H3C S5500-SI Series User manual

Operation Manual – MSTP
H3C S5500-SI Series Ethernet 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 ............................................................................................ 1-20
1.3.1 Configuring an MST Region.................................................................................. 1-20
1.3.2 Specifying the Root Bridge or a Secondary Root Bridge ...................................... 1-22
1.3.3 Configuring the Work Mode of MSTP Device ....................................................... 1-23
1.3.4 Configuring the Priority of the Current Device ...................................................... 1-24
1.3.5 Configuring the Maximum Hops of an MST Region.............................................. 1-25
1.3.6 Configuring the Network Diameter of a Switched Network................................... 1-26
1.3.7 Configuring Timers of MSTP................................................................................. 1-27
1.3.8 Configuring the Timeout Factor............................................................................. 1-29
1.3.9 Configuring the Maximum Transmission Rate of Ports......................................... 1-29
1.3.10 Configuring Ports as Edge Ports......................................................................... 1-30
1.3.11 Configuring Whether Ports Connect to Point-to-Point Links............................... 1-31
1.3.12 Configuring the Mode a Port Uses to Recognize/Send MSTP Packets ............. 1-33
1.3.13 Enabling the Output of Port State Transition Information ................................... 1-34
1.3.14 Enabling the MSTP Feature................................................................................ 1-34
1.4 Configuring Leaf Nodes ................................................................................................... 1-35
1.4.1 Configuring an MST Region.................................................................................. 1-35
1.4.2 Configuring the Work Mode of MSTP ................................................................... 1-35
1.4.3 Configuring the Timeout Factor............................................................................. 1-35
1.4.4 Configuring the Maximum Transmission Rate of Ports......................................... 1-35
1.4.5 Configuring Ports as Edge Ports........................................................................... 1-36
1.4.6 Configuring Path Costs of Ports............................................................................ 1-36
1.4.7 Configuring Port Priority ........................................................................................ 1-38
1.4.8 Configuring Whether Ports Connect to Point-to-Point Links................................. 1-39
1.4.9 Configuring the Mode a Port Uses to Recognize/Send MSTP Packets ............... 1-39
1.4.10 Enabling Output of Port State Transition Information ......................................... 1-39
1.4.11 Enabling the MSTP Feature................................................................................ 1-39
1.5 Performing mCheck ......................................................................................................... 1-40
1.5.1 Configuration Prerequisites................................................................................... 1-40
1.5.2 Configuration Procedure ....................................................................................... 1-40
1.5.3 Configuration Example.......................................................................................... 1-41
1.6 Configuring Digest Snooping ........................................................................................... 1-41

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1.6.1 Configuration Prerequisites................................................................................... 1-41
1.6.2 Configuration Procedure ....................................................................................... 1-42
1.6.3 Configuration Example.......................................................................................... 1-43
1.7 Configuring No Agreement Check................................................................................... 1-44
1.7.1 Prerequisites ......................................................................................................... 1-45
1.7.2 Configuration Procedure ....................................................................................... 1-45
1.7.3 Configuration Example.......................................................................................... 1-46
1.8 Configuring Protection Functions..................................................................................... 1-46
1.8.1 Configuration prerequisites ................................................................................... 1-47
1.8.2 Enabling BPDU Guard .......................................................................................... 1-47
1.8.3 Enabling Root Guard............................................................................................. 1-48
1.8.4 Enabling Loop Guard ............................................................................................ 1-49
1.8.5 Enabling TC-BPDU Attack Guard ......................................................................... 1-50
1.9 Displaying and Maintaining MSTP................................................................................... 1-51
1.10 MSTP Configuration Example ....................................................................................... 1-51

