
Spanning Tree Introduction: STP, RSTP, MSTP
©2008 Allied Telesis Inc. All rights reserved.
18.10
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Multiple Spanning Tree Protocol
(MSTP)
Conceptually, MSTP views the total bridged network as one that comprises a number of
Multiple Spanning Tree Regions
(MSTRs), where each region can contain up to 64 spanning trees
that operate locally, called
Multiple Spanning Tree Instances
(MSTIs). (This device supports up to
15 MSTIs.) The regions are linked by the Common Internal Spanning Tree (CIST). MSTP uses
BPDUs to exchange information between devices, and prevents loops for each MSTI and for
the CIST by selecting active and blocked paths, by the process described in
Table 18-1
.
If multiple links are aggregated together into dynamic (LACP) or static channel groups, the
spanning tree application is notified and considers the links as a single logical path.
Advantage of
MSTP over RSTP
MSTP is similar to RSTP, in that it provides loop resolution and rapid convergence. But it also
has the significant extra advantage of making it possible to have different forwarding paths for
different MST instances. This enables load balancing of network traffic across redundant links, so
that all the links in a network can be used by at least one MSTI, and no link is left completely
idle.