Table of Contents

Chapter 1 MSTP Configuration. 1-1

1.1 Introduction to MSTP. 1-1

1.1.1 MSTP Concepts. 1-1

1.1.2 How MSTP Works. 1-6

1.1.3 MSTP Implementation on the Switch. 1-11

1.2 MSTP Configuration Tasks. 1-12

1.2.1 Configuring the MST Region for a Switch. 1-13

1.2.2 Specifying the Switch as a Primary or a Secondary Root bridge. 1-15

1.2.3 Configuring the MSTP Running Mode. 1-16

1.2.4 Configuring the Bridge Priority for a Switch. 1-17

1.2.5 Configuring the Max Hops in an MST Region. 1-17

1.2.6 Configuring the Switched Network Diameter 1-18

1.2.7 Configuring the Time Parameters of a Switch. 1-19

1.2.8 Setting the Timeout Factor of a Specific Bridge. 1-20

1.2.9 Configuring the Max Transmission Speed on a Port 1-21

1.2.10 Configuring a Port as an Edge Port or Non-edge Port 1-22

1.2.11 Configuring the Path Cost of a Port 1-23

1.2.12 STP Path Cost Calculation Standards on STP port 1-24

1.2.13 Configuring the Priority of a Port 1-25

1.2.14 Configuring Whether a Port Is on a Point-to-Point Link. 1-26

1.2.15 Configuring the mCheck Variable of a Port 1-28

1.2.16 Configuring the Function of Clearing Dynamic ARP Entries. 1-29

1.2.17 Configuring the Switch Protection Functions. 1-29

1.2.18 Enabling/Disabling MSTP on the Device. 1-33

1.2.19 Disabling BPDU Packets from Flooding in the Default VLANs. 1-34

1.2.20 Enabling Single-Port Loop Detection. 1-35

1.2.21 Configuring the Switch to Support Standard or Legacy MSTP Packet Format 1-36

1.2.22 Configuring Digest Snooping. 1-36

1.2.23 Configuring Fast State Transition. 1-38

1.3 Displaying and Debugging MSTP. 1-41

1.4 Typical MSTP Configuration Examples. 1-43

1.4.1 Typical MSTP Configuration Example. 1-43

1.4.2 MSTP Hybrid Networking Configuration Example. 1-45

 

Chapter 1  MSTP Configuration

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

l           Introduction to MSTP

l           MSTP Configuration Tasks

l           Displaying and Debugging MSTP

l           Typical MSTP Configuration Examples

1.1  Introduction to MSTP

Multiple Spanning Tree Protocol (MSTP) is compatible with Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP).

STP is not fast in state transition. Even on a point-to-point link or an edge port, it has to take an interval twice as long as the forward delay before the port transits to the forwarding state.

RSTP converges fast, but has the following drawback like STP: all the network bridges in a LAN share one spanning tree and the redundant links cannot be blocked based on VLANs. Packets of all VLANs are forwarded along one spanning tree.

MSTP makes up for the drawback of STP and RSTP. It not only converges fast, but also allows the traffic of different VLANs to be distributed along their respective paths, which provides a better load-balance mechanism for the redundant links.

MSTP keeps a VLAN mapping table to associate VLANs with their spanning trees. Using MSTP, you can divide one switching network into multiple regions, each of which can have multiple spanning trees with each one independent of others. MSTP prunes the ring network into a loop-free tree to avoid the generation of loops and infinite circulations. It also provides multiple redundant paths for data forwarding to implement the load-balance mechanism of the VLAN data.

1.1.1  MSTP Concepts

There are 4 MST regions in Figure 1-1. Each region consists of four switches, all of which run MSTP. The following introduces the concept of MSTP with the help of this figure.

Figure 1-1 Basic MSTP concepts

I. MST region

Multiple Spanning Tree Regions: A multiple spanning tree region contains several switches and the network segments between them. These MSTP switches share the same region name, VLAN-spanning tree mapping configuration, and MSTP revision level configuration, and are connected directly. There can be several MST regions on a switching network. You can group several switches into a MST region, using MSTP configuration commands. For example, in Figure 1-1, the four switches in MST region A0 are configured with the same region name, the same VLAN mapping table (VLAN 1 is mapped to instance 1, VLAN 2 is mapped to instance 2, other VLANs is mapped to instance CIST), and the same revision level (not indicated in Figure 1-1).

II. VLAN mapping table

The VLAN mapping table is an attribute of MST region. It is used for describing the mapping relationship of VLANs and spanning tree instances (STIs). For example, in the VLAN mapping table of MST region A0 in Figure 1-1, VLAN 1 is mapped to instance 1, VLAN 2 is mapped to instance 2, and other VLANs are mapped to CIST.

In the same region, all the switches must have the same VLAN-to-STI mapping relationship. Otherwise, they cannot be in the same region.

III. IST

Internal Spanning Tree (IST): a spanning tree in a MSTP region. The IST and the Common Spanning Tree (CST), together make up a Common and Internal Spanning Tree (CIST) for the entire switching network. The IST in a MST region is a fragment of the CIST. For example, every MST region in Figure 1-1 has an IST, which is a fragment of CIST.

IV. CST

Common Spanning Tree (CST): a LAN has only one CST. CST connects the spanning trees of all MST regions. Regard every MST region as a “switch”, and the CST is generated by the computing of “switches” through STP/RSTP. For example, the red line in Figure 1-1 indicates the CST.

V. CIST

Common and Internal Spanning Tree (CIST): A single spanning tree made up of ISTs and CST. It connects all switches in a switching network. CIST of Figure 1-1 is composed of ISTs in all MST regions and the CST.

VI. MSTI

Multiple Spanning Tree Instance (MSTI): multiple spanning trees can be generated with MSTP in an MST region and independent of one another. Such a spanning tree is called an MSTI. As shown in Figure 1-1, every MST region has many STIs. Each STI corresponds to a VLAN and is called a MSTI.

VII. Region root

The region root refers to the root of the IST and MSTI of the MST region. The spanning trees in an MST region have different topology and their region roots may also be different. For example, the region root of the STI 1 is the switch B and that of the STI 2 is the switch C, as shown in Figure 1-1.

VIII. Common Root Bridge

The Common Root Bridge refers to the root bridge of CIST. For example, the common root bridge is a certain switch in A0, as shown in Figure 1-1.

IX. Edge port

The edge port refers to the port located at the MST region edge, connecting different MST regions, MST region and STP region, or MST region and RSTP region. For MSTP calculation, the edge port shall take the same role on MSTI and CIST instance. For example, as shown in Figure 1-1, if a switch in region A0 connects to the first port on a switch in region D0, and the common root bridge of the whole switching network is in A0, then this first port is an edge port of region D0.

X. Port role

In the process of MSTP calculation, a port can serve as a designated port, root port, master port, alternate port, or backup port.

l           The root port is the one through which the data are forwarded to the root.

l           The designated port is the one through which the data are forwarded to the downstream network segment or switch.

l           Master port is the port connecting the entire region to the Common Root Bridge and located on the shortest path between them.

l           An alternate port is a backup of the mater port, and also a backup port of a root port in the region. As a backup of the mater port, an alternate port will become a new master port after a master port is blocked.

l           If two ports of a switch are connected, there must be a loop. In this case, the switch blocks one of them. The blocked one is called a backup port.

A port can play different roles in different spanning tree instances.

As shown in Figure 1-2, the switch A, B, C, and D make up a MST region. Port 1 and 2 on switch A connects to the common root bridge; port 5 and 6 on switch C forms a loop; port 3 and 4 on switch D connects to other MST regions in the downstream direction.

Figure 1-2 Port roles

XI. TC packet

Topology change (TC) means the structure of the MSTP spanning tree changes due to some bridge change or some port change on the network. In software implementation, when a port state changes from discarding to forwarding, it means TC occurs.

The following section describes two kinds of STP packets:

1)         MSTP BPDU packet

MSTP modules communicate with each other among bridges by MSTP BPDU packets. The following figure shows the MSTP BPDU packet format:

Figure 1-3 MSTP BPDU packet format

Figure 1-4 MSTI information format of the last part in an MSTP BPDU

Besides field root bridge priority, root path cost, local bridge priority and port priority, the field flag which takes one byte in an instance is also used for role selection. The following figure describes the meaning of its eight bits:

Figure 1-5 Meaning of 1-byte Flags in a BPDU

The second and third bits together indicate MSTP port role.