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Chapter 1 MSTP Configuration
When configuring MSTP, go to these sections for information you are interested in:
zMSTP Overview
zConfiguring the Root Bridge
zConfiguring Leaf Nodes
zPerforming mCheck
zConfiguring Protection Functions
zDisplaying and Maintaining MSTP
1.1 MSTP Overview
1.1.1 Introduction to STP
I. Why STP?
The Spanning Tree Protocol (STP) was established based on the 802.1d standard of
IEEE to eliminate physical loops at the data link layer in a local area network (LAN).
Devices running this protocol detect loops in the network by exchanging information
with one another and eliminate loops by selectively blocking certain ports until the loop
structure is pruned into a loop-free network structure. This avoids proliferation and
infinite recycling of packets that would occur in a loop network and prevents
deterioration of the packet processing capability of network devices caused by
duplicate packets received.
In the narrow sense, STP refers to the STP protocol defined in IEEE 802.1d; in the
broad sense, it refers to the STP protocol defined in IEEE 802.1d and various
enhanced spanning tree protocols derived from the STP protocol.
II. Protocol Packets of STP
STP uses bridge protocol data units (BPDUs), also known as configuration messages,
as its protocol packets.
STP identifies the network topology by transmitting BPDUs between STP-compliant
network devices. BPDUs contain sufficient information for the network devices to
complete the spanning tree calculation.
In STP, BPDUs come in two types:
zConfiguration BPDUs, used for calculating spanning trees and maintaining the
spanning tree topology.
zTopology change notification (TCN) BPDUs, used for notifying concerned devices
of network topology changes, if any.

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III. Basic concepts in STP
1) Root bridge
A tree network must have a root; hence the concept of “root bridge” has been
introduced in STP.
There is one and only one root bridge in the entire network, and the root bridge can
change alone with changes of the network topology. Therefore, the root bridge is not
fixed.
Upon network convergence, the root bridge generates and sends out configuration
BPDUs at a certain interval, and other devices just forward the BPDUs. This
mechanism ensures topological stability.
2) Root port
On a non-root bridge device, the root port is the port nearest to the root bridge. The root
port is responsible for communication with the root bridge. A non-root-bridge device
has one and only one root port. The root bridge has no root port.
3) Designated bridge and designated port
The following table describes a designated bridge and a designated port.
Table 1-1 Description of designated bridge and designated port
Classification Designated bridge Designated port
For a device
The device directly
connected with this device
and responsible for
forwarding BPDUs
The port through which the
designated bridge
forwards BPDUs to this
device
For a LAN The device responsible for forwarding BPDUs to this
LAN segment
The port through which the
designated bridge
forwards BPDUs to this
LAN segment
Figure 1-1 shows designated bridges and designated ports. In the figure, AP1 and AP2,
BP1 and BP2, and CP1 and CP2 are ports on Device A, Device B, and Device C
respectively.
zIf Device A forwards BPDUs to Device B through AP1, the designated bridge for
Device B is Device A, and the designated port is the port AP1 on Device A.
zTwo devices are connected to the LAN: Device B and Device C. If Device B
forwards BPDUs to the LAN, the designated bridge for the LAN is Device B, and
the designated port is the port BP2 on Device B.

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LAN
Device A
Device B Device C
AP 2
BP 1
BP 2
CP 1
CP 2
AP 1
Figure 1-1 A schematic diagram of designated bridges and designated ports
IV. Path cost
Path cost is a reference value used for link selection in STP. By calculating the path
cost, STP selects relatively “robust” links and blocks redundant links, and finally prunes
the network into loop-free tree structure.
Note:
All the ports on the root bridge are designated ports.
V. How STP works
STP identifies the network topology by transmitting configuration BPDUs between
network devices. Configuration BPDUs contain sufficient information for network
devices to complete the spanning tree calculation. Important fields in a configuration
BPDU include:
zRoot bridge ID: consisting of root bridge priority and MAC address.
zRoot path cost: the cost of the shortest path to the root bridge.
zDesignated bridge ID: designated bridge priority plus MAC address.
zDesignated port ID, designated port priority plus port name.
zMessage age: age of the configuration BPDU while it propagates in the network.
zMax age: maximum age of the configuration BPDU maintained in the device.
zHello time: configuration BPDU interval.
zForward delay: forward delay of the port.

