specify the current device as the root bridge of a specific spanning tree	stp [ instance instance-id ] root primary [ bridge-diameter bridge-number ] [ hello-time centi-seconds ]	Required The device does not function as the root bridge by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Specify the current device as the root bridge of a specific spanning tree

stp [ instance instance-id ] root primary [ bridge-diameter bridge-number ] [ hello-time centi-seconds ]

Required

The device does not function as the root bridge by default

II. Specifying the current device as a secondary root bridge of a specific spanning tree

Follow these steps to specify the current device as a secondary root bridge of a specific spanning tree:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Specify the current device as a secondary root bridge of a specific spanning tree	stp [ instance instance-id ] root secondary [ bridge-diameter bridge-number ] [ hello-time centi-seconds ]	Required By default, a device does not function as a secondary root bridge

To do...

Use the command...

Remarks

Enter system view

system-view

—

Specify the current device as a secondary root bridge of a specific spanning tree

stp [ instance instance-id ] root secondary [ bridge-diameter bridge-number ] [ hello-time centi-seconds ]

Required

By default, a device does not function as a secondary root bridge

Note that:

l Upon specifying the current device as the root bridge or a secondary root bridge, you cannot change the priority of the device.

l You can configure the current device as the root bridge or a secondary root bridge of an MST instance, which is specified by instance instance-id in the command. If you set instance-id to 0, the current device will be the root bridge or a secondary root bridge of the CIST.

l The current device has independent roles in different instances. It can act as the root bridge or a secondary root bridge of one instance while it can also act as the root bridge or a secondary root bridge of another instance. However, the same device cannot be the root bridge and a secondary root bridge in the same instance at the same time.

l There is one and only one root bridge in effect in a spanning tree instance. If two or more devices have been designated to be root bridges of the same spanning tree instance, MSTP will select the device with the lowest MAC address as the root bridge.

l You can specify multiple secondary root bridges for the same instance. Namely, you can specify secondary root bridges for the same instance on two or more than two device.

l When the root bridge of an instance fails or is shut down, the secondary root bridge (if you have specified one) can take over the role of the instance. However, if you specify a new root bridge for the instance at this time, the secondary root bridge will not become the root bridge. If you have specified multiple secondary root bridges for an instance, when the root bridge fails, MSTP will select the secondary root bridge with the lowest MAC address as the new root bridge.

l When specifying the root bridge or a secondary root bridge, you can specify the network diameter and hello time. However, these two options are effective only for MST instance 0, namely the CIST. If you include these two options in your command for any other instance, your configuration can succeed, but they will not actually work. For the description of network diameter and hello time, refer to “Configuring the Network Diameter of a Switched Network” and “Configuring Timers of MSTP”.

l Alternatively, you can also specify the current device as the root bridge by setting by priority of the device to 0. For the device priority configuration, refer to “Configuring the Priority of the Current Device”.

III. Configuration example

# Specify the current device as the root bridge of MST instance 1 and a secondary root bridge of MST instance 2.

<Sysname> system-view

[Sysname] stp instance 1 root primary

[Sysname] stp instance 2 root secondary

1.3.3 Configuring the Work Mode of MSTP Device

MSTP and RSTP can recognize each other’s protocol packets, so they are mutually compatible. However, STP is unable to recognize MSTP packets. For hybrid networking with legacy STP devices and full interoperability with RSTP-compliant devices, MSTP supports three work modes: STP-compatible mode, RSTP mode, and MSTP mode.

l In STP-compatible mode, all ports of the device send out STP BPDUs,

l In RSTP mode, all ports of the device send out RSTP BPDUs. If the device detects that it is connected with a legacy STP device, the port connecting with the legacy STP device will automatically migrate to STP-compatible mode.

l In MSTP mode, all ports of the device send out MSTP BPDUs. If the device detects that it is connected with a legacy STP device, the port connecting with the legacy STP device will automatically migrate to STP-compatible mode.

I. Configuration procedure

Follow these steps to configure the MSTP work mode:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the work mode of MSTP	stp mode { stp \| rstp \| mstp }	Optional MSTP mode by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Configure the work mode of MSTP

stp mode { stp | rstp | mstp }

Optional

MSTP mode by default

II. Configuration example

# Configure MSTP to work in STP-compatible mode.

<Sysname> system-view

[Sysname] stp mode stp

1.3.4 Configuring the Priority of the Current Device

The priority of a device determines whether it can be elected as the root bridge of a spanning tree. A lower value indicates a higher priority. By setting the priority of a device to a low value, you can specify the device as the root bridge of spanning tree. An MSTP-compliant device can have different priorities in different MST instances.

I. Configuration procedure

Follow these steps to configure the priority of the current device:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the priority of the current device	stp [ instance instance-id ] priority priority	Optional 32768 by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Configure the priority of the current device

stp [ instance instance-id ] priority priority

Optional

32768 by default

Caution:

l Upon specifying the current device as the root bridge or a secondary root bridge, you cannot change the priority of the device.

l During root bridge selection, if all devices in a spanning tree have the same priority, the one with the lowest MAC address will be selected as the root bridge of the spanning tree.

