H3C S3100-52P User manual

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch Table of Contents
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Table of Contents
Chapter 1 MSTP Configuration....................................................................................................1-1
1.1 STP Overview .................................................................................................................... 1-1
1.2 MSTP Overview ...............................................................................................................1-11
1.2.1 Background of MSTP ............................................................................................ 1-11
1.2.2 Basic MSTP Terminologies................................................................................... 1-12
1.2.3 Principle of MSTP.................................................................................................. 1-17
1.2.4 MSTP Implementation on Switches ...................................................................... 1-18
1.2.5 STP-related Standards.......................................................................................... 1-18
1.3 Configuring Root Bridge .................................................................................................. 1-19
1.3.1 Configuration Prerequisites................................................................................... 1-20
1.3.2 Configuring an MST Region.................................................................................. 1-20
1.3.3 Specifying the Current Switch as a Root Bridge/Secondary Root Bridge............. 1-22
1.3.4 Configuring the Bridge Priority of the Current Switch ........................................... 1-23
1.3.5 Configuring How a Port Recognizes and Sends MSTP Packets.......................... 1-24
1.3.6 Configuring the MSTP Operation Mode................................................................ 1-26
1.3.7 Configuring the Maximum Hop Count of an MST Region..................................... 1-26
1.3.8 Configuring the Network Diameter of the Switched Network................................ 1-27
1.3.9 Configuring the MSTP Time-related Parameters.................................................. 1-28
1.3.10 Configuring the Timeout Time Factor ................................................................. 1-29
1.3.11 Configuring the Maximum Transmitting Rate on the Current Port...................... 1-30
1.3.12 Configuring the Current Port as an Edge Port .................................................... 1-31
1.3.13 Specifying Whether the Link Connected to a Port Is Point-to-point Link ............ 1-33
1.3.14 Enabling MSTP ................................................................................................... 1-34
1.4 Configuring Leaf Nodes ................................................................................................... 1-35
1.4.1 Configuration Prerequisites................................................................................... 1-36
1.4.2 Configuring the MST Region................................................................................. 1-36
1.4.3 Configuring How a Port Recognizes and Sends MSTP Packets.......................... 1-36
1.4.4 Configuring the Timeout Time Factor.................................................................... 1-36
1.4.5 Configuring the Maximum Transmitting Rate on the Current Port........................ 1-36
1.4.6 Configuring a Port as an Edge Port ...................................................................... 1-37
1.4.7 Configuring the Path Cost for a Port ..................................................................... 1-37
1.4.8 Configuring Port Priority ........................................................................................ 1-40
1.4.9 Specifying Whether the Link Connected to a Port Is a Point-to-point Link ........... 1-41
1.4.10 Enabling MSTP ................................................................................................... 1-41
1.5 Performing mCheck Operation ........................................................................................ 1-41
1.5.1 Configuration Prerequisites................................................................................... 1-41
1.5.2 Configuration Procedure ....................................................................................... 1-41
1.5.3 Configuration Example.......................................................................................... 1-42

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1.6 Configuring Guard Functions........................................................................................... 1-42
1.6.1 Introduction............................................................................................................ 1-42
1.6.2 Configuration Prerequisites................................................................................... 1-44
1.6.3 Configuring BPDU Guard...................................................................................... 1-44
1.6.4 Configuring Root Guard ........................................................................................ 1-45
1.6.5 Configuring Loop Guard........................................................................................ 1-46
1.6.6 Configuring TC-BPDU Attack Guard..................................................................... 1-46
1.6.7 Configuring BPDU Dropping ................................................................................. 1-47
1.7 Configuring Digest Snooping ........................................................................................... 1-48
1.7.1 Introduction............................................................................................................ 1-48
1.7.2 Configuring Digest Snooping ................................................................................ 1-48
1.8 Configuring Rapid Transition ........................................................................................... 1-50
1.8.1 Introduction............................................................................................................ 1-50
1.8.2 Configuring Rapid Transition................................................................................. 1-51
1.9 Configuring VLAN-VPN Tunnel ....................................................................................... 1-53
1.9.1 Introduction............................................................................................................ 1-53
1.9.2 Configuring VLAN-VPN tunnel .............................................................................. 1-53
1.10 STP Maintenance Configuration.................................................................................... 1-54
1.10.1 Introduction.......................................................................................................... 1-54
1.10.2 Enabling Log/Trap Output for Ports of MSTP Instance ...................................... 1-54
1.10.3 Configuration Example........................................................................................ 1-55
1.11 Enabling Trap Messages Conforming to 802.1d Standard ........................................... 1-55
1.12 Displaying and Maintaining MSTP................................................................................. 1-56
1.13 MSTP Configuration Example ....................................................................................... 1-56
1.14 VLAN-VPN tunnel Configuration Example .................................................................... 1-59