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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 name)
1) Specific calculation process of the STP algorithm
zInitial state
Upon initialization of a device, each port generates a BPDU with itself as the root bridge,
in which the root path cost is 0, designated bridge ID is the device ID, and the
designated port is the local port.
zSelection of the optimum configuration BPDU
Each device sends out its configuration BPDUs and receives configuration BPDUs
from other devices.
The process of selecting the optimum configuration BPDU is as follows:
Table 1-2 Selection of the optimum configuration BPDU
Step Description
1
Upon receiving a configuration BPDU on a port, the device performs the
following processing:
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:
Principle for configuration BPDU comparison:
zThe configuration BPDU that has the lowest root bridge ID has the highest priority.
zIf all the configuration BPDUs have the same root bridge ID, they will be compared
for their root path costs. If the root path cost in a configuration BPDU plus the path
cost corresponding to this port is S, the configuration BPDU with the smallest S
value has the highest priority.
zIf all configuration BPDUs have the same root path cost, they will be compared for
their designated bridge IDs, then their designated port IDs, and then the IDs of the
ports on which they are received. The smaller the ID, the higher message priority.
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 A non-root-ridge device regards the port on which it received the
optimum configuration BPDU as the root port.
2
Based on the configuration BPDU and the path cost of the root port, the
device calculates a designated port configuration BPDU for each of the
rest ports.
zThe root bridge ID is replaced with that of the configuration BPDU of
the root port.
zThe root path cost is replaced with that of the configuration BPDU of
the root port plus the path cost corresponding to the root port.
zThe designated bridge ID is replaced with the ID of this device.
zThe designated port ID is replaced with the ID of this port.

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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 different things according to the comparison result:
zIf the calculated configuration BPDU is superior, the device will
consider this port as the designated port, and the configuration
BPDU on the port will be replaced with the calculated configuration
BPDU, which will be sent out periodically.
zIf the configuration BPDU on the port is superior, the device will block
this port without updating its configuration BPDU, so that the port will
only receive BPDUs, but not send any, and will not forward data.
Note:
When the network topology is stable, only the root port and designated ports forward
traffic, while other ports are all in the blocked state – they only receive STP packets but
do not forward user traffic.
Once the root bridge, the root port on each non-root bridge and designated ports have
been successfully elected, the entire tree-shaped topology has been constructed.
The following is an example of how the STP algorithm works. The specific network
diagram is shown in Figure 1-2. In the feature, the priority of Device A is 0, the priority of
Device B is 1, the priority of Device C is 2, and the path costs of these links are 5, 10
and 4 respectively.
Figure 1-2 Network diagram for the STP algorithm
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 name BPDU of port
AP1 {0, 0, 0, AP1}
Device A
AP2 {0, 0, 0, AP2}
BP1 {1, 0, 1, BP1}
Device B
BP2 {1, 0, 1, BP2}
CP1 {2, 0, 2, CP1}
Device 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 port after
comparison
Device
A
zPort AP1 receives the configuration BPDU of Device
B {1, 0, 1, BP1}. Device A finds that the configuration
BPDU of the local port {0, 0, 0, AP1} is superior to
the configuration received message, and discards
the received configuration BPDU.
zPort AP2 receives the configuration BPDU of Device
C {2, 0, 2, CP1}. Device A finds that the BPDU of the
local port {0, 0, 0, AP2} is superior to the received
configuration BPDU, and discards the received
configuration BPDU.
zDevice A finds that both the root bridge and
designated bridge in the configuration BPDUs of all
its ports are Device A itself, so it assumes itself to be
the root bridge. In this case, it does not make any
change to the configuration BPDU of each port, and
starts sending out configuration BPDUs periodically.
AP1: {0, 0, 0,
AP1}
AP2: {0, 0, 0,
AP2}

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Device Comparison process BPDU of port
after
comparison
zPort BP1 receives the configuration BPDU of Device
A {0, 0, 0, AP1}. Device B finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {1, 0, 1, BP1}, and updates
the configuration BPDU of BP1.
zPort BP2 receives the configuration BPDU of Device
C {2, 0, 2, CP2}. Device B finds that the configuration
BPDU of the local port {1, 0, 1, BP2} is superior to
the received configuration BPDU, and discards the
received configuration BPDU.
BP1: {0, 0, 0,
AP1}
BP2: {1, 0, 1,
BP2}
Device
B
zDevice B compares the configuration BPDUs of all
its ports, and determines that the configuration
BPDU of BP1 is the optimum configuration BPDU.
Then, it uses BP1 as the root port, the configuration
BPDUs of which will not be changed.
zBased on the configuration BPDU of BP1 and the
path cost of the root port (5), Device B calculates a
designated port configuration BPDU for BP2 {0, 5, 1,
BP2}.
zDevice B compares the calculated configuration
BPDU {0, 5, 1, BP2} with the configuration BPDU of
BP2. If the calculated BPDU is superior, BP2 will act
as the designated port, and the configuration BPDU
on this port will be replaced with the calculated
configuration BPDU, which will be sent out
periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated
port BP2:
{0, 5, 1, BP2}