II. Configuration example

# Set the device priority in MST instance 1 to 4096.

<Sysname> system-view

[Sysname] stp instance 1 priority 4096

1.3.5 Configuring the Maximum Hops of an MST Region

By setting the maximum hops of an MST region, you can restrict the region size. The maximum hops setting configured on the regional root bridge will be used as the maximum hops of the MST region.

After a configuration BPDU leaves the root bridge of the spanning tree in the region, its hop count is decremented by 1 whenever it passes a device. When its hop count reaches 0, it will be discarded by the device that has received it. As a result, devices beyond the maximum hops are unable to take part in spanning tree computing, and thereby the size of the MST region is restricted.

I. Configuration procedure

Follow these steps to configure the maximum hops of the MST region

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the maximum hops of the MST region	stp max-hops hops	Optional 20 by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Configure the maximum hops of the MST region

stp max-hops hops

Optional

20 by default

& Note:

A larger maximum hops setting means a larger size of the MST region. Only the maximum hops configured on the regional root bridge can restrict the size of the MST region.

II. Configuration example

# Set the maximum hops of the MST region to 30.

<Sysname> system-view

[Sysname] stp max-hops 30

1.3.6 Configuring the Network Diameter of a Switched Network

Any two stations in a switched network are interconnected through specific paths, which are composed of a series of devices. Represented by the number of devices on a path, the network diameter is the path that comprises more devices than any other among these paths.

I. Configuration procedure

Follow these steps to configure the network diameter of the switched network:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the network diameter of the switched network	stp bridge-diameter bridge-number	Optional 7 by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Configure the network diameter of the switched network

stp bridge-diameter bridge-number

Optional

7 by default

Caution:

l Network diameter is a parameter that indicates network size. A bigger network diameter represents a larger network size.

l Based on the network diameter you configured, MSTP automatically sets an optimal hello time, forward delay, and max age for the device.

l The configured network diameter is effective for the CIST only, and not for MSTIs.

II. Configuration example

# Set the network diameter of the switched network to 6.

<Sysname> system-view

[Sysname] stp bridge-diameter 6

1.3.7 Configuring Timers of MSTP

MSTP involves three timers: forward delay, hello time and max age.

l Forward delay: the time a device will wait before changing states. A link failure can trigger a spanning tree computing process, and the spanning tree structure will change accordingly. However, as a new configuration BPDU cannot be propagated throughout the network immediately, if the new root port and designated port begin to forward data as soon as they are elected, a temporary loop may occur. For this reason, the protocol uses a state transition mechanism. Namely, a newly elected root port or designated port must wait twice the forward delay time before transitioning to the forwarding state, when the new configuration BPDU has been propagated throughout the network.

l Hello time is sued to detect whether a link is faulty. A device sends a hello packet to the devices around it at a regular interval of hello time to check whether any link is faulty.

l Max time is a used for determining whether a configuration BPDU has “expired”. A BPDU that has “expired” will be discarded by the device.

I. Configuration procedure

Follow these steps to configure the timers of MSTP:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the forward delay timer	stp timer forward-delay centi-seconds	Optional 1,500 centiseconds (15 seconds) by default
Configure the hello time timer	stp timer hello centi-seconds	Optional 200 centiseconds (2 seconds) by default
Configuring the max age timer	stp timer max-age centi-seconds	Optional 2,000 centiseconds (20 seconds) by default

These three timers set on the root bridge of the CIST apply on all the devices on the entire switched network.

Caution:

l The length of the forward delay time is related to the network diameter of the switched network. Typically, the larger the network diameter is, the longer the forward delay time should be. Note that if the forward delay setting is too small, temporary redundant paths may be introduced; if the forward delay setting is too big, it may take a long time for the network to resume connectivity. We recommend that you use the default setting.

l An appropriate hello time setting enables the device to timely detect link failures on the network without using excessive network resources. If the hello time is set too long, the device will take packet loss on a link for link failure and trigger a new spanning tree computing process; if the hello time is set too short, the device will send repeated configuration BPDUs frequently, which adds to the device burden and causes waste of network resources. We recommend that you use the default setting.

l If the max age time setting is too small, the network devices will frequently launch spanning tree computing and may take network congestion to a link failure; if the max age setting is too large, the network may fail to timely detect link failures and fail to timely launch spanning tree computing, thus reducing the auto-sensing capability of the network. We recommend that you use the default setting.

The setting of hello time, forward delay and max age must meet the following formulae; otherwise network instability will frequently occur.

l 2 × (forward delay – 1 second) ¦ max age

l Ma x age ¦ 2 × (hello time + 1 second)

We recommend that you specify the network diameter in the stp root primary command and let MSTP automatically calculate an optimal setting of these three timers.

II. Configuration example

# Set the forward delay to 1,600 centiseconds, hello time to 300 centiseconds, and max age to 2,100 centiseconds.

<Sysname> system-view

[Sysname] stp timer forward-delay 1600

[Sysname] stp timer hello 300

[Sysname] stp timer max-age 2100

1.3.8 Configuring the Timeout Factor

A device sends a BPDU to the devices around it at a regular interval of hello time to check whether any link is faulty. Typically, if a device does not receive a BPDU from the upstream device within nine times the hello time, it will assume that the upstream device has failed and start a new spanning tree computing process.

