H3C S9500 Series User manual

Operation Manual – MSTP
H3C S9500 Series Routing Switches Table of Contents
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Table of Contents
Chapter 1 MSTP Configuration....................................................................................................1-1
1.1 MSTP Overview ................................................................................................................. 1-1
1.1.1 Introduction to STP ................................................................................................. 1-1
1.1.2 Introduction to MSTP ............................................................................................ 1-11
1.1.3 Protocols and Standards....................................................................................... 1-18
1.2 Configuration Task List .................................................................................................... 1-18
1.3 Configuring the Root Bridge ............................................................................................ 1-19
1.3.1 Configuring an MST Region.................................................................................. 1-19
1.3.2 Specifying the Root Bridge or a Secondary Root Bridge ...................................... 1-21
1.3.3 Configuring the Work Mode of MSTP Device ....................................................... 1-23
1.3.4 Configuring the Priority of the Current Device ...................................................... 1-23
1.3.5 Configuring the Maximum Hops of an MST Region.............................................. 1-24
1.3.6 Configuring the Network Diameter of a Switched Network................................... 1-25
1.3.7 Configuring Timers of MSTP................................................................................. 1-26
1.3.8 Configuring the Timeout Factor............................................................................. 1-28
1.3.9 Configuring the Maximum Transmission Rate of Ports......................................... 1-28
1.3.10 Configuring Ports as Edge Ports......................................................................... 1-29
1.3.11 Configuring Whether Ports Connect to Point-to-Point Links............................... 1-30
1.3.12 Configuring the Mode a Port Uses to Recognize/Send MSTP Packets ............. 1-31
1.3.13 Enabling the MSTP Feature................................................................................ 1-33
1.4 Configuring Leaf Nodes ................................................................................................... 1-34
1.4.1 Configuring an MST Region.................................................................................. 1-34
1.4.2 Configuring the Work Mode of MSTP ................................................................... 1-34
1.4.3 Configuring the Timeout Factor............................................................................. 1-34
1.4.4 Configuring the Maximum Transmission Rate of Ports......................................... 1-34
1.4.5 Configuring Ports as Edge Ports........................................................................... 1-34
1.4.6 Configuring Path Costs of Ports............................................................................ 1-34
1.4.7 Configuring Port Priority ........................................................................................ 1-37
1.4.8 Configuring Whether Ports Connect to Point-to-Point Links................................. 1-38
1.4.9 Configuring the Mode a Port Uses to Recognize/Send MSTP Packets ............... 1-38
1.4.10 Enabling the MSTP Feature................................................................................ 1-38
1.5 Performing mCheck ......................................................................................................... 1-38
1.6 Configuring the VLAN Ignore Feature ............................................................................. 1-39
1.6.1 Introduction to the VLAN Ignore Feature .............................................................. 1-39
1.6.2 Configuration Procedure ....................................................................................... 1-40
1.6.3 Configuration Examples ........................................................................................ 1-40
1.7 Configuring Digest Snooping ........................................................................................... 1-41
1.7.1 Configuration Prerequisites................................................................................... 1-41

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1.7.2 Configuration Procedure ....................................................................................... 1-41
1.7.3 Configuration Examples ........................................................................................ 1-43
1.8 Configuring No Agreement Check................................................................................... 1-43
1.8.1 Configuration Prerequisites................................................................................... 1-45
1.8.2 Configuration Procedure ....................................................................................... 1-45
1.8.3 Configuration Examples ........................................................................................ 1-45
1.9 Configuring Protection Functions..................................................................................... 1-46
1.9.1 Configuration Prerequisites................................................................................... 1-48
1.9.2 Enabling the BPDU Guard Function ..................................................................... 1-48
1.9.3 Enabling the Root Guard Function........................................................................ 1-49
1.9.4 Enabling the Loop Guard Function ....................................................................... 1-49
1.9.5 Enabling the TC-BPDU Attack Guard Function .................................................... 1-50
1.10 Displaying and Maintaining MSTP................................................................................. 1-51
1.11 MSTP Configuration Examples...................................................................................... 1-51

