When configuring MSTP, go to these sections
for information you are interested in:
l
MSTP Overview
l
Configuring the Root Bridge
l
Configuring Leaf Nodes
l
Performing mCheck
l
Configuring Protection Functions
l
Displaying and Maintaining
MSTP
1.1 MSTP Overview
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:
l
Configuration BPDUs, used for calculating
spanning trees and maintaining the spanning tree topology.
l
Topology change notification (TCN) BPDUs, used for
notifying 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 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.
l
If 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.
l
Two 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
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.
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:
l
Root bridge ID: consisting of root bridge
priority and MAC address.
l
Root path cost: the cost of the shortest path to
the root bridge.
l
Designated bridge ID: designated bridge priority
plus MAC address.
l
Designated port ID, designated port priority
plus port name.
l
Message age: age of the configuration BPDU while
it propagates in the network.
l
Max age: maximum age of the configuration BPDU
maintained in the device.
l
Hello time: configuration BPDU interval.
l
Forward delay: forward delay of the port.
For the
convenience of description, the description and examples below involve only
four parts of a configuration BPDU:
l
Root bridge ID (in the form of device priority)
l
Root path cost
l
Designated bridge ID (in the form of device
priority)
l
Designated port ID (in the form of port name)
1)
Specific calculation process of the STP
algorithm
l
Initial 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.
l
Selection 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:
l
If 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.
l
If 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.
|
Principle
for configuration BPDU comparison:
l
The configuration BPDU that has the lowest root
bridge ID has the highest priority.
l
If 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.
l
If 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.
l
Selection 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.
l
Selection 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.
l
The root bridge ID is replaced with that of the
configuration BPDU of the root port.
l
The root path cost is replaced with that of
the configuration BPDU of the root port plus the path cost corresponding to
the root port.
l
The designated bridge ID is replaced with the
ID of this device.
l
The 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 of which the port role is to be defined, and does different things according
to the comparison result:
l If 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.
l If 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.
|
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
l
Initial 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
|
|
Device A
|
AP1
|
{0, 0, 0, AP1}
|
|
AP2
|
{0, 0, 0, AP2}
|
|
Device B
|
BP1
|
{1, 0, 1, BP1}
|
|
BP2
|
{1, 0, 1, BP2}
|
|
Device C
|
CP1
|
{2, 0, 2, CP1}
|
|
CP2
|
{2, 0, 2, CP2}
|
l
Comparison 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
|
l
Port 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.
l
Port 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.
l
Device 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}
|
|
Device B
|
l Port 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.
l Port 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}
|
|
l
Device 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.
l
Based 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}.
l
Device 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}
|
|
Device C
|
l Port 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.
l Port 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:
l The 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.
l Device 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}
|
|
l Next, 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.
l At 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}
|
|
By
comparison:
l Because 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.
l After 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.
Figure 1-3 The
final calculated spanning tree
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
l
Upon 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.
l
If 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.
l
If 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.
l
If 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.
3)
STP timers
STP calculations need three important
timing parameters: forward delay, hello time, and max age.
l
Forward 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.
l
Hello 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.
l
Max 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.
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.
l
In 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.
l
In 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:
l
MSTP 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.
l
MSTP divides a switched network into multiple
regions, each containing multiple spanning trees that are independent of one
another.
l
MSTP 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.
l
MSTP 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.
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:
l
All are MSTP-enabled,
l
They have the same region name,
l
They have the same VLAN-to-instance mapping
configuration,
l
They have the same MSTP revision level
configuration, and
l
They are physically linked with one another.
For example, all the devices in region A0
in Figure 1-4
have the same MST region configuration:
l
The same region name,
l
The 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
l
The 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.
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.
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.
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.
l
Root port: a port responsible for forwarding
data to the root bridge.
l
Designated port: a port responsible for
forwarding data to the downstream network segment or device.
l
A 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.
l
Alternate 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.
l
Backup 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.
Figure 1-5 Port roles
Figure 1-5 helps
understand these concepts. Where,
l
Devices A, B, C, and D constitute an MST region.
l
Port 1 and port 2 of device A connect to the
common root bridge.
l
Port 5 and port 6 of device C form a loop.
l
Port 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:
l
Forwarding: the port learns MAC addresses and
forwards user traffic;
l
Learning: the port learns MAC addresses but does
not forward user traffic;
l
Discarding: the port neither learns MAC
addresses nor forwards user traffic.
When in different MST instances, a port can be in different states.
l
The 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.
l
For 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 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:
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Within an MST region, the packet is forwarded
along the corresponding MSTI.
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Between 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:
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Root bridge hold
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Root bridge backup
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Root guard
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BPDU guard
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Loop guard
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TC-BPDU guard
MSTP is documented in:
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IEEE 802.1d: Spanning Tree Protocol
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