2)         TC packet

A TC packet is also an MSTP BPDU packet, but the lowest bit of its flags field is set to 1, which endows the TC packet with special meaning. So the TC packet has its special meaning. After receiving or detecting TC packets, a port will broadcast TC packets to tell the whole network the changed topology information at the fastest speed.

1.1.2  How MSTP Works

MSTP divides the entire Layer 2 network into several MST regions and calculates and generates CST for them. Multiple spanning trees are generated in a region and each of them is called an MSTI. The instance 0 is called IST, and others are called MSTI. Similar to RSTP, MSTP also use configuration messages to calculate and generate spanning trees, the difference is that it is the MSTP configuration information on the switches that is carried in the configuration messages.

I. CIST calculation

The CIST root is the highest-priority switch elected from the switches on the entire network through comparing their configuration BPDUs. MSTP calculates and generates IST in each MST region; at the same time it regards each MST region as a single "switch" and then calculates and generates the CST between the regions. The CST and ISTs together make up the CIST which connects all the switches in the whole switching network.

II. MSTI calculation

Inside an MST region, MSTP generates different MSTIs for different VLANs according to the association between VLAN and the spanning tree. The calculation process of MSTI is like that of RSTP.

The following introduces the calculation process of one MSTI.

The fundamental of STP is that the switches exchange a special kind of protocol packet (which is called configuration Bridge Protocol Data Units, or BPDU, in IEEE 802.1D) to decide the topology of the network. The configuration BPDU contains the information enough to ensure the switches to compute the spanning tree.

Table 1-1 describes the designated bridge and designated port.

Table 1-1 Description of the designated bridge and designate port

Classification	Designated bridge	Designated port
For a device	A device directly connected with the local device and responsible for forwarding BPDUs to the local device	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 1-6 Designated bridge and designated port

As illustrated in the Figure 1-6, Switch A forwards BPDUs to Switch B via port AP1. To Switch B, the designated bridge is Switch A and the designated port is AP1. In the figure, Switch B and Switch C are connected to the LAN and Switch B forwards BPDUs to LAN. Therefore, the designated bridge of LAN is Switch B and the designated port is BP2.

l           The specific calculation process of STP algorithm.

The following example illustrates the calculation process of STP.

Figure 1-7 illustrates the practical network.

Figure 1-7 Ethernet switch networking

To facilitate the descriptions, only the first four parts of the configuration BPDU are described in the example. They are root ID (expressed as switch priority), path cost to the root, designated bridge ID (expressed as switch priority) and the designated port ID (expressed as the port number). As illustrated Figure 1-7, the priorities of Switch A, B and C are 0, 1 and 2 and the path costs of their links are 5, 10 and 4 respectively.

1)         Initial state

When initialized, each port of the switches generates the configuration BPDU taking itself as the root with a root path cost as 0, designated bridge IDs as their own switch IDs and the designated ports as their ports.

Switch A:

Configuration BPDU of AP1: {0, 0, 0, AP1}

Configuration BPDU of AP2: {0, 0, 0, AP2}

Switch B:

Configuration BPDU of BP1: {1, 0, 1, BP1}

Configuration BPDU of BP2: {1, 0, 1, BP2}

Switch C:

Configuration BPDU of CP2: {2, 0, 2, CP2}

Configuration BPDU of CP1: {2, 0, 2, CP1}

2)         Select the optimum configuration BPDU

Every switch transmits its configuration BPDU to others. When a port receives a configuration BPDU with a lower priority than that of its own, the switch discards the message and keep the local BPDU unchanged. When the port receives a higher-priority configuration BPDU, the switch uses the content in the received configuration BPDU to change the content of the local BPDU of this port. Then the switch compares the configuration BPDU of this port to those of other ports on it to elect the optimum configuration BPDU.

The comparison rules are:

l           The configuration BPDU with a smaller root ID has a higher priority.

l           If the root IDs are the same, perform the comparison based on root path costs. The cost comparison is as follows: the path cost to the root recorded in the configuration BPDU plus the corresponding path cost of the local port is set as S, the configuration BPDU with a smaller S has a higher priority.

l           If the costs of path to the root are also the same, compare in sequence the designated bridge ID, designated port ID and the ID of the port via which the configuration BPDU was received.

For the convenience of expression, this example supposes that the optimum configuration BPDU can be elected just by the comparison of root IDs.

3)         Determine the root and designated ports, and update the configuration BPDU of designated ports.

The port receiving the optimum configuration BPDU is designated to be the root port, whose configuration BPDU remains unchanged. Switch calculates a designated port BPDU for every port: substituting the root ID with the root ID in the configuration BPDU of the root port, the cost of path to root with the value made by the root path cost plus the path cost corresponding to the root port, the designated bridge ID with the local switch ID and the designated port ID with the local port ID.

Switch compares the calculated BPDU with the BPDU of corresponding port. If the BPDU of corresponding port is better, the port is blocked, and the BPDU of the port remains unchanged. The port will not forward data and only receive but not send BPDU. If the calculated BPDU is better, the port will be the designated port, and the port BPDU will be modified by the calculated BPDU and sent out regularly.

The comparison process of each switch is as follows.

Switch A:

AP1 receives the configuration BPDU from Switch B and finds out that the local configuration BPDU priority is higher than that of the received one, so it discards the received configuration BPDU. The configuration BPDU is processed on the AP2 in a similar way. Thus Switch A finds itself the root and designated bridge in the configuration BPDU of every port. It regards itself as the root, retains the configuration BPDU of each port and transmits configuration BPDU to others regularly thereafter. By now, the configuration BPDUs of the two ports are as follows:

Configuration BPDU of AP1: {0, 0, 0, AP1}.

Configuration BPDU of AP2: {0, 0, 0, AP2}.

Switch B:

BP1 receives the configuration BPDU from Switch A and finds that the received BPDU has a higher priority than the local one, so it updates its configuration BPDU.

BP2 receives the configuration BPDU from Switch C and finds that the local BPDU priority is higher than that of the received one, so it discards the received BPDU.

By now, the configuration BPDUs of each port are as follows: Configuration BPDU of BP1: {0, 0, 0, AP1}, Configuration BPDU of BP2: {1, 0, 1, BP2}.

Switch B compares the configuration BPDUs of the ports and selects the BP1 BPDU as the optimum one because the current configuration BPDU {0, 5, 0, AP1} of BP1 has a higher priority than the configuration BPDU {1, 0, 1, BP2} of BP2. Thus BP1 is elected as the root port and the configuration BPDUs of Switch B ports are updated as follows.

The configuration BPDU of the root port BP1 retains as {0, 5, 0, AP1}. BP2 updates root ID with that in the optimum configuration BPDU, the path cost to root with 5, sets the designated bridge as the local switch ID and the designated port ID as the local port ID. Thus, the configuration BPDU becomes {0, 5, 1, BP2}.

Then, all the designated ports of Switch B transmit the configuration BPDUs regularly.

Switch C:

CP2 receives from the BP2 of Switch B the configuration BPDU {1, 0, 1, BP2} that has not been updated and then the updating process is launched. The configuration BPDU is updated as {1, 0, 1, BP2}.

CP1 receives the configuration BPDU {0, 0, 0, AP2} from Switch A and Switch C launches the updating. The configuration BPDU is updated as {0, 0, 0, AP2}.

Now, the configuration BPDU of CP1 is {0, 10, 0, AP2}, which has a higher priority than that of CP2. By comparison, CP1 configuration BPDU is elected as the optimum one. The CP1 is thus specified as the root port without modifying its configuration BPDU. However, CP2 will be blocked and its BPDU also remains unchanged, but it will not receive the data (excluding the STP packets) forwarded from Switch B until spanning tree calculation is launched again by some new events. For example, the link from Switch B to Switch C is down or the port receives any better configuration BPDU.