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Chapter 1 MSTP Configuration
When configuring MSTP, go to these sections for information you are interested in:
zMSTP Overview
zConfiguration Task List
zConfiguring the Root Bridge
zConfiguring Leaf Nodes
zPerforming mCheck
zConfiguring the VLAN Ignore Feature
zConfiguring Digest Snooping
zConfiguring No Agreement Check
zConfiguring Protection Functions
zDisplaying and Maintaining MSTP
zMSTP Configuration Examples
1.1 MSTP Overview
1.1.1 Introduction to STP
I. Functions of STP
The Spanning Tree Protocol (STP) was established based on the 802.1D standard of
IEEE to eliminate physical loops at the data link layer in a local area network (LAN).
Devices running this protocol detect loops in the network by exchanging information
with one another and eliminate loops by selectively blocking certain ports until the loop
structure is pruned into a loop-free network structure. This avoids proliferation and
infinite recycling of packets that would occur in a loop network and prevents
deterioration of device performance caused by duplicate packets received.
In the narrow sense, STP refers to the STP protocol defined in IEEE 802.1d; in the
broad sense, it refers to the STP protocol defined in IEEE 802.1d and various
enhanced spanning tree protocols derived from the STP protocol.
II. Protocol packets of STP
STP uses bridge protocol data units (BPDUs) for exchanging information. BPDU is also
known as configuration packets or configuration messages.
STP identifies the network topology by transmitting BPDUs between STP compliant
network devices. BPDUs contain sufficient information for the network devices to
complete the spanning tree computing.
In STP, BPDUs come in two types:

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zConfiguration BPDUs, used to calculate the spanning tree and maintain the
spanning tree topology.
zTopology change notification (TCN) BPDUs, used to notify concerned devices of
network topology changes, if any.
III. Basic concepts in STP
1) Root bridge
The root of a spanning tree is called the root bridge. In the entire network, there is only
one root bridge. Whichever bridge in the network can become the root bridge,
depending on the network topology.
Upon network convergence, the root bridge generates and sends out configuration
BPDUs at a certain interval and other devices just forward the BPDUs. This mechanism
ensures topological stability.
2) Root port
In an STP network, a root port is a port on a non-root bridge device. Among the ports on
an STP-enabled device, the root port has the lowest path cost to the root bridge.
The root port takes charge of communicating with the root bridge. A non-root-bridge
device has one and only one root port. The root bridge has no root port.
3) Designated bridge and designated port
Refer to the following table for the description of designated bridge and designated
port.
Table 1-1 Description of designated bridge and designated port
Classification Designated bridge Designated port
For a device
The device directly connected
with this device and responsible
for forwarding configuration
BPDUs
The port through which the
designated bridge forwards
configuration BPDUs to this
device
For a LAN The device responsible for forwarding configuration BPDUs
to this LAN segment
The port through which the
designated bridge forwards
configuration BPDUs to this
LAN segment

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LAN
AP1 AP2
Switch A
Switch B Switch C
BP1
BP2
CP1
CP2
Figure 1-1 A schematic diagram of designated bridges and designated ports
As shown in Figure 1-1, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on
Switch A, Switch B, and Switch C.
zIf Switch A forwards configuration BPDUs to Switch B through AP1, the
designated bridge for Switch B is Switch A, and the designated port is AP1 on
Switch A.
zTwo Switches are connected to the LAN: Switch B and Switch C. If Switch B
forwards configuration BPDUs to the LAN, the designated bridge for the LAN is
Switch B, and the designated port is BP2 on Switch B.
Note:
All the ports on the root bridge are designated ports.
4) Path cost
Path cost is a reference value used for link selection in STP. By calculating the path
cost, STP selects relatively “robust” links and blocks redundant links, and finally prunes
the network into a loop-free tree structure.
IV. How STP works
STP identifies the network topology by transmitting configuration BPDUs between
network devices.
Configuration BPDUs contain sufficient information for network devices to complete the
spanning tree computing. A configuration BPDU mainly contains the following
information:
zRoot bridge ID: formed by root bridge priority and MAC address
zRoot path cost
zDesignated bridge ID: formed by designated bridge priority and MAC address

