Table of Contents

Chapter 1 MSTP Configuration. 1-1

1.1 MSTP Overview. 1-1

1.1.1 Introduction to STP. 1-1

1.1.2 Introduction to MSTP. 1-11

1.1.3 Protocols and Standards. 1-18

1.2 Configuration Task List 1-18

1.3 Configuring the Root Bridge. 1-20

1.3.1 Configuring an MST Region. 1-20

1.3.2 Specifying the Root Bridge or a Secondary Root Bridge. 1-22

1.3.3 Configuring the Work Mode of MSTP Device. 1-23

1.3.4 Configuring the Priority of the Current Device. 1-24

1.3.5 Configuring the Maximum Hops of an MST Region. 1-25

1.3.6 Configuring the Network Diameter of a Switched Network. 1-26

1.3.7 Configuring Timers of MSTP. 1-27

1.3.8 Configuring the Timeout Factor 1-29

1.3.9 Configuring the Maximum Transmission Rate of Ports. 1-29

1.3.10 Configuring Ports as Edge Ports. 1-30

1.3.11 Configuring Whether Ports Connect to Point-to-Point Links. 1-31

1.3.12 Configuring the Mode a Port Uses to Recognize/Send MSTP Packets. 1-33

1.3.13 Enabling the Output of Port State Transition Information. 1-34

1.3.14 Enabling the MSTP Feature. 1-34

1.4 Configuring Leaf Nodes. 1-35

1.4.1 Configuring an MST Region. 1-35

1.4.2 Configuring the Work Mode of MSTP. 1-35

1.4.3 Configuring the Timeout Factor 1-35

1.4.4 Configuring the Maximum Transmission Rate of Ports. 1-35

1.4.5 Configuring Ports as Edge Ports. 1-36

1.4.6 Configuring Path Costs of Ports. 1-36

1.4.7 Configuring Port Priority. 1-38

1.4.8 Configuring Whether Ports Connect to Point-to-Point Links. 1-39

1.4.9 Configuring the Mode a Port Uses to Recognize/Send MSTP Packets. 1-39

1.4.10 Enabling Output of Port State Transition Information. 1-39

1.4.11 Enabling the MSTP Feature. 1-39

1.5 Performing mCheck. 1-40

1.5.1 Configuration Prerequisites. 1-40

1.5.2 Configuration Procedure. 1-40

1.5.3 Configuration Example. 1-41

1.6 Configuring Digest Snooping. 1-41

1.6.1 Configuration Prerequisites. 1-41

1.6.2 Configuration Procedure. 1-42

1.6.3 Configuration Example. 1-43

1.7 Configuring No Agreement Check. 1-44

1.7.1 Prerequisites. 1-45

1.7.2 Configuration Procedure. 1-45

1.7.3 Configuration Example. 1-46

1.8 Configuring Protection Functions. 1-46

1.8.1 Configuration prerequisites. 1-47

1.8.2 Enabling BPDU Guard. 1-47

1.8.3 Enabling Root Guard. 1-48

1.8.4 Enabling Loop Guard. 1-49

1.8.5 Enabling TC-BPDU Attack Guard. 1-50

1.9 Displaying and Maintaining MSTP. 1-51

1.10 MSTP Configuration Example. 1-51

 

Chapter 1  MSTP Configuration

When configuring MSTP, go to these sections for information you are interested in:

l           MSTP Overview

l           Configuring the Root Bridge

l           Configuring Leaf Nodes

l           Performing mCheck

l           Configuring Protection Functions

l           Displaying and Maintaining MSTP

1.1  MSTP Overview

1.1.1  Introduction to STP

I. Why STP?

The Spanning Tree Protocol (STP) was established based on the 802.1d standard of IEEE to eliminate physical loops at the data link layer in a local area network (LAN). Devices running this protocol detect loops in the network by exchanging information with one another and eliminate loops by selectively blocking certain ports until the loop structure is pruned into a loop-free network structure. This avoids proliferation and infinite recycling of packets that would occur in a loop network and prevents deterioration of the packet processing capability of network devices caused by duplicate packets received.

