Cisco 3032 Software Configuration Manual page 498

Understanding MSTP
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 19-1
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 19-1 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,
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.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
19-4
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 19
C
IST master
MST Region 3
Configuring MSTP
OL-12247-04