In a very stable network, this kind of spanning tree computing may occur because the upstream device is busy. In this case, you can avoid such unwanted spanning tree computing by lengthening the timeout time.

I. Configuration procedure

Follow these steps to configure the timeout factor:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Configure the timeout factor of the device	stp timer-factor number	Optional 3 by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Configure the timeout factor of the device

stp timer-factor number

Optional

3 by default

& Note:

l Timeout time = timeout factor × 3 × hello time.

l Typically, we recommend that you set the timeout factor to 5, or 6, or 7 for a stable network.

II. Configuration example

# Set the timeout factor to 6.

<Sysname> system-view

[Sysname] stp timer-factor 6

1.3.9 Configuring the Maximum Transmission Rate of Ports

The maximum transmission rate of a port refers to the maximum number of MSTP packets that the port can send within each hello time.

The maximum transmission rate of an Ethernet port is related to the physical status of the port and the network structure. You can make your configuration based on the actual networking condition.

I. Configuration procedure

Following these steps to configure the maximum transmission rate of a port or a group of ports:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Configure the maximum transmission rate of the port(s)		stp transmit-limit packet-number	Optional 10 by default

& Note:

If the maximum transmission rate setting of a port is too big, the port will send a large number of MSTP packets within each hello time, thus using excessive network resources. We recommend that you use the default setting.

II. Configuration example

# Set the maximum transmission rate of port Ethernet1/0/1 to 5.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp transmit-limit 5

1.3.10 Configuring Ports as Edge Ports

If a port directly connects to a user terminal rather than another device or a shared LAN segment, this port is regarded as an edge port. When the network topology changes, an edge port will not cause a temporary loop. Therefore, if you specify a port as an edge port, this port can transition rapidly from the blocked state to the forwarding state without delay.

I. Configuration procedure

Following these steps to specify a port or a group of ports as edge port(s):

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Configure the port(s) as edge port(s)		stp edged-port enable	Required All Ethernet ports are non-edge ports by default

& Note:

l With BPDU guard disabled, when a port set as an edge port receives a BPDU from another port, it will become a non-edge port again. In this case, you must reset the port before you can configure it to be an edge port again.

l If a port directly connects to a user terminal, configure it to be an edge port and enable BPDU guard for it. This enables the port to transition to the forwarding state while ensuring network security.

II. Configuration example

# Configure Ethernet1/0/1 to be an edge port.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp edged-port enable

1.3.11 Configuring Whether Ports Connect to Point-to-Point Links

A point-to-point link is a link directly connecting with two devices. If the two ports across a point-to-point link are root ports or designated ports, the ports can rapidly transition to the forwarding state by transmitting synchronization packets.

I. Configuration procedure

Following these steps to configure whether a port or a group of ports connect to point-to-point links:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Configure whether the port(s) connect to point-to-point links		stp point-to-point { auto \| force-false \| force-true }	Optional The default setting is auto; namely the device automatically detects whether an Ethernet port connects to a point-to-point link

& Note:

l As for aggregated ports, all ports can be configured as connecting to point-to-point links. If a port works in auto-negotiation mode and the negotiation result is full duplex, this port can be configured as connecting to a point-to-point link.

l If a port is configured as connecting to a point-to-point link, the setting takes effect for the port in all MST instances. If the physical link to which the port connects is not a point-to-point link and you force it to be a point-to-point link by configuration, your configuration may incur a temporary loop.

II. Configuration example

# Configure port Ethernet1/0/1 as connecting to a point-to-point link.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp point-to-point force-true

1.3.12 Configuring the MSTP Packet Format for Ports

A port support two types of MSTP packets:

l 802.1s-compliant standard format

l Compatible format

The default packet format setting is auto, namely a port recognizes the two MSTP packet formats automatically. You can configure the MSTP packet format to be used by a port through CLI. After your configuration, when working in MSTP mode, the port sends and receives only the packets of the standard format or packets of the formats compatible with the non-standard format. In this case, the port sends and receives only MSTP packets of the format you have configured.

I. Configuration procedure

Follow these steps to configure the MSTP packet format for a port or a group of ports:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Configure the MSTP packet format for the port(s)		stp compliance { auto \| dot1s \| legacy }	Optional auto by default

& Note:

l If the port is configured not to detect the packet format automatically while it works in the MSTP mode, and if it receives a packet in the format other than as configured, that port will become a designated port, and the port will remain in the discarding state to prevent the occurrence of a loop.

l If a port receives MSTP packets of different formats frequently, this means that the MSTP packet formation configuration contains error. In this case, if the port is working in MSTP mode, it will be disabled for protection. Those ports closed thereby can be restored only by the network administers.

II. Configuration example

# Configure port Ethernet1/0/1 to receive and send standard-format MSTP packets.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp compliance dot1s

1.3.13 Enabling the MSTP Feature

I. Configuration procedure

Follow these steps to enable the MSTP feature:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enable the MSTP feature for the device		stp enable	Required Whether a device is MSTP-enabled by default depends on the specific device model.
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Enable the MSTP feature for the port(s)		stp enable	Optional By default, MSTP is enabled for all ports after it is enabled for the device globally
Disable the MSTP feature for the port(s)		stp disable or undo stp	Optional To control MSTP flexibly, you can disable the MSTP feature for certain Ethernet ports so that these ports will not take part in spanning tree computing and thus to save the device’s CPU resources

& Note:

You must enable MSTP for the device before any other MSTP-related configuration can take effect.

