Cisco N5010P-N2K-BE, Software Configuration Manual

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S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
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Chapter 9 Configuring MST
Information About 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 as shown in
Figure 9-2 on page 9-6 . The CIST regional root is also the CIST root
if there is only one region in the network. If the CIST root is outside the region, the protocol selects one
of the MST switches at the boundary of the region 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 the path costs to the CIST root and to the CIST regional root set to zero. The
switch also initializes all of its MSTIs 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 the information that
is currently stored for the port, it relinquishes its claim as the CIST regional root.
During initialization, an MST 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. This action causes all
subregions to shrink except for the subregion that contains the true CIST regional root.
All switches in the MST region must agree on the same CIST regional root. Any two switches in the
region will only synchronize their port roles for an MSTI if they converge to a common CIST regional
root.
Spanning Tree Operations Between MST Regions
If you have multiple regions or 802.1 w or 802.1D STP instances within a network, MST establishes and
maintains the CST, which includes all MST regions and all 802.1w and 802.1D STP switches in the
network. The MSTIs 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 9-2 shows a network with three MST regions and an 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.