CP2 will receive the updated configuration BPDU, {0, 5, 1, BP2}, from Switch B. Since this configuration BPDU is better then the old one, the old BPDU will be updated to {0, 5, 1, BP2}.

Meanwhile, CP1 receives the configuration BPDU from Switch A but its configuration BPDU is not updated and retain {0, 10, 0, AP2}.

By comparison, {0, 9, 1, BP2}, the configuration BPDU of CP2, is elected as the optimum one. Thus, CP2 is elected as the root port, whose BPDU will not change, while CP1 is blocked, its BPDU is retained, and will not receive the data forwarded from Switch A until spanning tree calculation is triggered again by some changes. For example, the link from Switch B to Switch C is down or the port receives any better configuration BPDU

Thus, the spanning tree is stabilized. The tree with the root bridge A is illustrated in the Figure 1-8.

Figure 1-8 The final stabilized spanning tree

To facilitate the descriptions, the description of the example is simplified. For example, the root ID and the designated bridge ID in actual calculation should comprise both switch priority and switch MAC address. Designated port ID should comprise port priority and port ID. In the updating process of a configuration BPDU, other configuration BPDUs besides the first four items will make modifications according to certain rules. The basic calculation process is described below:

In addition, with identical priority, the path cost of an aggregation port is smaller than that of a non-aggregation port. Therefore, under identical root ID, path cost value and designated switch ID, the switch will generally select the aggregation port as the root port.

l           Configuration BPDU forwarding mechanism in STP:

Upon the initiation of the network, all the switches regard themselves as the roots. The designated ports send the configuration BPDUs of local ports at a regular interval of HelloTime. If it is the root port that receives the configuration BPDU, the switch will enable a timer to time the configuration BPDU as well as increase MessageAge carried in the configuration BPDU by certain rules. If a path goes wrong, the root port on this path will not receive configuration BPDUs any more and the old configuration BPDUs will be discarded due to timeout. Hence, recalculation of the spanning tree will be initiated to generate a new path to replace the failed one and thus restore the network connectivity.

However, the new configuration BPDU as now recalculated will not be propagated throughout the network right away, so the old root ports and designated ports that have not detected the topology change will still forward the data through the old path. If the new root port and designated port begin to forward data immediately after they are elected, an occasional loop may still occur. In STP, a transitional state mechanism is thus adopted to ensure the new configuration BPDU has been propagated throughout the network before the root port and designated port begin to send data again. That is, the root port and designated port should undergo a transitional state for a period of Forward Delay before they enter the forwarding state.

And thus, the packets of a VLAN will be forwarded along the following path: in the MST region, the packets will be forwarded along the corresponding MSTI; among the regions, the packets will be forwarded along the CST.

1.1.3  MSTP Implementation on the Switch

MSTP is compatible with STP and RSTP. The MSTP switch can recognize both the STP and RSTP packets and calculate the spanning tree with them. Besides the basic MSTP functions, H3C Ethernet Switch Series also provide some features easy to manage from users’ point of view. These features include root bridge hold, secondary root bridge, ROOT protection, BPDU protection, loop protection, hot swapping of the interface boards, master/slave switchover, and so on. Note that the spanning tree needs to be calculated again when a master/slave switchover occurs.

1.2  MSTP Configuration Tasks

MSTP configuration tasks include:

l           Configuring the MST Region for a Switch

l           Specifying the Switch as a Primary or a Secondary Root bridge

l           Configuring the MSTP Running Mode

l           Configuring the Bridge Priority for a Switch

l           Configuring the Max Hops in an MST Region

l           Configuring the Switched Network Diameter

l           Configuring the Time Parameters of a Switch

l           Setting the Timeout Factor of a Specific Bridge

l           Configuring the Max Transmission Speed on a Port

l           Configuring a Port as an Edge Port or Non-edge Port

l           Configuring the Path Cost of a Port

l           Configuring the Priority of a Port

l           Configuring Whether a Port Is on a Point-to-Point Link

l           Configuring the mCheck Variable of a Port

l           Configuring the Switch Protection Functions

l           Enabling/Disabling MSTP on the Device

l           Disabling BPDU Packets from Flooding in the Default VLANs

l           Enabling Single-Port Loop Detection

l           Configuring the Switch to Support Standard or Legacy MSTP Packet Format

l           Configuring Digest Snooping

l           Configuring Fast State Transition

MSTP must be enabled on the device before other related configurations can take effect. Before enabling MSTP, you can configure the related parameters of the device and Ethernet ports, which will take effect upon enabling MSTP and stay effective even after resetting MSTP. You can use the check region-configuration command to view newly configured MSTP region parameters. The display current-configuration command shows the parameters configured before MSTP is enabled. For those configured after MSTP is enabled, you can use the related display commands. For detailed information, refer to Displaying and Debugging MSTP.

 

 

1.2.1  Configuring the MST Region for a Switch

Which MST region a switch belongs to is determined with the configurations of the region name, VLAN mapping table, and MSTP revision level. You can perform the following configurations to put a switch into an MST region.

I. Entering MST region view

Perform the following configuration in system view.

Table 1-2 Enter MST region view

To do...	Use the command...
Enter MST region view (from system view)	stp region-configuration
Restore the default settings of MST region	undo stp region-configuration

 

II. Configuring parameters for the MST region

Perform the following configuration in MST region view.

Table 1-3 Configure the MST region for a switch

To do...	Use the command...
Configure the MST region name	region-name name
Restore the default MST region name	undo region-name
Configure VLAN mapping table	instance instance-id vlan vlan-list
Restore the default VLAN mapping table	undo instance instance-id [ vlan vlan-list ]
Configure the MSTP revision level of MST region	revision-level level
Restore the MSTP revision level of MST region	undo revision-level

 

An MST region can contain up to 49 spanning tree instances, among which the Instance 0 is IST and the Instances 1 through 48 are MSTIs. Upon the completion of the above configurations, the current switch is put into a specified MST region. Note that two switches belong to the same MST region only if they have been configured with the same MST region name, STI-VLAN mapping tables of an MST region, and the same MST region revision level.

Configuring the related parameters, especially the VLAN mapping table, of the MST region, will lead to the recalculation of spanning tree and network topology flapping. To bate such flapping, MSTP triggers to recalculate the spanning tree according to the configurations only if one of the following conditions is met:

l           A user manually activates the configured parameters related to the MST region, using the active region-configuration command.

l           A user enables MSTP using the stp enable command.

By default, the MST region name is the switch MAC address, all the VLANs in the MST region are mapped to the STI 0, and the MSTP region revision level is 0. You can restore the default settings of MST region, using the undo stp region-configuration command in system view.

III. Configuring the mapping relationship between a VLAN list and a Spanning Tree Instance

MSTP describes the mapping relationship between VLAN and Spanning Tree instances through the VLAN mapping table. You can use this command to configure the VLAN mapping table: each VLAN can be allocated to different Spanning Tree instances according to your configuration.

You cannot map one VLAN to different instances. When you map a mapped VLAN to a different MSTI, the previous mapping relationship will be automatically cancelled.

You can use the vlan-mapping modulo modulo command to configure VLAN-instance mappings quickly. This command maps a VLAN to a spanning tree instance whose ID is (VLAN ID-1) %modulo+1.

Perform the following configurations in MST region view.

Table 1-4 Map all the VLAN lists to the specific Spanning Tree instances

To do...	Use the command...
Map all the VLAN lists to the specific spanning tree instances uniformly as per the specified modulo number	vlan-mapping modulo modulo
Restore the default VLAN-instance mappings	undo vlan-mapping modulo

 

By default, all the VLAN lists are mapped to CIST, namely, Instance 0.

IV. Activating the MST region configuration, and exit the MST region view

Perform the following configuration in MST region view.

Table 1-5 Activate the MST region configuration and exit the MST region view

To do...	Use the command...
View the latest configuration information of the MST region, regardless whether the configuration is active or not	check region-configuration
Manually activate the MST region configuration	active region-configuration
Exit MST region view	quit

 

1.2.2  Specifying the Switch as a Primary or a Secondary Root bridge

MSTP can determine the spanning tree root through calculation. You can also specify the current switch as the root, using the command provided by the switch.

