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

Software configuration guide

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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.

for region 1 (A) is also the CST root. The IST master for region 2 (B) and the IST master for region 3

Figure 16-1 MST Regions, IST Masters, and the CST Root

Legacy 802.1D

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

shows a network with three MST regions and a legacy 802.1D switch (D). The IST master

MST Region 2

does not show additional MST instances for each region. Note that the topology of MST

and CST root

MST Region 1

Configuring MSTP

MST Region 3

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