Cisco ME 3400 Series, Software Configuration Manual

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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 15 Configuring MSTP
Understanding MSTP
Figure 15-1 shows a network with three MST regions and a legacy IEEE 802.1D switch (D). The IST
master for region 1 (A) is also the CST root. The IST master for region 2 (B) and the IST master for
region 3 (C) are the roots for their respective subtrees within the CST. The RSTP runs in all regions.
Figure 15-1 MST Regions, IST Masters, and the CST Root
Figure 15-1 does not show additional MST instances for each region. Note that the topology of MST
instances can be different from that of the IST for the same region.
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, port VLAN priority) can be configured on both the CST instance and the MST instance.
MSTP switches use Version 3 RSTP BPDUs or 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 802.1D
B
MST Region 2 MST Region 3
C
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