Operations Between Mst Regions - Cisco 3020 - Catalyst Blade Switch Configuration Manual

Understanding MSTP
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.

Operations Between MST Regions

If there are multiple regions or legacy IEEE 802.1D switches within the network, MSTP establishes and
maintains the CST, which includes all MST regions and all legacy 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 MSTP 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 14-1
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. The RSTP runs
in all regions.
Figure 14-1 MST Regions, CIST Masters, and CST Root
D
Legacy IEEE 802.1D
B
IST master
MST Region 2
Only the CST instance sends and receives BPDUs, and MST instances add their spanning-tree
information into the BPDUs to interact with neighboring switches and compute the final spanning-tree
topology. Because of this, the spanning-tree parameters related to BPDU transmission (for example,
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
14-4
shows a network with three MST regions and a legacy IEEE 802.1D switch (D). The CIST
IST master
A
and CST root
MST Region 1
Chapter 14
C
IST master
MST Region 3
Configuring MSTP
OL-8915-01