II. Configuration example

# Enable MSTP for the device and disable MSTP for port Ethernet1/0/1.

<Sysname> system-view

[Sysname] stp enable

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp disable

1.4 Configuring Leaf Nodes

1.4.1 Configuring an MST Region

Refer to ”Configuring an MST Region”.

1.4.2 Configuring the Work Mode of MSTP

Refer to “Configuring the Work Mode of MSTP”.

1.4.3 Configuring the Timeout Factor

Refer to “Configuring the Timeout Factor”.

1.4.4 Configuring the Maximum Transmission Rate of Ports

Refer to “Configuring the Maximum Transmission Rate of Ports”.

1.4.5 Configuring Ports as Edge Ports

Refer to “Configuring Ports as Edge Ports”.

1.4.6 Configuring Path Costs of Ports

Path cost is a parameter related to the rate of port-connected links. On an MSTP-compliant device, ports can have different priorities in different MST instances. Setting an appropriate path cost allows VLAN traffic flows to be forwarded along different physical links, thus to enable per-VLAN load balancing.

The device can automatically calculate the default path cost; alternatively, you can also configure the path cost for ports.

I. Specifying a standard that the device uses when calculating the default path cost

You can specify a standard for the device to use in automatic calculation for the default path cost. The device supports the following standards:

l dot1d-1998: The device calculates the default path cost for ports based on IEEE 802.1D-1998.

l dot1t: The device calculates the default path cost for ports based on IEEE 802.1t.

l legacy: The device calculates the default path cost for ports based on a private standard.

Follow these steps to specify a standard for the device to use when calculating the default path cost:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Specify a standard for the device to use when calculating the default path cost of the link connected with the device	stp pathcost-standard { dot1d-1998 \| dot1t \| legacy }	Optional The default standard used by the device depends on the specific device model.

To do...

Use the command...

Remarks

Enter system view

system-view

—

Specify a standard for the device to use when calculating the default path cost of the link connected with the device

stp pathcost-standard { dot1d-1998 | dot1t | legacy }

Optional

The default standard used by the device depends on the specific device model.

Table 1-7 Link speed vs. path cost

Link speed	Duplex state	802.1D-1998	802.1t	Private standard
0	—	65535	200,000,000	200,000
10 Mbps	Single Port Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	100 100 100 100	2,000,000 1,000,000 666,666 500,000	2,000 1,800 1,600 1,400
100 Mbps	Single Port Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	19 19 19 19	200,000 100,000 66,666 50,000	200 180 160 140
1000 Mbps	Single Port Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	4 4 4 4	20,000 10,000 6,666 5,000	20 18 16 14
10 Gbps	Single Port Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	2 2 2 2	2,000 1,000 666 500	2 1 1 1

Link speed

Duplex state

802.1D-1998

802.1t

Private standard

—

65535

200,000,000

200,000

10 Mbps

Single Port

Aggregated Link 2 Ports

Aggregated Link 3 Ports

Aggregated Link 4 Ports

100

2,000,000

1,000,000

666,666

500,000

2,000

1,800

1,600

1,400

100 Mbps

Single Port

Aggregated Link 2 Ports

Aggregated Link 3 Ports

Aggregated Link 4 Ports

200,000

100,000

66,666

50,000

200

180

160

140

1000 Mbps

Single Port

Aggregated Link 2 Ports

Aggregated Link 3 Ports

Aggregated Link 4 Ports

20,000

10,000

6,666

5,000

10 Gbps

Single Port

Aggregated Link 2 Ports

Aggregated Link 3 Ports

Aggregated Link 4 Ports

2,000

1,000

666

500

& Note:

In the calculation of the path cost value of an aggregated link, 802.1D-1998 does not take into account the number of ports in the aggregated link. Whereas, 802.1T takes the number of ports in the aggregated link into account. The calculation formula is: Path Cost = 200,000,000/link speed in 100 kbps, where link speed is the sum of the link speed values of the non-blocked ports in the aggregated link.

II. Configuring Path Costs of Ports

Follow these steps to configure the path cost of ports:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Configure the path cost of the port(s)		stp [ instance instance-id ] cost cost	Required By default, MSTP automatically calculates the path cost of each port

Caution:

l If you change the standard that the device uses in calculating the default path cost, the port path cost value set through the stp cost command will be out of effect.

l When the path cost of a port is changed, MSTP will re-compute the role of the port and initiate a state transition. If you use 0 as instance-id, you are setting the path cost of the CIST.

1.4.7 Configuring Port Priority

The priority of a port is an import basis that determines whether the port can be elected as the root port of device. If all other conditions are the same, the port with the highest priority will be elected as the root port.

On an MSTP-compliant device, a port can have different priorities in different MST instances, and the same port can play different roles in different MST instances, so that data of different VLANs can be propagated along different physical paths, thus implementing per-VLAN load balancing. You can set port priority values based on the actual networking requirements.

