Cisco ME 3400 Software Configuration Manual page 336

Understanding MSTP
Figure 15-1
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
Legacy 802.1D
Figure 15-1
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.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
15-4
shows a network with three MST regions and a legacy IEEE 802.1D switch (D). The IST
MST Regions, IST Masters, and the CST Root
D
B
IST master
MST Region 2
does not show additional MST instances for each region. Note that the topology of MST
IST master
A
and CST root
MST Region 1
Chapter 15 Configuring MSTP
C
IST master
MST Region 3
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