In the narrow sense, STP refers to the STP protocol defined in IEEE 802.1d; in the broad sense, it refers to the STP protocol defined in IEEE 802.1d and various enhanced spanning tree protocols derived from the STP protocol.

II. Protocol Packets of STP

STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol packets.

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 calculation.

In STP, BPDUs come in two types:

l           Configuration BPDUs, used for calculating spanning trees and maintaining the spanning tree topology.

l           Topology change notification (TCN) BPDUs, used for notifying 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 configuration BPDUs at a certain interval, and other devices just forward the BPDUs. This mechanism ensures topological stability.

2)         Root port

On a non-root bridge device, the root port is the port nearest to the root bridge. The root port is responsible for communication with 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

The following table describes a designated bridge and a 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 bridge 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 Device A, Device B, and Device C respectively.

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

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

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

IV. Path cost

Path cost is a reference value used for link selection in STP. By calculating the path cost, STP selects relatively “robust” links and blocks redundant links, and finally prunes the network into loop-free tree structure.

 

 

V. How STP works

STP identifies the network topology by transmitting configuration BPDUs between network devices. Configuration BPDUs contain sufficient information for network devices to complete the spanning tree calculation. 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 while it propagates in the network.

l           Max age: maximum age of the configuration BPDU maintained in the device.

l           Hello time: configuration BPDU interval.

l           Forward delay: forward delay of the port.

 

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

 

1)         Specific calculation process of the STP algorithm

l           Initial state

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

l           Selection of the optimum configuration BPDU

Each device sends out its configuration BPDUs 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.

 

Principle for configuration BPDU comparison:

 

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 its 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	A non-root-ridge device regards the port on which it received the optimum configuration BPDU as the root port.
2	Based on the configuration BPDU and the path cost of the root port, the device calculates a designated port configuration BPDU for each of 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 calculated configuration BPDU with the configuration BPDU on the port of which the port role is to be defined, and does different things according to the comparison result: l      If the calculated configuration BPDU is superior, the device will consider this port as the designated port, and the configuration BPDU on the port will be replaced with the calculated configuration BPDU, which will be sent out periodically. l      If the configuration BPDU on the port is superior, the device will block this port without updating its configuration BPDU, so that the port will only receive BPDUs, but not send any, and will not forward data.

 

 

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 Device A is 0, the priority of Device B is 1, the priority of Device C is 2, and the path costs of these links are 5, 10 and 4 respectively.

Figure 1-2 Network diagram for the 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
Device A	AP1	{0, 0, 0, AP1}
	AP2	{0, 0, 0, AP2}
Device B	BP1	{1, 0, 1, BP1}
	BP2	{1, 0, 1, BP2}
Device C	CP1	{2, 0, 2, CP1}
	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
Device A	l      Port AP1 receives the configuration BPDU of Device B {1, 0, 1, BP1}. Device 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 Device C {2, 0, 2, CP1}. Device 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      Device A finds that both the root bridge and designated bridge in the configuration BPDUs of all its ports are Device 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}
Device B	l      Port BP1 receives the configuration BPDU of Device A {0, 0, 0, AP1}. Device 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 Device C {2, 0, 2, CP2}. Device 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      Device 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), Device B calculates a designated port configuration BPDU for BP2 {0, 5, 1, BP2}. l      Device B compares the calculated configuration BPDU {0, 5, 1, BP2} with the configuration BPDU of BP2. If the calculated BPDU is superior, BP2 will act as the designated port, and the configuration BPDU on this port will be replaced with the calculated configuration BPDU, which will be sent out periodically.	Root port BP1: {0, 0, 0, AP1} Designated port BP2: {0, 5, 1, BP2}
Device C	l      Port CP1 receives the configuration BPDU of Device A {0, 0, 0, AP2}. Device 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 Device B {1, 0, 1, BP2} before the message was updated. Device 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 BPDU 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      Device C compares the calculated 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 calculated 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 Device B {0, 5, 1, BP2}. Because the received configuration BPDU is superior to its old one, Device C launches a BPDU update process. l      At the same time, port CP1 receives configuration BPDUs periodically from Device A. Device 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) plus 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 calculated 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 Device A until a spanning tree calculation process is triggered by a new condition, for example, the link from Device B to Device 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 Device A as the root bridge is stabilized, as shown in Figure 1-3.