I. Configuration procedure

Follow these steps to configure the priority of a port or a group of ports:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Configure port priority		stp [ instance instance-id ] port priority priority	Optional 128 for all Ethernet ports by default

& Note:

l When the priority of a port is changed, MSTP will re-compute the role of the port and initiate a state transition.

l Generally, a lower configured value priority indicates a higher priority of the port. If you configure the same priority value for all the Ethernet ports on the a device, the specific priority of a port depends on the index number of that port. Changing the priority of an Ethernet port triggers a new spanning tree computing process.

II. Configuration example

# Set the priority of port Ethernet1/0/1 to 16 in MST instance 1.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp instance 1 port priority 16

1.4.8 Configuring Whether Ports Connect to Point-to-Point Links

Refer to “Configuring Whether Ports Connect to Point-to-Point Links”.

1.4.9 Configuring the MSTP Packet Format for Ports

Refer to “Configuring the MSTP Packet Format for Ports”.

1.4.10 Enabling the MSTP Feature

Refer to “Enabling the MSTP Feature”.

1.5 Performing mCheck

Ports on an MSTP-compliant device have three working modes: STP compatible mode, RSTP mode, and MSTP mode.

In a switched network, if a port on the device running MSTP (or RSTP) connects to a device running STP, this port will automatically migrate to the STP-compatible mode. However, if the device running STP is removed, this will not be able to migrate automatically to the MSTP (or RSTP) mode, but will remain working in the STP-compatible mode. In this case, you can perform an mCheck operation to force the port to migrate to the MSTP (or RSTP) mode.

You can perform mCheck on a port through two approaches, which lead to the same result.

1.5.1 Configuration Prerequisites

MSTP has been correctly configured on the device.

1.5.2 Configuration Procedure

I. Perform global mCheck

Follow these steps to perform global mCheck:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Perform mCheck	stp mcheck	Required

II. Perform mCheck in Ethernet port view

Follow these steps to perform mCheck in Ethernet port view:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Perform mCheck	stp mcheck	Required

Caution:

The stp mcheck command is meaningful only when the device works in the MSTP (or RSTP) mode, not in the STP-compatible mode.

1.5.3 Configuration Example

# Perform mCheck on port Ethernet1/0/1.

1) Method 1: Perform mCheck globally.

<Sysname> system-view

[Sysname] stp mcheck

2) Method 2: Perform mCheck in Ethernet port view

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp mcheck

1.6 Configuring Digest Snooping

As defined in IEEE 802.1s, interconnected devices are in the same region only when the region related configuration (domain name, revision level, VLAN-to-instance mappings) on them is identical. An MSTP enabled device identifies devices in the same MST region via checking the configuration ID in BPDU packets. The configuration ID includes the region name, revision level, configuration digest that is in 16-byte length and is the result computed via the HMAC-MD5 algorithm based on VLAN-to-instance mappings.

Since MSTP implementations differ with vendors, the configuration digest computed using private key is different; hence different vendors’ devices in the same MST region can not communicate with each other.

Enabling the Digest Snooping feature on the associated port can make a device communicate with another vendor’s device in the same MST region.

1.6.1 Configuration Prerequisites

Associated devices of different vendors are interconnected and run MSTP.

1.6.2 Configuration Procedure

Follow these steps to configure Digest Snooping:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port or port group view	Enter Ethernet port view	interface interface-type interface-number	Choose either
Enter Ethernet port or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }	Choose either
Enable digest snooping on the interface or port group		stp config-digest-snooping	Required Not enabled by default
Exit to system view		quit	—
Enable global digest snooping		stp config-digest-snooping	Required Not enabled by default
Display configuration information		display stp	Available in any view

Caution:

l You can only enable the Digest Snooping feature on the device connected to another vendor’s device that use private key to compute the configuration digest.

l With the Digest Snooping feature enabled, comparison of configuration digest is not needed for in-the-same-region check, so the VLAN-to-instance mappings must be the same on associated ports.

l With global Digest Snooping enabled, modification of VLAN-to-instance mappings and removing of the current region configuration using the undo stp region-configuration command are not allowed. You can only modify the region name and revision level.

l You need to enable this feature both globally and on associated ports to make it take effect. It is recommended to enable the feature on all associated ports first and then globally, making all configured ports take effect, and disable the feature globally to disable it on all associated ports.

l It is not recommended to enable Digest Snooping on the MST region edge port to avoid loops.

l It is recommended to enable Digest Snooping first and then MSTP. Do not enable Digest Snooping when the network works well to avoid traffic interruption.

1.6.3 Configuration Example

I. Network requirements

l Switch A and B connect to a different vendor’s device and all of them are in the same region.

l Enable Digest Snooping on Switch A and B to make the three communicate with each other.

II. Network diagram

Figure 1-6 Digest Snooping configuration

III. Configuration procedure

1) Enable Digest Snooping on Switch A.

# Enable Digest Snooping on Ethernet1/0/1.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp config-digest-snooping

# Enable global Digest Snooping.

[Sysname-Ethernet1/0/1] quit

[Sysname] stp config-digest-snooping

2) Enable Digest Snooping on Switch B (similar with the above, omitted).

