Cisco Systems 3560 Manual De Usuario

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.

 shows a network with three MST regions and a legacy IEEE 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. The RSTP runs 
in all regions.

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, 
hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect 
all MST instances. Parameters related to the spanning-tree topology (for example, switch priority, port 
VLAN cost, and port VLAN priority) can be configured on both the CST instance and the MST instance.

MSTP switches use Version 3 RSTP BPDUs or IEEE 802.1D STP BPDUs to communicate with legacy 
IEEE 802.1D switches. MSTP switches use MSTP BPDUs to communicate with MSTP switches.

IST master
and CST root

IST master

IST master

A

MST Region 1

D

Legacy IEEE 802.1D

B

MST Region 2

MST Region 3

C

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