Spanning Tree Operations Between Mst Regions - Cisco Nexus 9000 Series Configuration Manual

Configuring MST Using Cisco NX-OS
IST, CIST, and CST
During initialization, an MST region might have many subregions, each with its own CIST regional root. As
devices 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 devices in the MST region must agree on the same CIST regional root. Any two devices 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 devices 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 devices 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 device to adjacent STP devices and MST regions.
This figure shows a network with three MST regions and an 802.1D device (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.
Figure 11: MST Regions, CIST Regional Roots, and CST Root
Only the CST instance sends and receives BPDUs. MSTIs add their spanning tree information into the BPDUs
(as M-records) to interact with neighboring devices within the same MST region and compute the final
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