Figure 1-3 The final calculated spanning tree

 

 

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 send 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 calculation process so that a new path is established to restore the network connectivity.

However, the newly calculated 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 along 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.

3)         STP timers

STP calculations need three important timing parameters: forward delay, hello time, and max age.

l           Forward delay is the delay time for device state transition. A path failure will cause re-calculation of the spanning tree, and the spanning tree structure will change accordingly. However, the new configuration BPDU as the calculation result cannot be propagated throughout the network immediately. If the newly elected root port and designated ports start to forward data right away, a temporary loop is likely to occur. For this reason, as a mechanism for state transition in STP, 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.

l           Hello time is the time interval at which a device sends hello packets to the surrounding devices to make sure that the paths are fault-free.

l           Max age is a parameter used to determine whether a configuration BPDU held in the device has expired. A configuration BPDU beyond the max age will be discarded.

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.

 

 

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. For description about VLANs, refer to VLAN Configuration.

MSTP features the following:

l           MSTP supports mapping VLANs to MST instances by means of a VLAN-to-instance mapping table. MSTP can save communication overheads and resource usage by mapping multiple VLANs to one MST instance.

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. Basic concepts in MSTP

Assume that all the four switches in Figure 1-4 are running MSTP. In light with the diagram, the following paragraphs will present some basic concepts of MSTP.

Figure 1-4 Basic concepts in MSTP

1)         MST region

A multiple spanning tree region (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.

For example, all the devices in region A0 in Figure 1-4 have the same MST region configuration:

l           The same region name,

l           The same VLAN-to-instance mapping (VLAN 1 is mapped to MST instance 1, VLAN 2 to MST instance 2, and the rest to the command and internal spanning tree (CIST). CIST refers to MST instance 0), and

l           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 (VLAN 1 is mapped to MST instance 1, VLAN 2 to MST instance 2, and the rest to CIST). MSTP achieves load balancing by means of the VLAN-to-instance mapping table.

3)         IST

Internal spanning tree (IST) is a spanning tree that runs in an MST region.

ISTs in all MST regions and 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 the given MST region.

In Figure 1-4, for example, the CIST has a section in each MST region, and this section is the IST in the respective MST region.

4)         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 calculated by these devices through STP or RSTP. For example, the red lines in Figure 1-4 describe the CST.

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 common root bridge is the root bridge of the CIST.

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

9)         Boundary port

A boundary 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 calculation, a boundary port assumes the same role on the CIST and on MST instances. Namely, if a boundary port is the 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 boundary port of region D0.

 

 

10)     Roles of ports

In the MSTP calculation process, port roles include root port, designated 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           A master port connects an MST region to the common root. The path from the master port to the common root is the shortest path between the MST region and the common root. In the CST, the master port is the root port of the region, which is considered as a node. The master port is a special boundary port. It is a root port in the IST/CIST while a master port in the other MSTIs.

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

l           Backup port: The backup port of designated ports. When a designated port is blocked, the backup port becomes a new designated port and starts forwarding data without delay. When a loop occurs while two ports of the same MSTP 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 states

In MSTP, port states fall into the following tree:

l           Forwarding: the port learns MAC addresses and forwards user traffic;

l           Learning: the port learns MAC addresses but does not forward user traffic; 

l           Discarding: the port neither learns MAC addresses nor forwards user traffic.

