H3C S9500 Series User manual

Operation Manual – MSTP
H3C S9500 Series Routing Switches Table of Contents
i
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

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Chapter 1 MSTP Configuration
When configuring MSTP, go to these sections for the information you are interested in:
zIntroduction to MSTP
zMSTP Configuration Tasks
zDisplaying and Debugging MSTP
zTypical 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 STPand 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.

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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

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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 inA0, 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, MSTregion and STP region, or MSTregion 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 regionA0 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.

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zThe root port is the one through which the data are forwarded to the root.
zThe designated port is the one through which the data are forwarded to the
downstream network segment or switch.
zMaster port is the port connecting the entire region to the Common Root Bridge
and located on the shortest path between them.
zAn 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.
zIf 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 switchA, B, C, and D make up a MST region. Port 1 and 2
on switch Aconnects 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.
Connecting to the
common root bridge
Edge port
Port 1 Port 2
Master port Alternate port
Designated port Port 3 Port 4
Port 5
A
BC
D
Port 6
Backup port
MST region
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

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MSTP modules communicate with eachother 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:
7 6 5 4 3 2 1 0
TcProposalLearningForwardingAgreementTcAck
7 6 5 4 3 2 1 0
TcProposalLearningForwardingAgreementTcAck
Figure 1-5 Meaning of 1-byte Flags in a BPDU

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The second and third bits together indicate MSTP port role.
2) TC packet
ATC 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 CSTfor 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

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Classification Designated bridge Designated port
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
LAN
AP1 AP2
Switch A
Switch B Switch C
BP1
BP2
CP1
CP2
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.
zThe specific calculation process of STP algorithm.
The following example illustrates the calculation process of STP.
Figure 1-7 illustrates the practical network.
AP1 AP2
Switch A
With priority 0
Switch B
With priority 1
Switch C
With priority 2
BP1
BP2 CP1
CP2
510
4
Figure 1-7 Ethernet switch networking

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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 bridgeID (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 ofAP1: {0, 0, 0, AP1}
Configuration BPDU ofAP2: {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:
zThe configuration BPDU with a smaller root ID has a higher priority.
zIf 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.
zIf 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.

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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 ofAP1: {0, 0, 0, AP1}.
Configuration BPDU ofAP2: {0, 0, 0, AP2}.
Switch B:
BP1 receives the configuration BPDU from SwitchA 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 portBP1 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}.

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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 bridgeAis illustrated in the
Figure 1-8.
AP1
Switch A
With priority 0
Switch B
With priority 1
Switch C
With priority 2
BP1
BP2
CP2
5
4
Figure 1-8 The final stabilized spanning tree

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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.
zConfiguration 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.

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1.2 MSTP Configuration Tasks
MSTP configuration tasks include:
zConfiguring the MST Region for a Switch
zSpecifying the Switch as a Primary or a Secondary Root bridge
zConfiguring the MSTP Running Mode
zConfiguring the Bridge Priority for a Switch
zConfiguring the Max Hops in an MST Region
zConfiguring the Switched Network Diameter
zConfiguring the Time Parameters of a Switch
zSetting the Timeout Factor of a Specific Bridge
zConfiguring the Max Transmission Speed on a Port
zConfiguring a Port as an Edge Port or Non-edge Port
zConfiguring the Path Cost of a Port
zConfiguring the Priority of a Port
zConfiguring Whether a Port Is on a Point-to-Point Link
zConfiguring the mCheck Variable of a Port
zConfiguring the Switch Protection Functions
zEnabling/Disabling MSTP on the Device
zDisabling BPDU Packets from Flooding in the Default VLANs
zEnabling Single-Port Loop Detection
zConfiguring the Switch to Support Standard or Legacy MSTP Packet Format
zConfiguring Digest Snooping
zConfiguring 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.
Note:
When GVRP and MSTP start on the switch simultaneously, GVRP packets will
propagate along CIST which is a spanning tree instance. In this case, if you want to
issue a certain VLAN through GVRP on the network, you should make sure that the
VLAN is mapped to CIST when configuring the VLAN mapping table of MSTP.
CIST is spanning tree instance 0.

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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:

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

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To do... Use the command...
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.

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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.
Note:
You can configure the current switch as the root of several STIs. However, it is not
necessary to specify two or more roots for an STI. In other words, do not specify the
root for an STI on two or more switches.
You can configure more than one secondary root for a spanning tree throughspecifying
the secondary STI root on two or more switches.
Generally, you are recommended to designate one primary root and more than one
secondary root for a spanning tree.
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 portconnected to it will
automatically transit from MSTP mode to STP-compatible mode. But the port cannot

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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.
Caution:
In the process of spanning tree root election, of two or more switches with the same
Bridge priorities, the one has a smaller MAC address is elected as the root.
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.

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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 MSTregion, 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.

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Note:
The stp bridge-diameter command configures the switched network diameter and
determines the three MSTP time parameters(Hello Time, Forward Delay, and Max Age)
accordingly.
1.2.7 Configuring the Time Parameters of a Switch
The switch has three time parameters, Forward Delay, Hello Time, and MaxAge.
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