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zDesignated port ID: formed by designated port priority and port name
zMessage age: age of the configuration BPDU while it is propagating in the network
zMax age: maximum time that a received configuration BPDU can be maintained
on the device
zHello time: interval to send configuration BPDUs
zForward delay: state transition delay of the port
1) A simplified STP computing model
Note:
For the convenience of description, the description and examples below involve only
four parts of a configuration BPDU:
zRoot bridge ID (in the form of device priority)
zRoot path cost
zDesignated bridge ID (in the form of device priority)
zDesignated port ID (in the form of port ID)
zInitial state
Upon initialization of a device, each port generates a configuration BPDU with itself as
the root, in which the root path cost is 0, designated bridge ID is the device ID, and the
designated port is the local port.
zSelection of the optimum configuration BPDU
Each device sends out its configuration BPDU and receives configuration BPDUs from
other devices.
The process of selecting the optimum configuration BPDU is as follows:
Table 1-2 Selection of the optimum configuration BPDU
Step Description
1
Upon receiving a configuration BPDU on a port, the device performs
the following processing:
zIf the received configuration BPDU has a lower priority than that
of the configuration BPDU generated by the port, the device will
discard the received configuration BPDU without doing any
processing on the configuration BPDU of this port.
zIf the received configuration BPDU has a higher priority than that
of the configuration BPDU generated by the port, the device will
replace the content of the configuration BPDU generated by the
port with the content of the received configuration BPDU.
2 The device compares the configuration BPDUs of all the ports and
chooses the optimum configuration BPDU.

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Note:
Rules for configuration BPDU comparison:
zThe configuration BPDU that has the lowest root bridge ID has the highest priority.
zFor configuration BPDUs with the same root bridge ID, they will be compared by
their root path costs. If the root path cost in a configuration BPDU plus the path cost
corresponding to this port is S, the configuration BPDU with the smallest S value
has the highest priority.
zFor configuration BPDUs with the same root bridge ID and the same root path cost,
they will be compared by their designated bridge IDs, then their designated port IDs,
and then the IDs of the ports through which they are received. The smaller the ID,
the higher the message priority.
zSelection of the root bridge
At network initialization, each STP-compliant device on the network assumes itself to
be the root bridge, with the root bridge ID being its own device ID. By exchanging
configuration BPDUs, the devices compare one another’s root bridge ID. The device
with the smallest root bridge ID is elected as the root bridge.
zSelection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 1-3 Selection of the root port and designated ports
Step Description
1 The root port is the port through which the optimum configuration BPDU
was received.
2
Based on the configuration BPDU and the path cost of the root port, the
device generates a designated port configuration BPDU for each of the
rest ports as follows:
zUsing the root bridge ID of the configuration BPDU of the root port as
the root bridge ID.
zUsing the sum of the root path cost of the configuration BPDU of the
root port and the path cost corresponding to the root port as the root
path cost.
zUsing the local device ID as the designated bridge ID.
zUsing the local port ID as the designated port ID.

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Step Description
3
The device compares the calculated configuration BPDU with the
configuration BPDU on the port of which the port role is to be defined,
and does the following according to the comparison result:
zIf the calculated configuration BPDU is superior, the device considers
this port as the designated port, and replaces the original
configuration BPDU with the calculated configuration BPDU. The new
configuration BPDU will be sent out periodically.
zIf the configuration BPDU on the port is superior, the device blocks
this port without updating its configuration BPDU. As a result, the port
can receive BPDUs, but cannot send BPDUs or forward data.
Note:
When the network topology is stable, only the root port and designated ports forward
traffic, while other ports are all in the blocked state – they only receive STP packets but
do not forward user traffic.
Once the root bridge, the root port on each non-root bridge and designated ports have
been successfully elected, the entire tree-shaped topology has been constructed.
The following is an example of how the STP algorithm works. The specific network
diagram is shown in Figure 1-2, where the priority of Switch A is 0, the priority of Switch
B is 1, the priority of Switch C is 2, and the path costs of the links are 5, 10 and 4.
Figure 1-2 Network diagram for STP algorithm
zInitial state of each device
The following table shows the initial state of each device.