1.7 Configuring No Agreement Check

Two types of packet are used for rapid state transition on designated RSTP and MSTP ports:

l Proposal: Packets sent by designated ports to request rapid transition

l Agreement: Packets used to acknowledge rapid transition requests

Both RSTP and MSTP switches can perform rapid transition operation on a designated port only when the port receives an agreement packet from the downstream switch. The differences between RSTP and MSTP switches are:

l For MSTP, the downstream device’s root port sends an agreement packet only after it receives an agreement packet from the upstream device.

l For RSTP, the down stream device sends an agreement packet regardless of whether an agreement packet from the upstream device is received.

Figure 1-7 and Figure 1-8 show the rapid state transition mechanism on MSTP and RSTP designated ports.

Figure 1-7 Rapid state transition mechanism on the MSTP designated port

Figure 1-8 Rapid state transition mechanism on the RSTP designated port

If the upstream device comes from another vendor, the rapid state transition implementation may be limited. For example, when the upstream device adopts RSTP, the downstream device adopts MSTP and does not support RSTP mode, the root port on the downstream device receives no agreement packet from the upstream device and thus sends no agreement packets to the upstream device. As a result, the designated port of the upstream switch fails to transit rapidly and can only change to the Forwarding state after a period twice the Forward Delay.

In this case, you can enable the No Agreement Check feature on the downstream device’s port to perform rapid state transition.

1.7.1 Prerequisites

l A device is the upstream one that is connected to another vendor’s MSTP supported device via a point-to-point link.

l Configure the same region name, revision level and VLAN-to-instance mappings on the two devices, making them in the same region.

1.7.2 Configuration Procedure

Following these steps to configure No Agreement Check:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port or port group view	Enter Ethernet port view	interface interface-type interface-number	Choose either
Enter Ethernet port or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }	Choose either
Enable No Agreement Check		stp no-agreement-check	Required Not enabled by default

& Note:

The No Agreement Check feature can only take effect on the root port or Alternate port after enabled.

1.7.3 Configuration Example

I. Network requirements

l Switch A connects to a device of another vendor that has different MSTP implementation.

l Another vendor’s device is the regional root bridge, and Switch A is the downstream device.

II. Network diagram

Figure 1-9 No Agreement Check configuration

III. Configuration procedure

# Enable No Agreement Check on Ethernet1/0/1.

<Sysname> system-view

[Sysname] interface Ethernet1/0/1

[Sysname-Ethernet1/0/1] stp no-agreement-check

1.8 Configuring Protection Functions

An MSTP-compliant device supports the following protection functions:

l BPDU guard

l Root guard

l Loop guard

l TC-BPDU attack guard

& Note:

l The support for the BPDU guard, root guard and loop guard functions depends on the specific device model.

l Among loop guard, root guard and edge port setting, only one function can take effect on the same port at the same time.

The purposes of these protection functions are as follows:

l BPDU guard

For access layer devices, the access ports generally connect directly with user terminals (such as PCs) or file servers. In this case, the access ports are configured as edge ports to allow rapid transition of these ports. When these ports receive configuration BPDUs, the system will automatically set these ports as non-edge ports and starts a new spanning tree computing process. This will cause network topology instability. Under normal conditions, these ports should not receive configuration BPDUs. However, if someone forges configuration BPDUs maliciously to attack the devices, network instability will occur.

MSTP provides the BPDU guard function to protect the system against such attacks. With the BPDU guard function enabled on the devices, when edge ports receive configuration BPDUs, the system will close these ports and notify the NMS that these ports have been closed by MSTP. Those ports closed thereby can be restored only by the network administers.

l Root guard

The root bridge and secondary root bridge of a panning tree should be located in the same MST region. Especially for the CIST, the root bridge and secondary root bridge are generally put in a high-bandwidth core region during network design. However, due to possible configuration errors or malicious attacks in the network, the legal root bridge may receive a configuration BPDU with a higher priority. In this case, the current root bridge will be superseded by another device, causing undesired change of the network topology. As a result of this kind of illegal topology change, the traffic that should go over high-speed links is drawn to low-speed links, resulting in network congestion.

To prevent this situation from happening, MSTP provides the root guard function to protect the root bridge. If the root guard function is enabled on a port, this port will keep playing the role of designated port on all MST instances. Once this port receives a configuration BPDU with a higher priority from an MST instance, it immediate sets that instance port to the listening state, without forwarding the packet (this is equivalent to disconnecting the link connected with this port). If the port receives no BPDUs with a higher priority within a sufficiently long time, the port will revert to its original state.

l Loop guard

By keeping receiving BPDUs from the upstream device, a device can maintain the state of the root port and other blocked ports. However, due to link congestion or unidirectional link failures, these ports may fail to receive BPDUs from the upstream device. In this case, the downstream device will reselect the port roles: those ports failed to receive upstream BPDUs will become designated ports and the blocked ports will transition to the forwarding state, resulting in loops in the switched network. The loop guard function can suppress the occurrence of such loops.