 

 

A port state is not exclusively associated with a port role. Table 1-6 lists the port state(s) supported by each port role (“√” indicates that the port supports this state, while “—“ indicates that the port does not support this state).

Table 1-6 Ports states supported by different port roles

Role State	Root port/Master port	Designated 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 calculated CST. Inside an MST region, multiple spanning trees are generated through calculation, each spanning tree called an MST instance. Among these MST instances, instance 0 is the IST, while all the others are MSTIs. Similar to STP, MSTP uses configuration BPDUs to calculate spanning trees. The only difference between the two protocols is that an MSTP BPDU carries the MSTP configuration on the device from which this BPDU is sent.

1)         CIST calculation

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

2)         MSTI calculation

Within an MST region, MSTP generates different spanning tree instances for different VLANs based on the VLAN-to-instance mappings. MSTP performs a separate calculation process, which is similar to spanning tree calculation in STP, for each spanning tree. For details, refer to How STP works.

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 calculation.

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

l           Root bridge hold

l           Root bridge backup

l           Root guard

l           BPDU guard

l           Loop guard

l           TC-BPDU guard

1.1.3  Protocols and Standards

MSTP is documented in:

l           IEEE 802.1d: Spanning Tree Protocol

l           IEEE 802.1w: Rapid Spanning Tree Protocol

l           IEEE 802.1s: Multiple Spanning Tree Protocol

1.2  Configuration Task List

Before configuring MSTP, 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.

Complete these tasks to configure MSTP:

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 Device	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 Mode a Port Uses to Recognize/Send MSTP Packets	Optional
	Enabling the Output of Port State Transition Information	Optional
	Enabling the MSTP Feature	Required
Configuring Leaf Nodes	Configuring an MST Region	Required
	Configuring the Work Mode of MSTP Device	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 Mode a Port Uses to Recognize/Send MSTP Packets	Optional
	Enabling the Output of Port State Transition Information	Optional
	Enabling the MSTP Feature	Required
Performing mCheck		Optional
Configuring Digest Snooping		Optional
Configuring No Agreement Check		Optional
Configuring Protection Functions		Optional

 

 

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	Optional The MST region name is the MAC address by default.
Configure the VLAN-to-instance mapping table	instance instance-id vlan vlan-list	Optional Use either command. All VLANs in an MST region are mapped to MST instance 0 by default.
	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.

 

 

The configuration of MST region–related parameters, especially the VLAN-to-instance mapping table, will cause MSTP to launch a new spanning tree calculation 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 calculation process when processing MST region–related configurations; instead, such configurations will take effect only if you:

l           activate the MST region–related parameters using 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 calculation. 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	Required By default, a device does not function as the root bridge.

 

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	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 devices.

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, the 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 the 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

 

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 the 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

 

 

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.

The regional root bridge always sends a configuration BPDU with a hop count set to the maximum value. When a switch receives this configuration BPDU, it decrements the hop count by 1 and uses the new hop count as the remaining hop count in the BPDUs it propagates. When the hop count of a BPDU reaches 0, it is discarded by the device that received it. Thus, devices beyond the reach of the maximum hop are unable to take part in spanning tree calculation, and thereby the size of the MST region is confined.

When a device becomes the root bridge of the CIST or MSTI of an MST region , the maximum hop in the configuration BPDUs generated by this device defines the network diameter of the spanning tree to define how far the spanning tree can reach in this MST region. All the devices other than the root bridge in the MST region use the maximum hop value set for the root bridge.

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

 

 

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

 

 

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. You can configure these three parameters for MSTP to calculate spanning trees.

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
Configure 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.

 

 

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           Max 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

After the network topology is stabilized, each non-root-bridge device forwards configuration BPDUs to the surrounding devices at the 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 calculation process.

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

 

 

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.

I. Configuration procedure

Follow 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 interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	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

 

 

II. Configuration example

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

<Sysname> system-view

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] 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 a network topology change occurs, 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

Follow 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 interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	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.