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Table 1-4 Initial state of each device
Device Port ID BPDU of the port
AP1 {0, 0, 0, AP1}
Switch A
AP2 {0, 0, 0, AP2}
BP1 {1, 0, 1, BP1}
Switch B
BP2 {1, 0, 1, BP2}
CP1 {2, 0, 2, CP1}
Switch C
CP2 {2, 0, 2, CP2}
zComparison process and result on each device
The following table shows the comparison process and result on each device.
Table 1-5 Comparison process and result on each device
Device Comparison process BPDU of the port
after comparison
Switch
A
zPort AP1 receives a configuration BPDU from
Switch B (that is, {1, 0, 1, BP1}). As the
configuration BPDU of the local port (that is, {0, 0,
0, AP1}) is superior to the received configuration
BPDU, the received configuration BPDU is
discarded.
zPort AP2 receives a configuration BPDU from
Switch C (that is, {2, 0, 2, CP1}). As the BPDU of
the local port (that is, {0, 0, 0, AP2}) is superior to
the received configuration BPDU, the received
configuration BPDU is discarded.
zSwitch A finds that both the root bridge and
designated bridge in the configuration BPDUs of
all its ports are Switch A itself, so it assumes itself
to be the root bridge. In this case, it does not
make any change to the configuration BPDU of
each port, and starts sending out configuration
BPDUs periodically.
AP1: {0, 0, 0, AP1}
AP2: {0, 0, 0, AP2}

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Device Comparison process BPDU of the port
after comparison
zPort BP1 receives a configuration BPDU from
Switch A (that is, {0, 0, 0, AP1}). As the received
configuration BPDU is superior to that of the local
port (that is, {1, 0, 1, BP1}), Switch B uses the
received configuration BPDU as the
configuration BPDU of BP1.
zPort BP2 receives a configuration BPDU from
Switch C (that is, {2, 0, 2, CP2}). As the
configuration BPDU of the local port (that is, {1, 0,
1, BP2}) is superior to the received configuration
BPDU, Switch B discards the received
configuration BPDU.
BP1: {0, 0, 0, AP1}
BP2: {1, 0, 1, BP2}
Switch
B
zSwitch B compares the configuration BPDUs of
all its ports, and determines that the configuration
BPDU of BP1 is the optimum one. So, BP1 acts
as the root port, the configuration BPDUs of
which remains unchanged.
zBased on the configuration BPDU of BP1 and the
path cost of the root port (5), Switch B generates
a designated port configuration BPDU for BP2
(that is, {0, 5, 1, BP2}).
zSwitch B compares the generated configuration
BPDU (that is, {0, 5, 1, BP2}) with the
configuration BPDU of BP2. As the former is
superior, BP2 acts as a designated port, and
Switch B sends the generated configuration
BPDU through BP2 periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated port
BP2:
{0, 5, 1, BP2}

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Device Comparison process BPDU of the port
after comparison
zPort CP1 receives a configuration BPDU from
Switch A (that is, {0, 0, 0, AP2}). As the received
configuration BPDU is superior to that of the local
port (that is, {2, 0, 2, CP1}), Switch C uses the
received configuration BPDU as the
configuration BPDU of CP1.
zPort CP2 receives a configuration BPDU from
Switch B (that is, {1, 0, 1, BP2}) before the
configuration BPDU is updated on BP2. As the
received configuration BPDU is superior to that of
the local port (that is, {2, 0, 2, CP2}), Switch C
uses the received configuration BPDU as the
configuration BPDU of CP2.
CP1: {0, 0, 0, AP2}
CP2: {1, 0, 1, BP2}
By comparison:
zThe configuration BPDUs of CP1 is the optimum
configuration BPDU, so CP1 acts as the root
port, the configuration BPDUs of which remains
unchanged.
zSwitch C generates a designated port
configuration BPDU (that is, {0, 10, 2, CP2}) and
compare it with the configuration BPDU of CP2.
As the former is superior, CP2 acts as a
designated port and Switch C sends the
generated configuration BPDU through CP2
periodically.
Root port CP1:
{0, 0, 0, AP2}
Designated port
CP2:
{0, 10, 2, CP2}
zNext, port CP2 receives the updated
configuration BPDU of Switch B (that is, {0, 5, 1,
BP2}). As the received configuration BPDU is
superior to the local one, Switch C launches a
BPDU update process.
zAt the same time, port CP1 receives
configuration BPDUs periodically from Switch A.
Switch C does not launch an update process
after comparison.
CP1: {0, 0, 0, AP2}
CP2: {0, 5, 1, BP2}
Switch
C
By comparison:
zBecause the root path cost of CP2 ( which is 9) is
smaller than the root path cost of CP1 (which is
10), the configuration BPDU of CP2 is the
optimum BPDU, and CP2 acts as the root port,
the configuration BPDUs of which remains
unchanged.
zAfter the comparison between the configuration
BPDU of CP1 and the generated designated port
configuration BPDU, port CP1 is blocked, with
the configuration BPDU of the port remaining
unchanged, and the port will not receive data
from Switch A until a spanning tree computing
process is triggered by a new condition, for
example, the link between Switch B and Switch C
becomes down.
Blocked port CP1:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}