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Chapter 1 MSTP Configuration
Go to these sections for information you are interested in:
zMSTP Overview
zConfiguring Root Bridge
zConfiguring Leaf Nodes
zPerforming mCheck Operation
zConfiguring Guard Functions
zConfiguring Digest Snooping
zConfiguring Rapid Transition
zConfiguring VLAN-VPN Tunnel
zSTP Maintenance Configuration
zEnabling Trap Messages Conforming to 802.1d Standard
zDisplaying and Maintaining MSTP
zMSTP Configuration Example
zVLAN-VPN tunnel Configuration Example
Note:
The following features were added:
zSTP Maintainability. Refer to STP Maintenance Configuration.
zDisplaying STP maintenance. Refer to Displaying and Maintaining MSTP.
zSending trap messages conforming to 802.1d standard. Refer to Enabling Trap
Messages Conforming to 802.1d Standard.
1.1 STP Overview
I. Functions of STP
Spanning tree protocol (STP) is a protocol conforming to IEEE 802.1d. It aims to
eliminate loops on data link layer in a local area network (LAN). Devices running this
protocol detect loops in the network by exchanging packets with one another and
eliminate the loops detected by blocking specific ports until the network is pruned into
one with tree topology. As a network with tree topology is loop-free, it prevents packets
in it from being duplicated and forwarded endlessly and prevents device performance
degradation.
Currently, in addition to the protocol conforming to IEEE 802.1d, STP also refers to the
protocols based on IEEE 802.1d, such as RSTP, and MSTP.

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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 to calculate spanning trees 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
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 periodically. Other devices just forward the configuration BPDUs received. This
mechanism ensures the topological stability.
2) Root port
On a non-root bridge device, the root port is the port with the lowest path cost to the root
bridge. The root port is used for 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 Designated bridge and designated port
Classification Designated bridge Designated port
For a device
A designated bridge is a device
that is directly connected to a
switch and is responsible for
forwarding BPDUs to this
switch.
The port through which
the designated bridge
forwards BPDUs to this
device

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Classification Designated bridge Designated port
For a LAN A designated bridge is a 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.
Figure 1-1 A schematic diagram of designated bridges and designated ports
Note:
All the ports on the root bridge are designated ports.
4) Path cost
Path cost is a value used for measuring link capacity. By comparing the path costs of
different links, STP selects the most robust links and blocks the other links to prune the
network into a tree.

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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 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: lifetime for the configuration BPDUs to be propagated within the
network.
zMax age, lifetime for the configuration BPDUs to be kept in a switch.
zHello time, configuration BPDU interval.
zForward delay, forward delay of the port.
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) Detailed calculation process of the STP algorithm
zInitial state
Upon initialization of a device, each device 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 BPDU and receives configuration BPDUs from
other devices.
The process of selecting the optimum configuration BPDU is as follows:

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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.
Note:
Principle for configuration BPDU comparison:
zThe configuration BPDU that has the lowest root bridge ID has the highest priority.
zIf all 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, the following configuration
BPDU priority is compared sequentially: designated bridge IDs, designated port IDs,
and then the IDs of the ports on which the configuration BPDUs are received. The
switch with a higher priority is elected as the root bridge.
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 bridge 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:

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Table 1-3 Selection of the root port and designated ports
Step Description
1 A non-root-bridge device takes the port on which the optimum
configuration BPDU was received 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.
3
The device compares the calculated configuration BPDU with the
configuration BPDU on the port whose role is to be determined,
and acts as follows based on the comparison result:
zIf the calculated configuration BPDU is superior, this port will
serve 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
stops updating the configuration BPDUs of the port and blocks
the port, so that the port only receives configuration BPDUs,
but does not forward data or send configuration BPDUs.
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. 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.

