Spanning Tree Algorithm
11-2
11
MSTP – When using STP or RSTP, it may be difficult to maintain a stable path
between all VLAN members. Frequent changes in the tree structure can easily
isolate some of the group members. MSTP (which is based on RSTP for fast
convergence) is designed to support independent spanning trees based on VLAN
groups. Using multiple spanning trees can provide multiple forwarding paths and
enable load balancing. One or more VLANs can be grouped into a Multiple Spanning
Tree Instance (MSTI). MSTP builds a separate Multiple Spanning Tree (MST) for
each instance to maintain connectivity among each of the assigned VLAN groups.
MSTP then builds a Internal Spanning Tree (IST) for the Region containing all
commonly configured MSTP bridges.
An MST Region consists of a group of interconnected bridges that have the same
MST Configuration Identifiers (including the Region Name, Revision Level and
Configuration Digest – see “Configuring Multiple Spanning Trees” on page 16). An
MST Region may contain multiple MSTP Instances. An Internal Spanning Tree (IST)
is used to connect all the MSTP switches within an MST region. A Common
Spanning Tree (CST) interconnects all adjacent MST Regions, and acts as a virtual
bridge node for communications with STP or RSTP nodes in the global network.
MSTP connects all bridges and LAN segments with a single Common and Internal
Spanning Tree (CIST). The CIST is formed as a result of the running spanning tree
algorithm between switches that support the STP, RSTP, MSTP protocols.
Region R
IST
(for this Region)
MST 1
MST 2
Region 1
Region 4
Region 2 Region 3
CIST
IST
Region 1
Region 4
Region 2 Region 3
CST
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