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After the comparison processes described in the table above, a spanning tree with
Switch A as the root bridge is stabilized, as shown in Figure 1-3.
Figure 1-3 A spanning tree with Switch A as the root bridge
Note:
To facilitate description, the spanning tree computing process in this example is
simplified, while the actual process is more complicated.
2) The BPDU forwarding mechanism in STP
zUpon network initiation, every switch regards itself as the root bridge, generates
configuration BPDUs with itself as the root, and sends the configuration BPDUs at
a regular interval of hello time.
zIf it is the root port that received the configuration BPDU and the received
configuration BPDU is superior to the configuration BPDU of the port, the device
will increase message age carried in the configuration BPDU by a certain rule and
start a timer to time the configuration BPDU while it sends out this configuration
BPDU through the designated port.
zIf the configuration BPDU received on the designated port is inferior to the
configuration BPDU of the local port, the port will immediately send out its superior
configuration BPDU in response.
zIf a path becomes faulty, the root port on this path will no longer receive new
configuration BPDUs and the old configuration BPDUs will be discarded due to
timeout. In this case, the device will generate a configuration BPDU with itself as
the root and send out the BPDU. This triggers a new spanning tree computing
process so that a new path is established to restore the network connectivity.

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However, the newly computed configuration BPDU will not be propagated throughout
the network immediately, so the old root ports and designated ports that have not
detected the topology change continue forwarding data through the old path. If the new
root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur.
3) STP timers
STP calculation involves three timing parameters: forward delay, hello time, and max
age.
zForward delay is the delay for port state transition. A path failure can cause
re-calculation of the spanning tree, and hence topology change. As it takes time
for the new configuration BPDU to propagate throughout the network, forwarding
data on the root port or designated port immediately upon its selection can cause a
temporary loop. To avoid this, STP defines that a newly elected root port or
designated port must wait twice the forward delay time before it begins to forward
data. The delay ensures that the new configuration BPDU has been propagated
throughout the network.
zHello time is the time interval at which a device sends hello packets to the adjacent
devices to ensure that the paths are fault-free.
zMax age determines how long a configuration BPDU can be held by the device.
The device discards a configuration BPDU once its age reaches the max age.
1.1.2 Introduction to MSTP
I. Why MSTP
1) Disadvantages of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or
designated port must wait twice the forward delay time before transitioning to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port.
The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows a
newly elected root port or designated port to enter the forwarding state much quicker
under certain conditions than in STP. As a result, it takes a shorter time for the network
to reach the final topology stability.

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Note:
zIn RSTP, a newly elected root port can enter the forwarding state rapidly if this
condition is met: The old root port on the device has stopped forwarding data and
the upstream designated port has started forwarding data.
zIn RSTP, a newly elected designated port can enter the forwarding state rapidly if
this condition is met: The designated port is an edge port (a port is an edge port if it
is not connected to the other devices directly or indirectly) or a port connected with a
point-to-point link. If the designated port is an edge port, it can enter the forwarding
state directly; if the designated port is connected with a point-to-point link, it can
enter the forwarding state immediately after the device undergoes handshake with
the downstream device and gets a response.
Although RSTP support rapid network convergence, it has the same drawback as STP
does: All bridges within a LAN share the same spanning tree, so redundant links cannot
be blocked based on VLANs, and the packets of all VLANs are forwarded along the
same spanning tree.
2) Features of MSTP
The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP and
RSTP. In addition to support for rapid network convergence, it also allows data flows of
different VLANs to be forwarded along their own paths, thus providing a better load
sharing mechanism for redundant links. For description about VLANs, refer to VLAN
Configuration in the Access Volume.
MSTP features the following:
zMSTP maintains a VLAN-to-MSTI mapping table that maps VLANs to MST
instances (MSTIs). MSTP can save communication overheads and resource
usage by mapping multiple VLANs to one MSTI.
zMSTP divides a switched network into multiple regions, each containing multiple
spanning trees that are independent of one another.
zMSTP prunes loop networks into a loop-free tree, thus avoiding proliferation and
endless recycling of packets in a loop network. In addition, it provides multiple
redundant paths for data forwarding, thus supporting load balancing of VLAN data.
zMSTP is compatible with STP and RSTP.
II. Basic concepts in MSTP
As shown in Figure 1-4, there are four multiple spanning tree (MST) regions, each
made up of four switches running MSTP. The following explains some basic MSTP
concepts based on this figure.