You can use the following commands to specify the current switch as the primary or secondary root of the spanning tree.

Perform the following configuration in system view.

Table 1-6 Specify the switch as a primary or a secondary root bridge

To do...	Use the command...
Specify the current switch as the primary root bridge of the specified spanning tree	stp [ instance instance-id ] root primary [ bridge-diameter bridgenum ] [ hello-time centi-seconds ]
Specify the current switch as the secondary root bridge of the specified spanning tree	stp [ instance instance-id ] root secondary [ bridge-diameter bridgenum ] [ hello-time centi-seconds ]
Specify current switch not to be the primary or secondary root	undo stp [ instance instance-id ] root

 

After a switch is configured as the primary root bridge or the secondary root bridge, users cannot modify the bridge priority of the switch.

You can configure the current switch as the primary or secondary root bridge of the STI (specified by the instance instance-id parameter). If the instance-id takes 0, the current switch is specified as the primary or secondary root bridge of the CIST.

The root types of a switch in different STIs are independent of one another. The switch can be a primary or secondary root of any STI. However, it cannot serve as both the primary and secondary roots of one STI.

If the primary root is down or powered off, the secondary root will take its place, unless you configure a new primary root. Of two or more configured secondary root bridges, MSTP selects the one with the smallest MAC address to take the place of the failed primary root.

When configuring the primary and secondary switches, you can also configure the network diameter and hello time of the specified switching network. For detailed information, refer to section Configuring the Switched Network Diameter and section Configuring the Time Parameters of a Switch.

 

 

By default, a switch is neither the primary root nor the secondary root of the spanning tree.

1.2.3  Configuring the MSTP Running Mode

MSTP and RSTP are compatible and they can recognize the packets of each other. However, STP cannot recognize MSTP packets. To implement the compatibility, MSTP provides two operation modes, STP-compatible mode and MSTP mode. In STP-compatible mode, the switch sends STP packets via every port. In MSTP mode, the switch ports send MSTP or STP packets (when connected to the STP switch) and the switch provides multiple spanning tree function.

You can use the following command to configure MSTP running mode. MSTP can intercommunicate with STP. If there is a STP switch in the switching network, you may use the command to configure the current MSTP to run in STP-compatible mode. Otherwise, configure it to run in MSTP mode.

Perform the following configuration in system view.

Table 1-7 Configure the MSTP running mode

To do...	Use the command...
Configure MSTP to run in STP-compatible mode	stp mode stp
Configure MSTP to run in MSTP mode	stp mode mstp
Restore the default MSTP running mode	undo stp mode

 

Generally, if there is a STP switch on the switching network, the port connected to it will automatically transit from MSTP mode to STP-compatible mode. But the port cannot automatically transit back to MSTP mode after the STP switch is removed. In this case, you can execute the stp mcheck command to restore the MSTP mode.

By default, the switch runs in MSTP mode.

1.2.4  Configuring the Bridge Priority for a Switch

Whether a switch can be elected as the spanning tree root depends on its Bridge priority. The switch configured with a smaller Bridge priority is more likely to become the root. An MSTP switch may have different priorities in different STIs.

You can use the following command to configure the Bridge priorities of the Designated bridge in different STIs.

Perform the following configuration in system view.

Table 1-8 Configure the Bridge priority for a switch

To do...	Use the command...
Configure the Bridge priority of the Designated bridge	stp [ instance instance-id ] priority priority
Restore the default Bridge priority of the Designated bridge	undo stp [ instance instance-id ] priority

 

When configuring the switch priority with the instance instance-id parameter as 0, you are configuring the CIST priority of the switch.

 

 

By default, the switch Bridge priority is 32768.

1.2.5  Configuring the Max Hops in an MST Region

The scale of MST region is limited by the max hops in an MST region, which is configured on the region root. As the BPDU travels from the spanning tree root, each time when it is forwarded by a switch, the max hops is reduced by 1. The switch discards the configuration BPDU with 0 hops left. This makes it impossible for the switch beyond the max hops to take part in the spanning tree calculation, thereby limiting the scale of the MST region.

You can use the following command to configure the max hops in an MST region.

Perform the following configuration in system view.

Table 1-9 Configure the max hops in an MST region

To do...	Use the command...
Configure the max hops in an MST region	stp max-hops hop
Restore the default max hops in an MST region	undo stp max-hops

 

The more the hops in an MST region, the larger the scale of the region is. Only the max hops configured on the region root can limit the scale of MST region. Other switches in the MST region also apply the configurations on the region root, even if they have been configured with max hops.

By default, the max hop of an MST is 20.

1.2.6  Configuring the Switched Network Diameter

Any two hosts on the switched network are connected with a specific path carried by a series of switches. Among these paths, the one passing more switches than all others is the network diameter, expressed as the number of passed switches.

You can use the following command to configure the diameter of the switched network.

Perform the following configuration in system view.

Table 1-10 Configure the switched network diameter

To do...	Use the command...
Configure the switched network diameter	stp bridge-diameter bridgenum
Restore the default switched network diameter	undo stp bridge-diameter

 

The network diameter is the parameter specifying the network scale. The larger the diameter is, the lager the scale of the network is.

When a user configures the network diameter on a switch, MSTP automatically calculates and sets the Hello Time, Forward-Delay and Max Age time of the switch to the desirable values.

Setting the network diameter takes effect on CIST only, but has no effect on MSTI.

By default, the network diameter is 7 and the three corresponding timers take the default values.

 

 

1.2.7  Configuring the Time Parameters of a Switch

The switch has three time parameters, Forward Delay, Hello Time, and Max Age.

Forward Delay is the switch state transition mechanism. The spanning tree will be recalculated upon link faults and its structure will change accordingly. However, the configuration BPDU recalculated cannot be immediately propagated throughout the network. The temporary loops may occur if the new root port and designated port forward data right after being elected. Therefore the protocol adopts a state transition mechanism. It takes a Forward Delay interval for the root port and designated port to transit from the learning state to forwarding state. The Forward Delay guarantees a period of time during which the new configuration BPDU can be propagated throughout the network.

The switch sends Hello packet periodically at an interval specified by Hello Time to check if there is any link fault.

Max Age specifies when the configuration BPDU will expire. The switch will discard the expired configuration BPDU.

You can use the following command to configure the time parameters for the switch.

Perform the following configuration in system view.

Table 1-11 Configure the time parameters of a switch

To do...	Use the command...
Configure Forward Delay on the switch	stp timer forward-delay centi-seconds
Restore the default Forward Delay of the switch	undo stp timer forward-delay
Configure Hello Time on the switch	stp timer hello centi-seconds
Restore the default Hello Time on the switch	undo stp timer hello
Configure Max Age on the switch	stp timer max-age centi-seconds
Restore the default Max Age on the switch	undo stp timer max-age

 

Every switch on the switching network adopts the values of the time parameters configured on the root bridge of the CIST.

 

 

To avoid frequent network flapping, the values of Hello Time, Forward Delay and Max Age should guarantee the following formulas.

2 x (Forward-Delay – 1 second) >= Max Age

Max Age >= 2 x (Hello Time + 1 second)

You are recommended to use the stp bridge-diameter command to specify the network diameter and Hello Time of the switching network, and then MSTP will automatically calculate and give the rather desirable values.

By default, Forward Delay is 15 seconds, Hello Time is 2 seconds, and Max Age is 20 seconds.

1.2.8  Setting the Timeout Factor of a Specific Bridge

A switch transmits hello packet regularly to the adjacent bridges to check if there is link failure. Typically, if the switch does not receive the STP packets from the upstream switch during 3 times the hello time, the switch considers that the upstream switch is dead and initiates a new spanning tree calculation process. In a topologically stable network, this kind of spanning tree recalculation may take place when the upstream is busy. In this case, you can set the timeout interval to a bigger value to avoid this kind of unwanted recalculation.

You can use the following command to set the multiple value of hello time of a specified bridge.

Perform the following configurations in system view.

Table 1-12 Setting the timeout factor of a specific switch

To do...	Use the command...
Set the timeout factor of a specified switch	stp timer-factor number
Restore the default timeout factor	undo stp timer-factor

 

It is recommended to set 5, 6 or 7 as the timeout factor in the steady network.

