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17-4
Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 17 Configuring MSTP
Understanding MSTP
For correct operation, all switches in the MST region must agree on the same CIST regional root.
Therefore, any two switches in the region only synchronize their port roles for an MST instance if they
converge to a common CIST regional root.
Operations Between MST Regions
If there are multiple regions or legacy IEEE 802.1D switches within the network, MSTP establishes and
maintains the CST, which includes all MST regions and all legacy STP switches in the network. The
MST instances combine with the IST at the boundary of the region to become the CST.
The IST connects all the MSTP switches in the region and appears as a subtree in the CIST that
encompasses the entire switched domain. The root of the subtree is the CIST regional root. The MST
region appears as a virtual switch to adjacent STP switches and MST regions.
Figure 17-1 shows a network with three MST regions and a legacy IEEE 802.1D switch (D). The CIST
regional root for region 1 (A) is also the CIST root. The CIST regional root for region 2 (B) and the CIST
regional root for region 3 (C) are the roots for their respective subtrees within the CIST. The RSTP runs
in all regions.
Figure 17-1 MST Regions, CIST Masters, and CST Root
IST master
and CST root
IST master IST master
A
MST Region 1
D
Legacy IEEE 802.1D
B
MST Region 2 MST Region 3
C
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