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Figure 1-4 Basic concepts in MSTP
1) MST region
An MST region is composed of multiple devices in a switched network and network
segments among them. These devices have the following characteristics:
zAll are MSTP-enabled.
zThey have the same region name.
zThey have the same VLAN-to-MSTI mapping configuration.
zThey have the same MSTP revision level configuration.
zThey are physically linked with one another.
In area A0 in Figure 1-4, for example, all devices have the same MST region
configuration:
zThe same region name.
zThe same VLAN-to-MSTI mapping (VLAN 1 is mapped to MSTI 1, VLAN 2 to
MSTI 2, and the rest to the common and internal spanning tree (CIST), that is,
MSTI 0).
zThe same MSTP revision level (not shown in the figure).

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Note:
Multiple MST regions can exist in a switched network. You can use an MSTP command
to group multiple devices to the same MST region.
2) VLAN-to-MSTI mapping table
As an attribute of an MST region, the VLAN-to-MSTI mapping table describes the
mapping relationships between VLANs and MSTIs. In Figure 1-4, for example, the
VLAN-to-MSTI mapping table of region A0 describes that the same region name, the
same VLAN-to-MSTI mapping (VLAN 1 is mapped to MSTI 1, VLAN 2 to MSTI 2, and
the rest to CIST.
3) IST
Internal spanning tree (IST) is a spanning tree that runs in an MST region. ISTs in all
MST regions and the common spanning tree (CST) jointly constitute the common and
internal spanning tree (CIST) of the entire network. An IST is a section of the CIST in an
MST region. In Figure 1-4, for example, the CIST has a section in each MST region,
and this section is the IST in each MST region.
4) CST
The CST is a single spanning tree that connects all MST regions in a switched network.
If you regard each MST region as a “device”, the CST is a spanning tree computed by
these devices through STP or RSTP. The red lines in Figure 1-4 describe the CST.
5) CIST
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that
connects all devices in a switched network. In Figure 1-4, for example, the ISTs in all
MST regions plus the inter-MST-region CST constitute the CIST of the entire network.
6) MSTI
Multiple spanning trees can be generated in an MST region through MSTP, one
spanning tree being independent of another. Each spanning tree is referred to as a
multiple spanning tree instance (MSTI). In Figure 1-4, for example, multiple spanning
tree can exist in each MST region, each spanning tree corresponding to a VLAN. These
spanning trees are called MSTIs.
7) Regional root bridge
The root bridge of the IST or an MSTI within an MST region is the regional root bridge of
the MST or that MSTI. Based on the topology, different spanning trees in an MST region
may have different regional roots. For example, in region D0 in Figure 1-4, the regional
root of instance 1 is device B, while that of instance 2 is device C.
8) Common root bridge
The common root bridge is the root bridge of the CIST.

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In Figure 1-4, for example, the common root bridge is a device in region A0.
9) Boundary port
A boundary port is a port that connects an MST region to another MST configuration, or
to a single spanning-tree region running STP, or to a single spanning-tree region
running RSTP.
During MSTP computing, a boundary port assumes the same role on the CIST and on
MSTIs. Namely, if a boundary port is master port on the CIST, it is also the master port
on all MSTIs within this region. In Figure 1-4, for example, if a device in region A0 is
interconnected with the first port of a device in region D0 and the common root bridge of
the entire switched network is located in region A0, the first port of that device in region
D0 is the boundary port of region D0.
Note:
Currently, the S9500 series routing switches are not capable of recognizing boundary
ports. When an S9500 series routing switch is connected to a third party’s device that
supports boundary port recognition, the third party’s device may malfunction in
recognizing a boundary port.
10) Roles of ports
MSTP calculation involves these port roles: root port, designated port, master port,
alternate port, and backup port.
zRoot port: a port responsible for forwarding data to the root bridge.
zDesignated port: a port responsible for forwarding data to the downstream network
segment or device.
zMaster port: A port on the shortest path from the entire region to the common root
bridge, connecting the MST region to the common root bridge.
zAlternate port: The standby port for the root port or master port. When the root port
or master port is blocked, the alternate port becomes the new root port or master
port.
zBackup port: The backup port of designated ports. When a designated port is
blocked, the backup port becomes the new designated port and starts forwarding
data without delay. When a loop occurs while two ports of the same MSTP device
are interconnected, the device blocks either of the two ports, whichever is the
backup port.
A port can assume different roles in different MSTIs.