If a loop guard–enabled port fails to receive BPDUs from the upstream device, and if the port took part in STP computing, all the instances on the port, no matter what roles they play, will be set to, and stay in, the Discarding state.

l TC-BPDU attack guard

Generally, a switch removes the local MAC address table and then updates the ARP address table based on STP instances according to the updated MAC address table upon receiving a TC-BPDU. If a malicious user forges TC-BPDUs to attack a switch, the switch will receive a large amount of TC-BPDUs in a short period and will thus be busy in removing local MAC address tables and updating the ARP address table, which will affect STP calculation and occupy large amount of network bandwidth. As a result, the CPU occupancy stays high for the switch.

With the TC-BPDU guard function enabled, the switch performs the operation of removing the local MAC address table only once in a specific period of time after it receives TC-BPDUs. The period is determined by a timer with the default setting of 10 seconds. If the switch receives more TC-BPDUs within this period, the switch performs another operation of removing MAC address entries once and triggers the timer. Such a mechanism prevents the switch from removing MAC address tables frequently and prevents degradation of STP calculation speed and network stability.

1.8.1 Configuration prerequisites

MSTP has been correctly configured on the device.

1.8.2 Enabling BPDU Guard

& Note:

The support for this feature depends on the specific device model.

I. Configuration procedure

Following these steps to enable BPDU guard:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable the BPDU guard function for the device	stp bpdu-protection	Required Disabled by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Enable the BPDU guard function for the device

stp bpdu-protection

Required

Disabled by default

II. Configuration example

# Enable BPDU protection.

<Sysname> system-view

[Sysname] stp bpdu-protection

1.8.3 Enabling Root Guard

& Note:

The support for this feature depends on the specific device model.

I. Configuration procedure

Follow these steps to enable root guard:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Enable the root guard function for the ports(s)		stp root-protection	Required Disabled by default

1.8.4 Enabling Loop Guard

& Note:

l The support for this feature depends on the specific device model.

l We recommend that you enable loop guard if your device supports this function.

I. Configuration procedure

Follow these steps to enable loop guard:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet port view or port group view	Enter Ethernet port view	interface interface-type interface-number	User either command Configured in Ethernet port view, the setting is effective on the current port only; configured in port group view, the setting is effective on all ports in the port group
Enter Ethernet port view or port group view	Enter port group view	port-group { manual port-group-name \| aggregation agg-id }
Enable the loop guard function for the ports(s)		stp loop-protection	Required Disabled by default

1.8.5 Enabling TC-BPDU Attack Guard

I. Configuration procedure

Follow these steps to enable TC-BPDU attack guard

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable the TC-BPDU attack guard function	stp tc-protection enable	Optional Enabled by default

To do...

Use the command...

Remarks

Enter system view

system-view

—

Enable the TC-BPDU attack guard function

stp tc-protection enable

Optional

Enabled by default

& Note:

We recommend that this function should not be disabled.

1.9 Displaying and Maintaining MSTP

To do...	Use the command...	Remarks
View the status information and statistics information of MSTP	display stp [ instance instance-id ] [ interface interface-list \| slot slot-number ] [ brief ]	Available in any view
View the MST region configuration information that has taken effect	display stp region-configuration	Available in any view
Clear the statistics information of MSTP	reset stp [ interface interface-list ]	Available in user view

& Note:

The slot slot-number in display stp [ instance instance-id ] [ interface interface-list | slot slot-number ] [ brief ] is only available on distributed devices.

1.10 MSTP Configuration Example

I. Network requirements

Configure MSTP so that packets of different VLANs are forwarded along different spanning trees. The specific configuration requirements are as follows:

l All devices on the network are in the same MST regions.

l Packets of VLAN 10 are forwarded along MST region 1, those of VLAN 30 are forwarded along MST instance 3, those of VLAN 40 are forwarded along MST instance 4, and those of VLAN 20 are forwarded along MST instance 0.

l Switch A and Switch B are convergence layer devices, while Switch C and Switch D are access layer devices. VLAN 10 and VLAN 30 are terminated on the convergence layer devices, and VLAN 40 is terminated on the access layer devices, so the root bridges of MST instance 1 and MST instance 3 are Switch A and Switch B respectively, while the root bridge of MST instance 4 is Switch C.

II. Network diagram

Figure 1-10 Network diagram for MSTP configuration

& Note:

“Permit:“ beside each link in the figure is followed by the VLANs the packets of which are permitted to pass this link.