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Device Comparison process BPDU of port
after
comparison
zPort CP1 receives the configuration BPDU of Device
A {0, 0, 0, AP2}. Device C finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {2, 0, 2, CP1}, and updates
the configuration BPDU of CP1.
zPort CP2 receives the configuration BPDU of port
BP2 of Device B {1, 0, 1, BP2} before the message
was updated. Device C finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {2, 0, 2, CP2}, and updates
the configuration BPDU of CP2.
CP1: {0, 0, 0,
AP2}
CP2: {1, 0, 1,
BP2}
By comparison:
zThe configuration BPDU of CP1 is elected as the
optimum configuration BPDU, so CP1 is identified as
the root port, the configuration BPDUs of which will
not be changed.
zDevice C compares the calculated designated port
configuration BPDU {0, 10, 2, CP2} with the
configuration BPDU of CP2, and CP2 becomes the
designated port, and the configuration BPDU of this
port will be replaced with the calculated
configuration BPDU.
Root port CP1:
{0, 0, 0, AP2}
Designated
port CP2:
{0, 10, 2, CP2}
zNext, port CP2 receives the updated configuration
BPDU of Device B {0, 5, 1, BP2}. Because the
received configuration BPDU is superior to its old
one, Device C launches a BPDU update process.
zAt the same time, port CP1 receives configuration
BPDUs periodically from Device A. Device C does
not launch an update process after comparison.
CP1: {0, 0, 0,
AP2}
CP2: {0, 5, 1,
BP2}
Device
C
By comparison:
zBecause the root path cost of CP2 (9) (root path cost
of the BPDU (5) plus path cost corresponding to CP2
(4)) is smaller than the root path cost of CP1 (10)
(root path cost of the BPDU (0) + path cost
corresponding to CP2 (10)), the BPDU of CP2 is
elected as the optimum BPDU, and CP2 is elected
as the root port, the messages of which will not be
changed.
zAfter comparison between the configuration BPDU
of CP1 and the calculated designated port
configuration BPDU, port CP1 is blocked, with the
configuration BPDU of the port remaining
unchanged, and the port will not receive data from
Device A until a spanning tree calculation process is
triggered by a new condition, for example, the link
from Device B to Device C becomes down.
Blocked port
CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
After the comparison processes described in the table above, a spanning tree with
Device A as the root bridge is stabilized, as shown in Figure 1-3.

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Figure 1-3 The final calculated spanning tree
Note:
To facilitate description, the spanning tree calculation 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 has a lower priority than
the configuration BPDU of the local port, the port will immediately send out its
better 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 sends out the BPDU. This triggers a new spanning tree calculation
process so that a new path is established to restore the network connectivity.
However, the newly calculated configuration BPDU will not be propagated throughout
the network immediately, so the old root ports and designated ports that have not
detected the topology change continue forwarding data along the old path. If the new
root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur.

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3) STP timers
STP calculations need three important timing parameters: forward delay, hello time,
and max age.
zForward delay is the delay time for device state transition. A path failure will cause
re-calculation of the spanning tree, and the spanning tree structure will change
accordingly. However, the new configuration BPDU as the calculation result
cannot be propagated throughout the network immediately. If the newly elected
root port and designated ports start to forward data right away, a temporary loop is
likely to occur. For this reason, as a mechanism for state transition in STP, a newly
elected root port or designated port requires twice the forward delay time before
transitioning to the forwarding state, when the new configuration BPDU has been
propagated throughout the network.
zHello time is the time interval at which a device sends hello packets to the
surrounding devices to make sure that the paths are fault-free.
zMax age is a parameter used to determine whether a configuration BPDU held in
the device has expired. A configuration BPDU beyond the max age will be
discarded.
1.1.2 Introduction to MSTP
I. Why MSTP
1) Disadvantages of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or
designated port must wait twice the forward delay time before transitioning to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port, which
directly connects to a user terminal rather than to another device or a shared LAN
segment.
The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows a
newly elected root port or designated port to enter the forwarding state much quicker
under certain conditions than in STP. As a result, it takes a shorter time for the network
to reach the final topology stability.

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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 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.
MSTP features the following:
zMSTP supports mapping VLANs to MST instances by means of a
VLAN-to-instance mapping table. MSTP can save communication overheads and
resource usage by mapping multiple VLANs to one MST instance.
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
in the data forwarding process.
zMSTP is compatible with STP and RSTP.
II. Basic concepts in MSTP
Assume that all the four switches in Figure 1-4 are running MSTP. In light with the
diagram, the following paragraphs will present some basic concepts of MSTP.