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Figure 1-2 Network diagram for STP algorithm
zInitial state of each device
The following table shows the initial state of each device.
Table 1-4 Initial state of each device
Device Port name BPDU of port
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.

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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}
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 BPDUs 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) + 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 sends 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 generates configuration BPDUs with itself as the
root bridge and sends configuration BPDUs and TCN BPDUs. This triggers a new
spanning tree calculation 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 through the old path. If the new

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root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur.
3) STP timers
The following three time parameters are important for STP calculation:
zForward delay, the period a device waits before state transition.
A link failure triggers a new round of spanning tree calculation and results in changes of
the spanning tree. However, as new configuration BPDUs cannot be propagated
throughout the network immediately, if the new root port and designated port begin to
forward data as soon as they are elected, loops may temporarily occur.
For this reason, the protocol uses a state transition mechanism. Namely, a newly
elected root port and the designated ports must go through a period, which is twice the
forward delay time, before they transit to the forwarding state. The period allows the
new configuration BPDUs to be propagated throughout the entire network.
zHello time, the interval for sending hello packets. Hello packets are used to check
link state.
A switch sends hello packets to its neighboring devices at a regular interval (the hello
time) to check whether the links are faulty.
zMax time, lifetime of the configuration BPDUs stored in a switch. A configuration
BPDU that has “expired” is discarded by the switch.
1.2 MSTP Overview
1.2.1 Background of MSTP
I. 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 transiting to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port (an edge
port refers to a port that 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, the state of a root port can transit fast under the following conditions: the
old root port on the device has stopped forwarding data and the upstream
designated port has started forwarding data.
zIn RSTP, the state of a designated port can transit fast under the following
conditions: 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.
RSTP supports rapid convergence. Like STP, it is of the following disadvantages: all
bridges in a LAN are on the same spanning tree; redundant links cannot be blocked by
VLAN; the packets of all VLANs are forwarded along the same spanning tree.
II. 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.
MSTP features the following:
zMSTP supports mapping VLANs to MST instances (MSTIs) by means of a
VLAN-to-MSTI mapping table. MSTP introduces instance (integrates multiple
VLANs into a set) and can bind multiple VLANs to an instance, thus saving
communication overhead and improving resource utilization.
zMSTP divides a switched network into multiple regions, each containing multiple
spanning trees that are independent of one another.
zMSTP prunes a ring network into a network with tree topology, preventing packets
from being duplicated and forwarded in a network endlessly. Furthermore, it offers
multiple redundant paths for forwarding data, and thus achieves load balancing for
forwarding VLAN data.
zMSTP is compatible with STP and RSTP.
1.2.2 Basic MSTP Terminologies
Figure 1-4 illustrates basic MSTP terms (assuming that MSTP is enabled on each
switch in this figure).

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CST
BPDU
Region A0:
VLAN 1 mapped to MSTI 1
VLAN 2 mapped to MSTI 2
Other VLANs mapped to CIST
BPDU BPDU
A
D
CB
Region B0:
VLAN 1 mapped to MSTI 1
VLAN 2 mapped to MSTI 2
Other VLANs mapped to CIST
Region C0:
VLAN 1 mapped to MSTI 1
VLAN 2 and 3 mapped to MSTI 2
Other VLANs mapped to CIST
Region D0:
VLAN 1 mapped to MSTI 1, B
as the regional root bridge
VLAN 2 mapped to MSTI 2, C
as the regional root bridge
Other VLANs mapped to CIST
Figure 1-4 Basic MSTP terminologies
I. MST region
A multiple spanning tree region (MST region) comprises multiple
physically-interconnected MSTP-enabled switches and the corresponding network
segments connected to these switches. These switches have the same region name,
the same VLAN-to-MSTI mapping configuration and the same MSTP revision level.
A switched network can contain multiple MST regions. You can group multiple switches
into one MST region by using the corresponding MSTP configuration commands.
As shown in Figure 1-4, all the switches in region A0 are of the same MST
region-related configuration, including:
zRegion name
zVLAN-to-MSTI mapping (that is, VLAN 1 is mapped to MSTI 1, VLAN 2 is mapped
to MSTI 2, and the other VLANs are mapped to CIST.)
zMSTP revision level (not shown in Figure 1-4)
II. MSTI
A multiple spanning tree instance (MSTI) refers to a spanning tree in an MST region.
Multiple spanning trees can be established in one MST region. These spanning trees
are independent of each other. For example, each region in Figure 1-4 contains
multiple spanning trees known as MSTIs. Each of these spanning trees corresponds to
a VLAN.