III. Configuration procedure

1) Configuration on Switch A

# Configure an MST region.

<Sysname> system-view

[Sysname] stp region-configuration

[Sysname-mst-region] region-name example

[Sysname-mst-region] instance 1 vlan 10

[Sysname-mst-region] instance 3 vlan 30

[Sysname-mst-region] instance 4 vlan 40

[Sysname-mst-region] revision-level 0

# Activate MST region configuration manually.

[Sysname-mst-region] active region-configuration

# Define Switch A as the root bridge of MST instance 1.

[Sysname] stp instance 1 root primary

# View the MST region configuration information that has taken effect.

[Sysname] display stp region-configuration

Oper configuration

Format selector :0

Region name :example

Revision level :0

Instance Vlans Mapped

0 1 to 9, 11 to 29, 31 to 39, 41 to 4094

1 10

3 30

4 40

2) Configuration on Switch B

# Configure an MST region.

<Sysname> system-view

[Sysname] stp region-configuration

[Sysname-mst-region] region-name example

[Sysname-mst-region] instance 1 vlan 10

[Sysname-mst-region] instance 3 vlan 30

[Sysname-mst-region] instance 4 vlan 40

[Sysname-mst-region] revision-level 0

# Activate MST region configuration manually.

[Sysname-mst-region] active region-configuration

# Define Switch B as the root bridge of MST instance 3.

[Sysname] stp instance 3 root primary

# View the MST region configuration information that has taken effect.

[Sysname] display stp region-configuration

Oper configuration

Format selector :0

Region name :example

Revision level :0

Instance Vlans Mapped

0 1 to 9, 11 to 29, 31 to 39, 41 to 4094

1 10

3 30

4 40

3) Configuration on Switch C

# Configure an MST region.

<Sysname> system-view

[Sysname] stp region-configuration

[Sysname-mst-region] region-name example

[Sysname-mst-region] instance 1 vlan 10

[Sysname-mst-region] instance 3 vlan 30

[Sysname-mst-region] instance 4 vlan 40

[Sysname-mst-region] revision-level 0

# Activate MST region configuration manually.

[Sysname-mst-region] active region-configuration

# Define Switch C as the root bridge of MST instance 4.

[Sysname] stp instance 4 root primary

# View the MST region configuration information that has taken effect.

[Sysname] display stp region-configuration

Oper configuration

Format selector :0

Region name :example

Revision level :0

Instance Vlans Mapped

0 1 to 9, 11 to 29, 31 to 39, 41 to 4094

1 10

3 30

4 40

4) Configuration on Switch D

# Configure an MST region.

<SwitchD> system-view

[SwitchD] stp region-configuration

[SwitchD-mst-region] region-name example

[SwitchD-mst-region] instance 1 vlan 10

[SwitchD-mst-region] instance 3 vlan 30

[SwitchD-mst-region] instance 4 vlan 40

[SwitchD-mst-region] revision-level 0

# Activate MST region configuration manually.

[SwitchD-mst-region] active region-configuration

# View the MST region configuration information that has taken effect.

[SwitchD] display stp region-configuration

Oper configuration

Format selector :0

Region name :example

Revision level :0

Instance Vlans Mapped

0 1 to 9, 11 to 29, 31 to 39, 41 to 4094

1 10

3 30

4 40

H3C S3610[5510] Series Ethernet Switches Operation Manual-Release 0001-(V1.02)

1.1 MSTP Overview

I. Functions of STP

II. Protocol Packets of STP

III. Basic concepts in STP

IV. How STP works

I. Why MSTP

II. Some concepts in MSTP

III. How MSTP works

IV. Implementation of MSTP on devices

1.2 Configuration Task List

1.3 Configuring the Root Bridge

1.3.1 Configuring an MST Region

I. Configuration procedure

II. Configuration example

1.3.2 Specifying the Root Bridge or a Secondary Root Bridge

I. Specifying the current device as the root bridge of a specific spanning tree

II. Specifying the current device as a secondary root bridge of a specific spanning tree

III. Configuration example

1.3.3 Configuring the Work Mode of MSTP Device

I. Configuration procedure

II. Configuration example

1.3.4 Configuring the Priority of the Current Device

I. Configuration procedure

II. Configuration example

1.3.5 Configuring the Maximum Hops of an MST Region

I. Configuration procedure

II. Configuration example

1.3.6 Configuring the Network Diameter of a Switched Network

I. Configuration procedure

II. Configuration example

1.3.7 Configuring Timers of MSTP

I. Configuration procedure

II. Configuration example

1.3.8 Configuring the Timeout Factor

I. Configuration procedure

II. Configuration example

1.3.9 Configuring the Maximum Transmission Rate of Ports

I. Configuration procedure

II. Configuration example

1.3.10 Configuring Ports as Edge Ports

I. Configuration procedure

II. Configuration example

1.3.11 Configuring Whether Ports Connect to Point-to-Point Links

I. Configuration procedure

II. Configuration example

1.3.12 Configuring the MSTP Packet Format for Ports

I. Configuration procedure

II. Configuration example

1.3.13 Enabling the MSTP Feature

I. Configuration procedure

II. Configuration example

1.4 Configuring Leaf Nodes

1.4.1 Configuring an MST Region

1.4.2 Configuring the Work Mode of MSTP

1.4.3 Configuring the Timeout Factor

1.4.4 Configuring the Maximum Transmission Rate of Ports

1.4.5 Configuring Ports as Edge Ports

1.4.6 Configuring Path Costs of Ports

I. Specifying a standard that the device uses when calculating the default path cost

II. Configuring Path Costs of Ports

1.4.7 Configuring Port Priority

I. Configuration procedure

II. Configuration example

1.4.8 Configuring Whether Ports Connect to Point-to-Point Links

1.4.9 Configuring the MSTP Packet Format for Ports

1.4.10 Enabling the MSTP Feature

1.5 Performing mCheck

I. Perform global mCheck

II. Perform mCheck in Ethernet port view

1.6 Configuring Digest Snooping

I. Network requirements

II. Network diagram

III. Configuration procedure

1.7 Configuring No Agreement Check

I. Network requirements

II. Network diagram

III. Configuration procedure

1.8 Configuring Protection Functions

1.8.1 Configuration prerequisites