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Figure 1-4 Basic concepts in MSTP
1) MST region
A multiple spanning tree region (MST region) is composed of multiple devices in a
switched network and network segments among them. These devices have the
following characteristics:
zAll are MSTP-enabled,
zThey have the same region name,
zThey have the same VLAN-to-instance mapping configuration,
zThey have the same MSTP revision level configuration, and
zThey are physically linked with one another.
For example, all the devices in region A0 in Figure 1-4 have the same MST region
configuration:
zThe same region name,
zThe same VLAN-to-instance mapping (VLAN 1 is mapped to MST instance 1,
VLAN 2 to MST instance 2, and the rest to the command and internal spanning
tree (CIST). CIST refers to MST instance 0), and
zThe same MSTP revision level (not shown in the figure).
Multiple MST regions can exist in a switched network. You can use an MSTP command
to group multiple devices to the same MST region.

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

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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 calculation, a boundary port assumes the same role on the CIST and on
MST instances. Namely, if a boundary port is the master port on the CIST, it is also the
master port on all MST instances within this region. In Figure 1-4, for example, if a
device in region A0 is interconnected with the first port of a device in region D0 and the
common root bridge of the entire switched network is located in region A0, the first port
of that device in region D0 is the boundary port of region D0.
Note:
Currently, the device is not capable of recognizing boundary ports. When the device
interworks with 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
In the MSTP calculation process, port roles include root port, designated port, master
port, alternate port, backup port, and so on.
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.
zA master port connects an MST region to the common root. The path from the
master port to the common root is the shortest path between the MST region and
the common root. In the CST, the master port is the root port of the region, which is
considered as a node. The master port is a special boundary port. It is a root port
in the IST/CIST while a master port in the other MSTIs.
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 a new designated port and starts forwarding
data without delay. When a loop occurs while two ports of the same MSTP device
are interconnected, the device will block either of the two ports, and the backup
port is that port to be blocked.
A port can assume different roles in different MST instances.

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Connecting to the
common root bridge
Edge ports
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 1-5 Port roles
Figure 1-5 helps understand these concepts. Where,
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 forward user traffic;
zDiscarding: the port neither learns MAC addresses nor forwards user traffic.

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Note:
When in different MST instances, a port can be in different states.
zThe role a boundary port plays in an MSTI is consistent with the role it plays in the
CIST. The master port, which is a root port in the CIST while a master port in the
other MSTIs, is an exception.
zFor example, in Figure 1-5, port 1 on switch A is a boundary port. It is a root port in
the CIST while a master port in all the other MSTIs in the region.
A port state is not exclusively associated with a port role. Table 1-6 lists the port state(s)
supported by each port role (“√” indicates that the port supports this state, while
“—“ indicates that the port does not support this state).
Table 1-6 Ports states supported by different port roles
Role
State
Root
port/Master
port
Designated
port Alternate port Backup port
Forwarding √√— —
Learning √√— —
Discarding √√√√
III. How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are
interconnected by a calculated CST. Inside an MST region, multiple spanning trees are
generated through calculation, each spanning tree called an MST instance. Among
these MST instances, instance 0 is the IST, while all the others are MSTIs. Similar to
STP, MSTP uses configuration BPDUs to calculate spanning trees. The only difference
between the two protocols is that an MSTP BPDU carries the MSTP configuration on
the device from which this BPDU is sent.
1) CIST calculation
By comparison of configuration BPDUs, the device with the highest priority is elected as
the root bridge of the CIST. MSTP generates an IST within each MST region through
calculation, and, at the same time, MSTP regards each MST region as a single device
and generates a CST among these MST regions through calculation. The CST and
ISTs constitute the CIST of the entire network.
2) MSTI calculation
Within an MST region, MSTP generates different spanning tree instances for different
VLANs based on the VLAN-to-instance mappings. MSTP performs a separate

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calculation process, which is similar to spanning tree calculation in STP, for each
spanning tree. For details, refer to How STP works.
In MSTP, a VLAN packet is forwarded along the following paths:
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 calculation.
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
zTC-BPDU guard
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 configuring MSTP, you need to know the position of each device in each MST
instance: root bridge or leave node. In each instance, one, and only one device acts as
the root bridge, while all others as leaf nodes.
Complete these tasks to configure MSTP: