Spanning Tree Operations Between Mst Regions - Cisco Nexus 3000 series Configuration Manual

Configuring Multiple Spanning Tree
IST, CIST, and CST
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.
The following figure 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.
Figure 15: 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 switches and compute the final spanning tree topology. Because
of this, the spanning tree parameters related to the BPDU transmission (for example, hello time, forward time,
max-age, and max-hops) are configured only on the CST instance but affect all MSTIs. You can configure
Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)
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