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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 21 Configuring MSTP
Information About Configuring MSTP
Figure 21-1 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 communica te with MSTP switches.
IEEE 802.1s Terminology
Some MST naming conventions used in Cisco’s prestandard implementation have been changed to
identify some internal or regional parameters. These parameters are significant only within an MST
region, as opposed to external parameters that are relevant to the who le network. Because the CIST is
the only spanning-tree instance that spans the whole network, only the CIST parameters require the
external rather than the internal or regional qualifiers.
• The CIST root is the root switch for the unique instance that spans the whole network, the CIST.
• The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an
MST region. Remember that an MST region looks like a single switch for the CIST. The CIST
external root path cost is the root path cost calculated between these virtual switches and switches
that do not belong to any region.
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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