Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual page 244

Understanding MST
Spanning Tree Operation Within an MST Region
The IST connects all the MST switches in a region. When the IST converges, the root of the IST becomes
the CIST regional root (called the IST master before the implementation of the 802.1s standard) as shown
in Figure 17-10 on page
in the network. If the CIST root is outside the region, one of the MST switches at the boundary of the
region is selected as the CIST regional root.
When an MST switch initializes, it sends BPDUs that identify itself as the root of the CIST and the CIST
regional root, with both of the path costs to the CIST root and to the CIST regional root set to zero. The
switch also initializes all of its MST instances and claims to be the root for all of them. If the switch
receives superior MST root information (lower switch ID, lower path cost, and so forth) than currently
stored for the port, it relinquishes its claim as the CIST regional root.
During initialization, a region might have many subregions, each with its own CIST regional root. As
switches receive superior IST information from a neighbor in the same region, they leave their old
subregions and join the new subregion that contains the true CIST regional root, which causes all
subregions to shrink except for the one that contains the true CIST regional root.
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.
Spanning Tree Operations Between MST Regions
If there are multiple regions or 802.1D switches within the network, MST establishes and maintains the
CST, which includes all MST regions and all 802.1D 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 MST 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 17-10
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.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
17-20
17-21. The CIST regional root is also the CIST root if there is only one region
shows a network with three MST regions and an 802.1D switch (D). The CIST regional
Chapter 17 Configuring STP and MST
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