 

 

II. Configuration example

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

<Sysname> system-view

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] 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 after a proposal-agreement handshake process.

I. Configuration procedure

Follow 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 interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	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.

 

 

II. Configuration example

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

<Sysname> system-view

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] stp point-to-point force-true

1.3.12  Configuring the Mode a Port Uses to Recognize/Send MSTP Packets

A port can send/recognize MSTP packets of two formats:

l           802.1s-compliant standard format, and

l           Compatible format

By default, the packet format recognition mode of a port is auto, namely the port automatically distinguishes the two MSTP packet formats, and determines the format of packets it will send based on the recognized format. You can configure the MSTP packet format to be used by a port. After the configuration, when working in MSTP mode, the port sends and receives only MSTP packets of the format you have configured to communicate with devices that send the same format of packets.

I. Configuration procedure

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

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Configure the mode the port uses to recognize/send MSTP packets		stp compliance { auto | dot1s | legacy }	Optional auto by default

 

 

II. Configuration example

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

<Sysname> system-view

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] stp compliance dot1s

1.3.13  Enabling the Output of Port State Transition Information

In a large-scale, MSTP-enabled network, there are a large number of MSTP instances, so ports may frequently transition from one state to another. In this situation, you can enable the device to output the port state transition information of all SPT instances or the specified SPT instance so as to monitor the port states in real time.

Follow these steps to enable output of port state transition information:

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable output of port state transition information of all instances or a particular instance	stp port-log { all | instance instance-id }	Optional Enabled by default

 

1.3.14  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 Disabled by default
Enter Ethernet interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Enable the MSTP feature on the port(s)		stp enable	Optional MSTP is disabled on ports by default and automatically enabled on all ports after it is enabled globally on the device.

 

 

II. Configuration example

# Enable MSTP for the device and disable MSTP on port GigabitEthernet1/0/1.

<Sysname> system-view

[Sysname] stp enable

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] stp disable

1.4  Configuring Leaf Nodes

1.4.1  Configuring an MST Region

Refer to Configuring an MST Region in the section about root bridge configuration.

1.4.2  Configuring the Work Mode of MSTP

Refer to Configuring the Work Mode of MSTP Device in the section about root bridge configuration.

1.4.3  Configuring the Timeout Factor

Refer to Configuring Timers of MSTP in the section about root bridge configuration.

1.4.4  Configuring the Maximum Transmission Rate of Ports

Refer to Configuring the Maximum Transmission Rate of Ports in the section about root bridge configuration.

1.4.5  Configuring Ports as Edge Ports

Refer to Configuring Ports as Edge Ports in the section about root bridge configuration.

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 legacy by default

 

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

 

 

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 interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	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.

 

 

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 interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Configure the port priority		stp [ instance instance-id ] port priority priority	Optional 128 for all Ethernet ports by default.

 

 

II. Configuration example

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

<Sysname> system-view

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] 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 in the section about root bridge configuration.

1.4.9  Configuring the Mode a Port Uses to Recognize/Send MSTP Packets

Refer to Configuring the Mode a Port Uses to Recognize/Send MSTP Packets in the section about root bridge configuration.

1.4.10  Enabling Output of Port State Transition Information

Refer to Enabling the Output of Port State Transition Information in the section about root bridge configuration.

1.4.11  Enabling the MSTP Feature

Refer to Enabling the MSTP Feature in the section about root bridge configuration.

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. Performing mCheckglobally

Follow these steps to perform global mCheck:

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

 

II. Performing mCheck in Ethernet interface view

Follow these steps to perform mCheck in Ethernet interface view:

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

 

 

1.5.3  Configuration Example

# Perform mCheck on port GigabitEthernet 1/0/1.

1)         Method 1: Perform mCheck globally.

<Sysname> system-view

[Sysname] stp mcheck

2)         Method 2: Perform mCheck in Ethernet interface view.

