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Cisco IE 3000 Switch Software Configuration Guide
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Chapter 19 Configuring MSTP
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
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.
Figure 19-1
MST Regions, CIST Masters, and CST Root
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.
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