By default, the timeout factor of the switch is 3.

1.2.9  Configuring the Max Transmission Speed on a Port

The max transmission speed on a port specifies how many MSTP packets will be transmitted via the port every Hello Time.

The max transmission speed on a port is limited by the physical state of the port and the network structure. You can configure it according to the network conditions.

You can configure the max transmission speed on a port in the following ways.

I. Configuration in system view

Perform the following configuration in system view.

Table 1-13 Configure the max transmission speed on a port

To do...	Use the command...
Configure the max transmission speed on a port	stp interface interface-list transmit-limit packetnum
Restore the default max transmission speed on a port	undo stp interface interface-list transmit-limit

 

II. Configuration in Ethernet port view

Perform the following configuration in Ethernet port view.

Table 1-14 Configure the max transmission speed on a port

To do...	Use the command...
Configure the max transmission speed on a port	stp transmit-limit packetnum
Restore the default max transmission speed on a port	undo stp transmit-limit

 

You can configure the max transmission speed on a port with either of the earlier-mentioned measures. For more about the commands, refer to the Command Manual.

This parameter only takes a relative value without units. If it is set too large, too many packets will be transmitted during every Hello Time and too many network resources will be occupied. The default value is recommended.

By default, the max transmission speed on every Ethernet port of the switch is 10.

1.2.10  Configuring a Port as an Edge Port or Non-edge Port

An edge port refers to the port not directly connected to any switch or indirectly connected to a switch over the connected network.

You can configure a port as an edge port or non-edge port in the following ways.

I. Configuration in system view

Perform the following configuration in system view.

Table 1-15 Configure a port as an edge port or a non-edge port

To do...	Use the command...
Configure a port as an edge port	stp interface interface-list edged-port enable
Configure a port as a non-edge port	stp interface interface-list edged-port disable
Restore the default setting of the port as a non-edge port	undo stp interface interface-list edged-port

 

II. Configuration in Ethernet port view

Perform the following configuration in Ethernet port view.

Table 1-16 Configure a port as an edge port or a non-edge port

To do...	Use the command...
Configure a port as an edge port	stp edged-port enable
Configure a port as a non-edge port	stp edged-port disable
Restore the default setting of the port as a non-edge port	undo stp edged-port

 

You can configure a port as an edge port or a non-edge port with either of the earlier-mentioned measures.

After configured as an edge port, the port can fast transit from blocking state to forwarding state without any delay. You can only set the port connecting with the terminal as an edge port. The configuration of this parameter takes effect on all the STIs. In other words, if a port is configured as an edge port or non-edge port, it is configured the same on all the STIs.

If BPDU protection is enabled on the switch, the edged port is disabled when it receives BPDU packets from the user. Only the network administrators can enable the port.

By default, all the Ethernet ports of the switch have been configured as non-edge ports.

 

 

 

1.2.11  Configuring the Path Cost of a Port

Path Cost is related to the speed of the link connected to the port. On the MSTP switch, a port can be configured with different path costs for different STIs. Thus the traffic from different VLANs can run over different physical links, thereby implementing the VLAN-based load-balancing.

You can configure the path cost of a port in the following ways.

I. Configuration in system view

Perform the following configuration in system view.

Table 1-17 Configure the path cost of a port

To do...	Use the command...
Configure the path cost of a port	stp interface interface-list [ instance instance-id ] cost cost
Restore the default path cost of a port	undo stp interface interface-list [ instance instance-id ] cost

 

II. Configuration in Ethernet port view

Perform the following configuration in Ethernet port view.

Table 1-18 Configure the path cost of a port

To do...	Use the command...
Configure the path cost of a port	stp [ instance instance-id ] cost cost
Restore the default path cost of a port	undo stp [ instance instance-id ] cost

 

You can configure the path cost of a port with either of the earlier-mentioned measures. Upon the change of path cost of a port, MSTP will recalculate the port role and transit the state. When instance-id takes 0, it indicates to set the path cost on the CIST.

By default, MSTP is responsible for calculating the path cost of a port.

1.2.12  STP Path Cost Calculation Standards on STP port

Path cost is a parameter related to the link rate of a port. On an MSTP-enabled switch, a port can have different path costs in different MST instances. Setting appropriate path costs can make different VLANs’ traffic flows along different physical links, thus implementing load balancing based on VLANs.

In calculating the path cost value of an aggregated link, 802.1D-1998 does not take into account the number of ports in the aggregation group, whereas 802.1T takes this number into account. The calculation formula is as follows: Path Cost = 200,000,000/link speed in 100 Kbps, where the link speed is the sum of the speed values of the non-blocked ports in the aggregation group. The following table shows port rates and corresponding path costs.

Table 1-19 Port rates and corresponding path costs

Port rate	Duplex status	802.1D-1998	IEEE 802.1t	Legacy standard
0	—	65535	200,000,000	200,000
10 Mbps	Half-Duplex Full-Duplex Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	100 100 95 95 95	2,000,000 2,000,000 1,000,000 666,666 500,000	2,000 2,000 1,800 1,600 1,400
100 Mbps	Half-Duplex Full-Duplex Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	19 19 15 15 15	200,000 200,000 100,000 66,666 50,000	200 200 180 160 140
1000 Mbps	Full-Duplex Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	4 3 3 3	20,000 10,000 6,666 5,000	20 18 16 14
10 Gbps	Full-Duplex Aggregated Link 2 Ports Aggregated Link 3 Ports Aggregated Link 4 Ports	2 1 1 1	2,000 1,000 666 500	2 1 1 1

 

You can specify the intended standard by using the following commands.

Perform the following configuration in system view.

Table 1-20 Specifying the standard to be followed in path cost calculation

To do...	Use the command...
Specify the standard to be adopted when the switch calculates the default path cost for the connected link	stp pathcost-standard { dot1d-1998 | dot1t legacy}
Restore the default standard to be used	undo stp pathcost-standard

 

By default, the switch calculates the default path cost of a port by the legacy standard.

1.2.13  Configuring the Priority of a Port

For spanning tree calculation, the port priority is an importance factor to determine if a port can be elected as the root port. With other things being equal, the port with the highest priority will be elected as the root port. On the MSTP switch, a port can have different priorities in different STIs and plays different roles respectively. Thus the traffic from different VLANs can run over different physical links, thereby implementing the VLAN-based load-balancing.

You can configure the port priority in the following ways.

I. Configuration in system view

Perform the following configuration in system view.

Table 1-21 Configure the port priority

To do...	Use the command...
Configure the port priority	stp interface interface-list instance instance-id port priority priority
Restore the default port priority	undo stp interface interface-list instance instance-id port priority

 

II. Configuration in Ethernet port view

Perform the following configuration in Ethernet port view.

Table 1-22 Configure the port priority

To do...	Use the command...
Configure the port priority	stp [ instance instance-id ] port priority priority
Restore the default port priority	undo stp [ instance instance-id ] port priority

 

You can configure the port priority with either of the earlier-mentioned measures. Upon the change of port priority, MSTP will recalculate the port role and transit the state. Generally, a smaller value represents a higher priority. If all the Ethernet ports of a switch are configured with the same priority value, the priorities of the ports will be differentiated by the index number. The change of Ethernet port priority will lead to spanning tree recalculation. You can configure the port priority according to actual networking requirements.

By default, the priority of all the Ethernet ports is 128.

1.2.14  Configuring Whether a Port Is on a Point-to-Point Link

A point-to-point link directly connects two switches.

You can configure a port or port as being on a point-to-point link or point-to-point links in the following ways.

I. Configuration in system view

Perform the following configuration in system view.

Table 1-23 Configure all ports as being (being not) on point-to-point links

To do...	Use the command...
Configure all ports as being on point-to-point links	stp interface interface-list point-to-point force-true
Configure all ports as being not on point-to-point links	stp interface interface-list point-to-point force-false
Configure MSTP to automatically detect whether a port is on a point-to-point link	stp interface interface-list point-to-point auto
Configure MSTP to automatically detect whether a port is on a point-to-point link, as defaulted	undo stp interface interface-list point-to-point

 

II. Configuration in Ethernet port view

Perform the following configuration in Ethernet port view.