<Sysname> system-view

[Sysname] interface GigabitEthernet 1/0/1

[Sysname-GigabitEthernet1/0/1] 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 by 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 calculated via the HMAC-MD5 algorithm based on VLAN-to-instance mappings.

Since MSTP implementations differ with vendors, the configuration digest calculated 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 interface or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Enable digest snooping on the interface or port group		stp config-digest-snooping	Required Not enabled by default
Return to system view		quit	—
Enable global digest snooping		stp config-digest-snooping	Required Not enabled by default

 

 

1.6.3  Configuration Example

I. Network requirements

l           Device A and Device B connect to a third-party’s router and all the routers are in the same region.

l           Enable Digest Snooping on Device A and Device B so that the three routers can communicate with one another.

II. Network diagram

Figure 1-6 Digest Snooping configuration

III. Configuration procedure

1)         Enable Digest Snooping on Device A

# Enable Digest Snooping on GigabitEthernet1/0/1.

<DeviceA> system-view

[DeviceA] interface GigabitEthernet 1/0/1

[DeviceA-GigabitEthernet1/0/1] stp config-digest-snooping

# Enable global Digest Snooping.

[DeviceA-GigabitEthernet1/0/1] quit

[DeviceA] stp config-digest-snooping

2)         Enable Digest Snooping on Device B (the same as above, omitted)

1.7  Configuring No Agreement Check

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

l           Proposal: sent by designated ports to request rapid transition

l           Agreement: 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 of a designated port in MSTP

Figure 1-8 Rapid state transition of a designated port in RSTP

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

Follow these steps to configure No Agreement Check:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet interface or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Enable No Agreement Check		stp no-agreement-check	Required Not enabled by default

 

 

1.7.3  Configuration Example

I. Network requirements

l           Device A connects to a third-party’s device that has different MSTP implementation. Both switches are in the same region.

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

II. Network diagram

Figure 1-9 No Agreement Check configuration

III. Configuration procedure

# Enable No Agreement Check on GigabitEthernet1/0/1 of Device A.

<DeviceA> system-view

[DeviceA] interface GigabitEthernet 1/0/1

[DeviceA-GigabitEthernet1/0/1] 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

 

 

1.8.1  Configuration prerequisites

MSTP has been correctly configured on the device.

1.8.2  Enabling 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 start a new spanning tree calculation process. This will cause a change of network topology. 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, MSTP 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.

 

 

Follow these steps to enable BPDU guard:

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

 

1.8.3  Enabling 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, legal 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 immediately 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 twice the forwarding delay, the port will revert to its original state.

 

 

Follow these steps to enable root guard:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Enable the root guard function on the port(s)		stp root-protection	Required Disabled by default

 

1.8.4  Enabling 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 calculation, all the instances on the port, no matter what roles they play, will be set to, and stay in, the Discarding state.

 

 

Follow these steps to enable loop guard:

To do...		Use the command...	Remarks
Enter system view		system-view	—
Enter Ethernet interface view or port group view	Enter Ethernet interface view	interface interface-type interface-number	Required Use either command. Configurations made in Ethernet interface view will take effect on the current port only; configurations made in port group view will take effect on all ports in the port group.
	Enter port group view	port-group { manual port-group-name | aggregation agg-id }
Enable the loop guard function for the port(s)		stp loop-protection	Required Disabled by default

 

1.8.5  Enabling TC-BPDU Attack Guard

When receiving a TC-BPDU (a PDU used as notification of topology change), the device will delete the corresponding forwarding address entry. If someone forges TC-BPDUs to attack the device, the device will receive a larger number of TC-BPDUs within a short time, and frequent deletion operations bring a big burden to the device and hazard network stability.

With the TC-BPDU guard function enabled, the device limits the maximum number of times of immediately deleting forwarding address entries within 10 seconds after it receives TC-BPDUs to the value set with the stp tc-protection threshold command (assume the value is X). At the same time, the system monitors whether the number of TC-BPDUs received within that period of time is larger than X. If so, the device will perform another deletion operation after that period of time elapses. This prevents frequent deletion of forwarding address entries.