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III. VLAN-to-MSTI mapping table
A VLAN-to-MSTI mapping table is maintained for each MST region. The table is a
collection of mappings between VLANs and MSTIs. For example, in Figure 1-4, the
VLAN-to-MSTI mapping table of region A0 contains these mappings: VLAN 1 to MSTI 1;
VLAN 2 to MSTI 2, and other VLANs to CIST. In an MST region, load balancing is
implemented according to the VLAN-to-MSTI mapping table.
IV. IST
An internal spanning tree (IST) is a spanning tree in an MST region.
ISTs together with the common spanning tree (CST) form the common and internal
spanning tree (CIST) of the entire switched network. An IST is a special MSTI; it is a
branch of CIST in the MST region.
In Figure 1-4, each MST region has an IST, which is a branch of the CIST.
V. CST
A CST is a single spanning tree in a switched network that connects all MST regions in
the network. If you regard each MST region in the network as a “switch”, then the CST
is the spanning tree generated by STP or RSTP running on the "switches".
VI. CIST
A CIST is the spanning tree in a switched network that connects all switches in the
network. It comprises the ISTs and the CST.
In Figure 1-4, the ISTs in the MST regions and the CST connecting the MST regions
form the CIST.
VII. Region root
A region root is the root of the IST or an MSTI in an MST region. Different spanning
trees in an MST region may have different topologies and thus have different region
roots.
In region D0 shown in Figure 1-4, the region root of MSTI 1 is switch B, and the region
root of MSTI 2 is switch C.
VIII. Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the network
shown in Figure 1-4 is a switch in region A0.
IX. Port role
MSTP calculation involves the following port roles: root port, designated port, master
port, region boundary port, alternate port, and backup port.
zA root port is used to forward packets to the root.

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zA designated port is used to forward packets to a downstream network segment or
switch.
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.
zA region boundary port is located on the boundary of an MST region and is used to
connect one MST region to another MST region, an STP-enabled region or an
RSTP-enabled region.
zAn alternate port is a secondary port of a root port or master port and is used for
rapid transition. With the root port or master port being blocked, the alternate port
becomes the new root port or master port.
zA backup port is the secondary port of a designated port and is used for rapid
transition. With the designated port being blocked, the backup port becomes the
new designated port fast and begins to forward data seamlessly. When two ports
of an MSTP-enabled switch are interconnected, the switch blocks one of the two
ports to eliminate the loop that occurs. The blocked port is the backup port.
In Figure 1-5, switch A, switch B, switch C, and switch D form an MST region. Port 1
and port 2 on switch A connect upstream to the common root. Port 5 and port 6 on
switch C form a loop. Port 3 and port 4 on switch D connect downstream to other MST
regions. This figure shows the roles these ports play.
Note:
zA port can play different roles in different MSTIs.
zThe role a region 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 region boundary port. It is a root
port in the CIST while a master port in all the other MSTIs in the region.

