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Configuring Spanning Tree
MST Instance
The MST instance is a spanning tree running in the MST region. Multiple MST instances can
be established in one MST region and they are independent of each other. As is shown in
Figure 1-4, there are three instances in a region, and each instance has its own root bridge.
Figure 1-4 MST Region
VLAN-Instance Mapping
VLAN-Instance Mapping describes the mapping relationship between VLANs and
instances. Multiple VLANs can be mapped to a same instance, but one VLAN can be
mapped to only one instance. As Figure 1-4 shows, VLAN 3 is mapped to instance 1, VLAN
4 and VLAN 5 are mapped to instance 2, the other VLANs are mapped to the IST.
IST
The Internal Spanning Tree (IST), which is a special MST instance with an instance ID 0. By
default, all the VLANs are mapped to IST.
CST
The Common Spanning Tree (CST), that is the spanning tree connecting all MST regions.
As is shown in Figure 1-3, region1-region 4 are connected by the CST.
CIST
The Common and Internal Spanning Tree (CIST), comprising IST and CST. CIST is the
spanning tree that connects all the switches in the network.
1.3
STP Security
STP Security prevents the loops caused by wrong configurations or BPDU attacks. It
contains Loop Protect, Root Protect, BPDU Protect, BPDU Filter and TC Protect functions.
B
Instance 1 (root bridge: A)
Instance 2 (root bridge: B)
IST (root bridge: C)
Blocked port
A
VLAN 3
Instance 1
VLAN 4-5
Other VLANs
Spanning Tree
C
Instance 2
IST
Configuration Guide
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