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Figure 1-5 Port roles
In Figure 1-5,
zDevices A, B, C, and D constitute an MST region.
zPort 1 and port 2 of device A connect to the common root bridge.
zPort 5 and port 6 of device C form a loop.
zPort 3 and port 4 of device D connect downstream to other MST regions.
11) Port states
In MSTP, port states fall into the following tree:
zForwarding: the port learns MAC addresses and forwards user traffic;
zLearning: the port learns MAC addresses but does not forwards user traffic;
zDiscarding: the port neither learns MAC addresses nor forwards user traffic.
Note:
When in different MSTIs, a port can be in different states.
A port state is not exclusively associated with a port role. Table 1-6 lists the port state(s)
supported by each port role (“√” indicates that the port supports this state, while
“-“ indicates that the port does not support this state).
Table 1-6 Ports states supported by different port roles
Role
State
Root
port/Master
port
Designated
port Alternate port Backup port
Forwarding √√— —
Learning √√— —

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Role
State
Root
port/Master
port
Designated
port Alternate port Backup port
Discarding √√√√
III. How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are
interconnected by a computed CST. Inside an MST region, multiple spanning trees are
generated through computing, each spanning tree called an MSTI. Among these
MSTIs, instance 0 is the IST, while all the others are MSTIs. Similar to STP, MSTP uses
configuration BPDUs to compute spanning trees. The only difference between the two
protocols being in that what is carried in an MSTP BPDU is the MSTP configuration on
the device from which this BPDU is sent.
1) CIST calculation
By comparison of configuration BPDUs, one device with the highest priority is elected
as the root bridge of the CIST. MSTP generates an IST within each MST region through
computing, and, at the same time, MSTP regards each MST region as a single device
and generates a CST among these MST regions through computing. The CST and
ISTs constitute the CIST of the entire network.
2) MSTI calculation
Within an MST region, MSTP generates different MSTIs for different VLANs based on
the VLAN-to-MSTI mappings.
MSTP performs a separate computing process, which is similar to spanning tree
computing in STP, for each spanning tree. For details, refer to How STP works in
section 1.1.1.
In MSTP, a VLAN packet is forwarded along the following paths:
zWithin an MST region, the packet is forwarded along the corresponding MSTI.
zBetween two MST regions, the packet is forwarded along the CST.
IV. Implementation of MSTP on devices
MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be
recognized by devices running MSTP and used for spanning tree computing.
In addition to basic MSTP functions, many management-facilitating special functions
are provided, as follows:
zRoot bridge hold
zRoot bridge backup
zRoot guard
zBPDU guard
zLoop guard

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zTC-BPDU guard
zSupport for hot swapping of interface cards and active/standby changeover.
1.1.3 Protocols and Standards
MSTP is documented in:
zIEEE 802.1D: Spanning Tree Protocol
zIEEE 802.1w: Rapid Spanning Tree Protocol
zIEEE 802.1s: Multiple Spanning Tree Protocol
1.2 Configuration Task List
Before configuration, you need to know the position of each device in each MSTI: root
bridge or leave node. In each instance, one, and only one device acts as the root bridge,
while all others as leaf nodes.
Task Remarks
Configuring an MST Region Required
Specifying the Root Bridge or a Secondary Root Bridge Optional
Configuring the Work Mode of MSTP Device Optional
Configuring the Priority of the Current Device Optional
Configuring the Maximum Hops of an MST Region Optional
Configuring the Network Diameter of a Switched
Network Optional
Configuring Timers of MSTP Optional
Configuring the Timeout Factor Optional
Configuring the Maximum Transmission Rate of Ports Optional
Configuring Ports as Edge Ports Optional
Configuring Whether Ports Connect to Point-to-Point
Links Optional
Configuring the Mode a Port Uses to Recognize/Send
MSTP Packets Optional
Configuring
the Root
Bridge
Enabling the MSTP Feature Required