Cisco 7604 Configuration Manual page 250
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Understanding MST
Spanning Tree Operation Within an MST Region
The IST connects all the MST routers 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 19-1 on page
the network. If the CIST root is outside the region, one of the MST routers at the boundary of the region
is selected as the CIST regional root.
When an MST router 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
router also initializes all of its MST instances and claims to be the root for all of them. If the router
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
routers 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 routers in the MST region must agree on the same CIST regional root.
Therefore, any two routers 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 routers within the network, MST establishes and maintains the
CST, which includes all MST regions and all 802.1D STP routers 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 routers 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 router to adjacent STP routers and MST regions.
Figure 19-1
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.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
19-4
19-5. The CIST regional root is also the CIST root if there is only one region in
shows a network with three MST regions and an 802.1D router (D). The CIST regional root
Chapter 19
Configuring Standard-Compliant IEEE MST
OL-4266-08
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