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Connecting to the
common root bridge
Region boundary 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
X. Port state
In MSTP, a port can be in one of the following three states:
zForwarding state. Ports in this state can forward user packets and receive/send
BPDU packets.
zLearning state. Ports in this state can receive/send BPDU packets but do not
forward user packets.
zDiscarding state. Ports in this state can only receive BPDU packets.
Port roles and port states are not mutually dependent. Table 1-6 lists possible
combinations of port states and port roles.
Table 1-6 Combinations of port states and port roles
Root/mast
er port Designate d port
Region
Boundary
port
Alternate
port Backup port
Forwarding √√√— —
Learning √√√— —
Discarding √√√√√
Port
role
Port
state

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1.2.3 Principle of MSTP
MSTP divides a Layer 2 network into multiple MST regions. The CSTs are generated
between these MST regions, and multiple spanning trees (also called MSTIs) can be
generated in each MST region. As well as RSTP, MSTP uses configuration BPDUs for
spanning tree calculation. The only difference is that the configuration BPDUs for
MSTP carry the MSTP configuration information on the switches.
I. Calculate the CIST
Through comparing configuration BPDUs, the switch of the highest priority in the
network is selected as the root of the CIST. In each MST region, an IST is calculated by
MSTP. At the same time, MSTP regards each MST region as a switch to calculate the
CSTs of the network. The CSTs, together with the ISTs, form the CIST of the network.
II. Calculate an MSTI
In an MST region, different MSTIs are generated for different VLANs based on the
VLAN-to-MSTI mappings. Each spanning tree is calculated independently, in the same
way as how STP/RSTP is calculated.
III. Implement STP algorithm
In the beginning, each switch regards itself as the root, and generates a configuration
BPDU for each port on it as a root, with the root path cost being 0, the ID of the
designated bridge being that of the switch, and the designated port being itself.
1) Each switch sends out its configuration BPDUs and operates in the following way
when receiving a configuration BPDU on one of its ports from another switch:
zIf the priority of the configuration BPDU is lower than that of the configuration
BPDU of the port itself, the switch discards the BPDU and does not change the
configuration BPDU of the port.
zIf the priority of the configuration BPDU is higher than that of the configuration
BPDU of the port itself, the switch replaces the configuration BPDU of the port with
the received one and compares it with those of other ports on the switch to obtain
the one with the highest priority.
2) Configuration BPDUs are compared as follows:
zThe smaller the root ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
zFor configuration BPDUs with the same root IDs, the path costs are compared.
Suppose S is the sum of the root path costs and the corresponding path cost of the
port. The less the S value is, the higher the priority of the configuration BPDU is.
zFor configuration BPDUs with both the same root ID and the same root path cost,
the designated bridge ID, designated port ID, the ID of the receiving port are
compared in turn.
3) A spanning tree is calculated as follows:

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zDetermining the root bridge
Root bridges are selected by configuration BPDU comparing. The switch with the
smallest root ID is chosen as the root bridge.
zDetermining the root port
For each switch in a network, the port on which the configuration BPDU with the highest
priority is received is chosen as the root port of the switch.
zDetermining the designated port
First, the switch calculates a designated port configuration BPDU for each of its ports
using the root port configuration BPDU and the root port path cost, with the root ID
being replaced with that of the root port configuration BPDU, root path cost being
replaced with the sum of the root path cost of the root port configuration BPDU and the
path cost of the root port, the ID of the designated bridge being replaced with that of the
switch, and the ID of the designated port being replaced with that of the port.
The switch then compares the calculated configuration BPDU with the original
configuration BPDU received from the corresponding port on another switch. If the
latter takes precedence over the former, the switch blocks the local port and keeps the
port's configuration BPDU unchanged, so that the port can only receive configuration
messages and cannot forward packets. Otherwise, the switch sets the local port to the
designated port, replaces the original configuration BPDU of the port with the
calculated one and advertises it regularly.
1.2.4 MSTP Implementation on Switches
MSTP is compatible with both STP and RSTP. That is, MSTP-enabled switches can
recognize the protocol packets of STP and RSTP and use them for spanning tree
calculation. In addition to the basic MSTP functions, H3C series switches also provide
the following functions for users to manage their switches.
zRoot bridge hold
zRoot bridge backup
zRoot guard
zBPDU guard
zLoop guard
zTC-BPDU attack guard
zBPDU packet drop
1.2.5 STP-related Standards
STP-related standards include the following.
zIEEE 802.1D: spanning tree protocol
zIEEE 802.1w: rapid spanning tree protocol
zIEEE 802.1s: multiple spanning tree protocol