Table 1-24 Configure the port as being (being not) on a point-to-point link

To do...	Use the command...
Configure the port as being on a point-to-point link	stp point-to-point force-true
Configure the port as being not on a point-to-point link	stp point-to-point force-false
Configure MSTP to automatically detect whether the port is on a point-to-point link	stp point-to-point auto
Configure MSTP to automatically detect whether the port is on a point-to-point link, as defaulted	undo stp point-to-point

 

When some port role conditions are met, ports on point-to-point links can quickly transit to forwarding state through the proposal-agreement mechanism, thus reducing the unnecessary forwarding delay. If the auto mode is specified, MSTP will automatically detect whether the current Ethernet port is on a point-to-point link.

 

 

This configuration takes effect on the CIST and all the MSTIs. Note that a temporary loop may be introduced if you configure a port as being on a point-to-point link by force while it is physically not.

By default, the parameter is set to auto.

1.2.15  Configuring the mCheck Variable of a Port

The port of an MSTP switch operates in either STP-compatible or MSTP mode.

Suppose a port of an MSTP switch on a switching network is connected to an STP switch, the port will automatically transit to operate in STP-compatible mode. However, the port stays in STP-compatible mode and cannot automatically transit back to MSTP mode when the STP switch is removed. In this case, you can perform mCheck operation to transit the port to MSTP mode by force.

You can use the following measure to perform mCheck operation on a port.

I. Configuration in system view

Perform the following configuration in system view.

Table 1-25 Configure the mCheck variable of a port

To do...	Use the command...
Perform mCheck operation on a port	stp [ interface interface-list ] mcheck

 

 

II. Configuration in Ethernet port view

Perform the following configuration in Ethernet port view.

Table 1-26 Configure the mCheck variable of a port

To do...	Use the command...
Perform mCheck operation on a port	stp mcheck

 

You can configure mCheck variable on a port with either of the earlier-mentioned measures. Note that the command can be used only if the switch runs MSTP. The command does not make any sense when the switch runs in STP-compatible mode.

1.2.16  Configuring the Function of Clearing Dynamic ARP Entries

Perform the following configuration in Ethernet port view or system view.

Table 1-27 Configure the function of clearing dynamic ARP entries

To do...	Use the command...
Enable the function of clearing dynamic ARP entries on the switch or on a port	stp reset-arp enable
Disable the function of clearing dynamic ARP entries on the switch or on a port	stp reset-arp disable
Enable the root protection function (system view)	stp interface interface-list root-protection

 

Note: If you enable the function of clearing dynamic ARP entries in system view, the function takes effect on all ports of the system. If you enable the function of clearing dynamic ARP entries in port view, the function takes effect only on the specified port.

1.2.17  Configuring the Switch Protection Functions

An MSTP switch provides BPDU protection, Root protection functions, loop protection and TC-protection.

I. BPDU protection

For an access device, the access port is generally directly connected to the user terminal (for example, PC) or a file server, and the access port is set to an edge port to implement fast transition. When such a port receives BPDU packet, the system will automatically set it as a non-edge port and recalculate the spanning tree, which causes the network topology flapping. In normal cases, these ports will not receive STP BPDU. If someone forges BPDU to attack the switch, the network will flap. BPDU protection function is used against such network attacks.

II. Root protection

The primary and secondary root bridges of the spanning tree, especially those of CIST, shall be located in the same region. It is because the primary and secondary roots of CIST are generally placed in the core region with a high bandwidth in network design. In case of configuration error or malicious attack, the legal primary root may receive the BPDU with a higher priority and then loose its place, which causes network topology change errors. Due to the illegal change, the traffic supposed to travel over the high-speed link may be pulled to the low-speed link and congestion will occur on the network. Root protection function is used against such problems.

III. Loop protection

The root port and other blocked ports maintain their states according to the BPDUs send by uplink switch. Once the link is blocked or has trouble, then the ports cannot receive BPDUs and the switch will select root port again. In this case, the downstream switch selects the port role again. The downstream bridge port that cannot receive BPDUs becomes specific port and the blocked port is transferred to the forwarding state. As a result, a link loop is generated. The loop protection function can prohibit such loop.

 

 

IV. TC-protection

As a general rule, the switch deletes the corresponding entries in the MAC address table and ARP table upon receiving TC-BPDU packets. Under malicious attacks of TC-BPDU packets, the switch shall receive a great number of TC-BPDU packets in a very short period. Too frequent delete operations shall consume huge switch resources and bring great risk to network stability.

When the protection from TC-BPDU packet attack is enabled, the switch just perform one delete operation in a specified period (generally, 15 seconds) after receiving TC-BPDU packets, as well as monitoring whether it receives TC-BPDU packets during this period. Even if it detects a TC-BPDU packet is received in a period shorter than the specified interval, the switch shall not run the delete operation till the specified interval is reached. This can avoid frequent delete operations on the MAC address table and ARP table.

You can use the following command to configure the protection functions of the switch.

Perform the following configuration in corresponding configuration modes.

Table 1-28 Configure the switch protection function

To do...	Use the command...
Configure BPDU protection of the switch (in system view)	stp bpdu-protection
Restore the disabled BPDU protection state as defaulted (in system view)	undo stp bpdu-protection
Configure Root protection of the switch (in system view)	stp interface interface-list root-protection
Restore the disabled Root protection state as defaulted (in system view)	undo stp interface interface-list root-protection
Configure Root protection of the switch (in Ethernet port view)	stp root-protection
Restore the disabled Root protection state as defaulted (in Ethernet port view)	undo stp root-protection
Configure loop protection function of the switch (in Ethernet port view)	stp loop-protection
Restore the disabled loop protection state, as defaulted (in Ethernet port view)	stp loop-protection
Enable the loop protection function of the switch (in system view)	stp interface interface-list loop-protection
Restore the disabled loop protection state, as defaulted (in system view)	undo stp interface interface-list loop-protection
Configure TC protection of the switch (in system view)	stp tc-protection enable
Disable TC protection (in system view)	stp tc-protection disable

 

 

By default, only the protection from TC-BPDU packet attack is enabled on the switch. BPDU protection, Root protection and loop protection are disabled.

After configured with BPDU protection, the switch will disable the edge port through MSTP which receives a BPDU, and notify the network manager at same time. These ports can be resumed by the network manager only.

The port configured with Root protection only plays a role of designated port on every instance. Whenever such port receives a higher-priority BPDU, that is, it is about to turn into non-designated port, it will be set to listening state and not forward packets any more (as if the link to the port is disconnected). If the port has not received any higher-priority BPDU for a certain period of time thereafter, it will resume the normal state.

For one port, only one configuration can be effective among loop protection, Root protection and Edge port configuration at the same moment.

 

 

1.2.18  Enabling/Disabling MSTP on the Device

You need to enable MSTP on the device before any other MSTP-related configurations can take effect. The MSTP feature takes effect on a port only after you enable MSTP both globally and on the port.

After you enable MSTP globally, the system automatically runs the stp non-flooding command and drops BPDU packets entering ports not enabled with MSTP. To avoid dropping such BPDU packets, you can use the undo stp non-flooding command.

After you disable MSTP globally (regardless of whether MSTP is enabled or disabled on individual ports), the system automatically runs the undo stp non-flooding command, and BPDU packets are forwarded as common multicast packets.

By default, MSTP is disabled globally but enabled on individual ports.

I. Enabling MSTP globally

You can use the following command to enable MSTP on the device.

Perform the following configuration in system view.

Table 1-29 Enable/Disable MSTP on a device

To do...	Use the command...
Enable MSTP on a device	stp enable
Disable MSTP on a device	stp disable
Restore the disable state of MSTP, as defaulted	undo stp

 

II. Enabling MSTP on a port

You can use the following command to enable/disable MSTP on a port. You may disable MSTP on some Ethernet ports of a switch to spare them from spanning tree calculation. This is a measure to flexibly control MSTP operation and save the CPU resources of the switch.

Note that, disabling MSTP on Ethernet ports may result in redundant paths.

MSTP can be enabled/disabled on a port through the following ways.

l           Configuration in system view

Table 1-30 Enable/Disable MSTP on a port

To do...	Use the command...
Enable MSTP on a port	stp interface interface-list enable
Disable MSTP on a port	stp interface interface-list disable

 

l           Configuration in Ethernet port view

Table 1-31 Enable/Disable MSTP on a port

To do...	Use the command...
Enable MSTP on a port	stp enable
Disable MSTP on a port	stp disable

 

By default, MSTP is enabled on all the ports after it is enabled on the device.

1.2.19  Disabling BPDU Packets from Flooding in the Default VLANs

If STP (spanning tree protocol) is not enabled, or if STP is disabled on a port though it is enabled globally, the BPDU packets through the STP-disabled port will be broadcast in the default VLAN, and these BPDU packets will affect the STP operation on other ports. The stp non-flooding command can discard the BPDU packets entering the STP-disabled port of the interface card, and thus prohibiting the BPDU packets from being broadcast in the VLAN.

Table 1-32 Disable BPDU packets from being broadcast on the default VLANs

To do...	Use the command...	Remarks
Enter system view	system-view	—
Disable BPDU packets from being broadcast on STP-disabled ports	stp non-flooding [ slot slotnum ]	By default, BPDU packets are broadcast on STP-disabled ports.

 

 

1.2.20  Enabling Single-Port Loop Detection

The common loop detection function cannot find out a downstream loop if the switch has only one port connected to the downstream loop. But the STP single-port loop detection can detect this kind of downstream loop and shut down the detected port causing the loop.

Table 1-33 Enable the STP single-port loop detection function

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Enable the STP single-port loop detection function	stp loop-detection	By default, the STP loop detection function is disabled.

 

 

1.2.21  Configuring the Switch to Support Standard or Legacy MSTP Packet Format

Currently, S9500 series routing switches support legacy and standard MSTP packet format.

When a switch connects to H3C devices, the switch will adopt the legacy STP packet format. When connecting to devices supporting standard STP, the switch will adopt standard STP packet format.

In addition, ports of the switch are self-adaptive. That is, the switch can automatically adjust the outbound packet format according to the inbound packet format.

Table 1-34 Enable a port to send and process MSTP packets in specified format

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure the current port to send and process MSTP packets in specified format	stp compliance { legacy | dot1s | auto }	By default, a port operates in auto mode.

 

1.2.22  Configuring Digest Snooping

I. Introduction to digest snooping

According to IEEE 802.1s, two interconnected switches can communicate with each other through multiple spanning tree instances (MSTIs) in a multiple spanning tree protocol (MSTP) region only when they are configured with the same region settings. With MSTP, interconnected switches determine whether or not they are in the same region by checking the configuration IDs of the bridge protocol data units (BPDUs) between them. (A configuration ID comprises information such as region ID, configuration digest.)

As switches of some manufacturers come with some private protocols concerning spanning trees, a switch of this type cannot communicate with other switches in an MSTP region even if it is configured with the same MSTP region settings as other switches in the MSTP region.

Digest snooping addresses this interoperability problem. Digest snooping enables a switch to track and maintain configuration digests of other switches that are in the same region and come from other manufacturers by examining their BPDUs. It also enables the switch to insert corresponding configuration digests in its BPDUs destined for these switches. In this way, switches of different manufacturers are capable of communicating with each other in an MSTP region.

Note that:

1)         When implementing digest snooping in an MSTP region, make sure that the region configurations of the switches of different manufacturers are exactly the same to prevent possible broadcast storm caused by otherwise inconsistent mapping relationships between VLANs and VPN instances of each switch.

2)         To change the configuration of a region with one or multiple of its switches being digest snooping-enabled, be sure to disable digest snooping on these switches first to prevent possible broadcast storm caused by otherwise inconsistent mapping relationships between VLANs and VPN instances of each switch.

3)         A digest snooping-enabled switch always keeps the latest configuration digests it receives. A configuration digest remains valid even if the corresponding port goes down.

II. Configuration procedure

Table 1-35 Configure digest snooping

Configuration step	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Enable digest snooping on the port	stp config-digest-snooping	Required. Digest snooping is disabled by default on the port.
Quit Ethernet port view	quit	—
Enable digest snooping globally	stp config-digest-snooping	Required. Digest snooping is disabled by default.
Display current configuration information	display current-configuration	This command can be executed in any view.

 

 

III. Digest snooping configuration example

1)         Network requirements

All switches in Figure 1-9 are MSTP-enabled and have the same region configuration. All the switches except switch A are H3C switches.

2)         Network diagram

Figure 1-9 Network diagram for digest snooping configuration

3)         Configuration procedure

# Switch B is directly connected to Switch A. Enable digest snooping on GigabitEthernet 1/1/1 and GigabitEthernet 1/1/2.

<SwitchB> system-view

System View: return to User View with Ctrl+Z.

[SwitchB] interface GigabitEthernet1/1/1

[SwitchB-GigabitEthernet1/1/1] stp config-digest-snooping

[SwitchB-GigabitEthernet1/1/1] quit

[SwitchB] interface GigabitEthernet1/1/2

[SwitchB-GigabitEthernet1/1/2] stp config-digest-snooping

[SwitchB-GigabitEthernet1/1/2] quit

# Finally, you need to enable digest snooping globally on Switch B.

[SwitchB] stp config-digest-snooping

After the above configuration, all the switches in the MSTP region can communicate with each other through MSTI.

1.2.23  Configuring Fast State Transition

I. Introduction to fast state transition

The fast transition mechanism of designated ports in RSTP and MSTP involves two types of protocol packets:

l           Proposal: Requests for fast transition.

l           Agreement: Accepts the fast transition proposal of the opposite end.

RSTP and MSTP request that a designated port of the upstream switch can perform fast transition after receiving the agreement packet from the downstream switch. RSTP and MSTP are different in the following:

l           In MSTP, the upstream switch sends an agreement packet to the downstream switch first. After receiving the agreement packet, the downstream switch sends an agreement packet to the upstream switch.

l           In RSTP, the upstream switch does not send an agreement packet to the downstream switch.

Figure 1-10 and Figure 1-11 show the designated port fast transition mechanisms in RSTP and MSTP.

Figure 1-10 Fast state transition of designated port in RSTP

Figure 1-11 Fast state transition of designated port in MSTP

There is a certain limit on fast transition cooperation of RSTP and MSTP. For example, the upstream switch runs RSTP and the downstream switch runs MSTP, and the downstream MSTP does not support the compatible RSTP mode. As the root port of the downstream MSTP-enabled switch cannot receive the agreement packet from the upstream switch, the downstream switch does not send the agreement packet to the upstream switch. As a result, the designated port of the upstream switch cannot implement fast transition. Instead, the state becomes forwarding state after double forward delay.

In practice, since private protocols related to STP are configured on the switch of another vendor and the designated port state transition mechanism is similar to that of RSTP, the designated ports cannot implement fast state transition when these switches serving as upstream switches interconnect with the S9500 series routing switch that runs MSTP.

To avoid such problem, S9500 series routing switches provide fast transition. When the S9500 series routing switch interconnects with the switches from another vendor, you can enable fast transition on the port of the S9500 switch that serves as the downstream switch. If this port is a root port, after receiving the proposal packet from the designated port of the upstream switch, the port sends the agreement packet to the upstream switch initiatively rather than sends the agreement packet after receiving the agreement packet. As a result, the upstream switch can perform fast state transition on the designated port.

II. Configuring fast state transition

1)         Configuration prerequisites

The S9500 series routing switch Switch A serves as the downstream switch and the switch from another vendor serves as the upstream switch. They have been connected correctly, as shown in Figure 1-12.

A private protocol related to STP is enabled on the upstream switch. The designated port state transition mechanism is similar to that of RSTP.

MSTP is enabled on Switch A.

Figure 1-12 Network diagram for fast state transition

2)         Configuration Tasks

l           Configuring fast transition in system view

Table 1-36 Configure fast transition in system view

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enable fast transition	stp interface interface-type interface-number no-agreement-check	Required By default, port fast transition is disabled.