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
Configure the maximum number of times the device deletes forwarding address entries within a certain period of time immediately after it receives TC-BPDUs	stp tc-protection threshold number	Optional 6 by default

 

 

1.9  Displaying and Maintaining MSTP

To do...	Use the command...	Remarks
View the information about abnormally blocked ports	display stp abnormal-port	Available in any view
View the information about ports blocked by STP protection actions	display stp down-port	Available in any view
View the information of port role calculation history for the specified MSTP instance or all MSTP instances	display stp [ instance instance-id ] history	Available in any view
View the statistics of TC/TCN BPDUs sent and received by all ports in the specified MSTP instance or all MSTP instances	display stp [ instance instance-id ] tc	Available in any view
View the status information and statistics information of MSTP	display stp [ instance instance-id ] [ interface interface-list ] [ brief ]	Available in any view
View the information about MST region configuration in effect	display stp region-configuration	Available in any view
View root bridge information of all MSTP instances	display stp root	Available in any view
Clear the statistics information of MSTP	reset stp [ interface interface-list ]	Available in user view

 

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 region.

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           Device A and Device B are convergence layer devices, while Device C and Device 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 Device A and Device B respectively, while the root bridge of MST instance 4 is Device C.

II. Network diagram

Figure 1-10 Network diagram for MSTP configuration

 

 

III. Configuration procedure

1)         Configuration on Device A

# Enter MST region view.

<DeviceA> system-view

[DeviceA] stp region-configuration

# Configure the region name, VLAN-to-instance mappings and revision level of the MST region.

[DeviceA-mst-region] region-name example

[DeviceA-mst-region] instance 1 vlan 10

[DeviceA-mst-region] instance 3 vlan 30

[DeviceA-mst-region] instance 4 vlan 40

[DeviceA-mst-region] revision-level 0

# Activate MST region configuration manually.

[DeviceA-mst-region] active region-configuration

[DeviceA-mst-region] quit

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

[DeviceA] stp instance 1 root primary

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

[DeviceA] 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 Device B

# Enter MST region view.

<DeviceB> system-view

[DeviceB] stp region-configuration

# Configure the region name, VLAN-to-instance mappings and revision level of the MST region.

[DeviceB-mst-region] region-name example

[DeviceB-mst-region] instance 1 vlan 10

[DeviceB-mst-region] instance 3 vlan 30

[DeviceB-mst-region] instance 4 vlan 40

[DeviceB-mst-region] revision-level 0

# Activate MST region configuration manually.

[DeviceB-mst-region] active region-configuration

[DeviceB-mst-region] quit

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

[DeviceB] stp instance 3 root primary

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

[DeviceB] 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 Device C

# Enter MST region view.

<DeviceC> system-view

[DeviceC] stp region-configuration

[DeviceC-mst-region] region-name example

# Configure the region name, VLAN-to-instance mappings and revision level of the MST region.

[DeviceC-mst-region] instance 1 vlan 10

[DeviceC-mst-region] instance 3 vlan 30

[DeviceC-mst-region] instance 4 vlan 40

[DeviceC-mst-region] revision-level 0

# Activate MST region configuration manually.

[DeviceC-mst-region] active region-configuration

[DeviceC-mst-region] quit

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

[DeviceC] stp instance 4 root primary

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

[DeviceC] 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 Device D

# Enter MST region view.

<DeviceD> system-view

[DeviceD] stp region-configuration

[DeviceD-mst-region] region-name example

# Configure the region name, VLAN-to-instance mappings and revision level of the MST region.

[DeviceD-mst-region] instance 1 vlan 10

[DeviceD-mst-region] instance 3 vlan 30

[DeviceD-mst-region] instance 4 vlan 40

[DeviceD-mst-region] revision-level 0

# Activate MST region configuration manually.

[DeviceD-mst-region] active region-configuration

[DeviceD-mst-region] quit

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

[DeviceD] 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