Operations Between Mst Regions - Cisco WS-C3560-48PS-S Software Configuration Manual

Understanding MSTP

Operations Between MST Regions

If there are multiple regions or legacy 802.1D switches within the network, MSTP establishes and
maintains the CST, which includes all MST regions and all legacy STP switches in the network. The
MST instances combine with the IST at the boundary of the region to become the CST.
The IST connects all the MSTP switches in the region and appears as a subtree in the CST that
encompasses the entire switched domain, with the root of the subtree being the IST master. The MST
region appears as a virtual switch to adjacent STP switches and MST regions.
Figure 16-1
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 16-1 MST Regions, IST Masters, and the CST Root
Legacy 802.1D
Figure 16-1
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 802.1D
switches. MSTP switches use MSTP BPDUs to communicate with MSTP switches.
Catalyst 3560 Switch Software Configuration Guide
16-4
shows a network with three MST regions and a legacy 802.1D switch (D). The IST master
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 16 Configuring MSTP
C
IST master
MST Region 3
78-16156-01