 

l           Configuring fast transition in Ethernet port view

Table 1-37 Configure fast transition in Ethernet port view

To do...	Use the command...	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Enable fast transition	stp no-agreement-check	Required By default, port fast transition is disabled.

 

 

1.3  Displaying and Debugging MSTP

After the above configuration, execute the display command in any view to display the running of the MSTP configuration, and to verify the effect of the configuration. Execute the reset stp [ interface interface-list ] command in user view to clear the statistics of MSTP module. Execute the debugging command in user view to debug the MSTP module

Table 1-38 Display and debug MSTP

To do...	Use the command...
Display the MSTP information about the current switch	display stp
Display the configuration information about the current port and the switch	display stp [ instance instance-id ] [ interface interface-list | slot slot-num ] [ brief ]
Display the current configurations of the specified service board	display stp slot number [ brief ]
Display the configuration information about the region	display stp region-configuration
Display TC statistics	display stp [ instance instanceid ] tc { all | detected | received | sent }
Clear the MSTP statistics information	reset stp [ interface interface-list ]
Enable event debugging of MSTP for a specified port	debugging stp [ interface interface-list ] { lacp-key | packet | event }
Disable debugging of MSTP for a specified port	undo debugging stp [ interface interface-list ] { packet | event }
Enable event debugging of MSTP	debugging stp event
Disable event debugging of MSTP	undo debugging stp event
Enable packet debugging of MSTP	debugging stp packet
Disable packet debugging of MSTP	undo debugging stp packet
Enable global debugging	debugging stp all
Disable global debugging	undo debugging stp all
Enable instance debugging of MSTP	debugging stp instance instance-id
Disable instance debugging of MSTP	undo debugging stp instance instance-id
Enable STP global error or event debugging	debugging stp { global-error | global-event }
Disable STP global error or event debugging	undo debugging stp { global-error | global-event }
Enable MD5 summary debugging of Lacp protocol	debugging stp lacp-key
Disable MD5 summary debugging of Lacp protocol	undo debugging stp lacp-key
Enable debugging of the state machine	debugging stp state-machine
Disable debugging of the state machine	undo debugging stp state-machine
Enable debugging of the port information state machine	debugging stp state-machine pim
Disable debugging of the port information state machine	undo debugging stp state-machine pim
Enable debugging of the state machine for port role selection	debugging stp state-machine prs
Disable debugging of the state machine for port role selection	undo debugging stp state-machine prs
Enable debugging of the state machine for port role transition	debugging stp state-machine prt
Disable debugging of the state machine for port role transition	undo debugging stp state-machine prt
Enable debugging of the state machine for port state transition	debugging stp state-machine pst
Enable debugging of the state machine for port state transition	undo debugging stp state-machine pst
Enable debugging of the topology change state machine	debugging stp state-machine tcm
Disable debugging of the topology change state machine	undo debugging stp state-machine tcm
Enable debugging of the state machine for port protocol transition	debugging stp state-machine ppm
Disable debugging of the state machine for port protocol transition	undo debugging stp state-machine ppm
Enable debugging of the port transport state machine	debugging stp state-machine ptx
Disable debugging of the port transport state machine	undo debugging stp state-machine ptx
Enable debugging of the state machine for topology change protection	debugging stp state-machine tcpm
Disable debugging of the state machine for topology change protection	undo debugging stp state-machine tcpm

 

1.4  Typical MSTP Configuration Examples

1.4.1  Typical MSTP Configuration Example

I. Network requirements

MSTP provides different forwarding paths for packets of different VLANs. The configurations are as follows: all the switches in the network belong to the same MST region, packets of VLAN 10 travels along instance 1, packets of VLAN 30 travels along instance 3, packets of VLAN 40 travels along instance 4, and that of VLAN 20 travels along instance 0.

In the following network diagram, Switch A and Switch B are devices of the convergence layer, Switch C and Switch D are devices of the access layer. VLAN 10 and 30 function at the distribution and access layers, and VLAN 40 functions at the access layer only. So the root of instance 1 can be configured as Switch A, root of instance 3 can be Switch B, and root of instance 4 can be Switch C.

II. Network diagram

Figure 1-13 Network diagram for MSTP configuration

 

 

III. Configuration procedure

1)         Configurations on Switch A

# Configure the MST region.

<H3C> system-view

[H3C] stp region-configuration

[H3C-mst-region] region-name example

[H3C-mst-region] instance 1 vlan 10

[H3C-mst-region] instance 3 vlan 30

[H3C-mst-region] instance 4 vlan 40

[H3C-mst-region] revision-level 0

# Manually activate MST region configuration.

[H3C-mst-region] active region-configuration

# Specify Switch A as the root of instance 1.

[H3C] stp instance 1 root primary

2)         Configurations on Switch B

# Configure the MST region.

[H3C] stp region-configuration

[H3C-mst-region] region-name example

[H3C-mst-region] instance 1 vlan 10

[H3C-mst-region] instance 3 vlan 30

[H3C-mst-region] instance 4 vlan 40

[H3C-mst-region] revision-level 0

# Manually activate MST region configuration.

[H3C-mst-region] active region-configuration

# Specify Switch B as the root of instance 3.

[H3C] stp instance 3 root primary

3)         Configurations on Switch C

# Configure the MST region.

[H3C] stp region-configuration

[H3C-mst-region] region-name example

[H3C-mst-region] instance 1 vlan 10

[H3C-mst-region] instance 3 vlan 30

[H3C-mst-region] instance 4 vlan 40

[H3C-mst-region] revision-level 0

# Manually activate MST region configuration.

[H3C-mst-region] active region-configuration

# Specify Switch C as the root of instance 4.

[H3C] stp instance 4 root primary

4)         Configurations on Switch D

# Configure the MST region.

[H3C] stp region-configuration

[H3C-mst-region] region-name example

[H3C-mst-region] instance 1 vlan 10

[H3C-mst-region] instance 3 vlan 30

[H3C-mst-region] instance 4 vlan 40

[H3C-mst-region] revision-level 0

# Manually activate MST region configuration.

[H3C-mst-region] active region-configuration

1.4.2  MSTP Hybrid Networking Configuration Example

I. Network requirements

l           Four switches form a mesh network. Three are S9500 series routing switches supporting MSTP and the other is an RSTP-capable switch from another vendor.

l           The ports and VLANs are configured as shown in Figure 1-14.

l           Configure the four switches to perform overall STP calculation in the network.

II. Network diagram

Figure 1-14 Network diagram for MSTP hybrid networking

 

 

III. Configuration procedure

1)         Configurations on S9500 A

# Enable MSTP in system view.

<S9500A> system-view

[S9500A] stp enable

# Configure the VLAN-to-MSTI mapping and region name of the MSTP region. Activate the MSTP region configuration.

[S9500A] stp region-configuration

[S9500A-mst-region] instance 1 vlan 100

[S9500A-mst-region] instance 2 vlan 200

[S9500A-mst-region] region-name h3c

[S9500A-mst-region] active region-configuration

# Configure S9500 A as the root bridge of the instances.

[S9500A] stp instance 0 root primary

[S9500A] stp instance 1 root primary

[S9500A] stp instance 2 root primary

2)         Configurations on S9500 B

# Enable MSTP in system view.

<S9500B> system-view

[S9500B] stp enable

# Configure the VLAN-to-MSTI mapping and region name of the MSTP region. Activate the MSTP region configuration.

[S9500B] stp region-configuration

[S9500B-mst-region] instance 1 vlan 100

[S9500B-mst-region] instance 2 vlan 200

[S9500B-mst-region] region-name h3c

[S9500B-mst-region] active region-configuration

# Configure S9500 B as the secondary root bridge of the instances.

[S9500B] stp instance 0 root secondary

[S9500B] stp instance 1 root secondary

[S9500B] stp instance 2 root secondary

3)         Configurations on S9500 C

Enable STP globally. Perform the configuration on S9500 C similar to those on S9500 B. For configuration procedure, refer to section Configurations on S9500 B.

4)         Configurations on Switch

Enable RSTP globally. Disable STP on the port connecting to the PC terminal or configure the port as an edge port. The configuration procedure is omitted.