Allied Telesis AT-GS950/48, User Manual

Page: 391 / 410

AT-GS950/48 Web Interface User Guide
391
Summary of Guidelines
Careful planning is essential for the successful implementation of MSTP. This
section reviews all of the rules and guidelines mentioned in earlier sections
and contains a few extra ones:

The AT-GS950/48 switch can support up to 32 multiple
spanning tree instances, including the CIST, at a time.
 An MSTI can contain any number of VLANs.
 A VLAN can belong to only one MSTI at a time.
 An MSTI ID can be from 1 to 31.
 The CIST ID value cannot be changed.
 A switch port can belong to more than one spanning tree
instance at a time. This allows you to assign a port as an
untagged and tagged member of VLANs that belong to
different MSTIs. What makes this possible is a port’s ability
to be in different MSTP states for different MSTIs
simultaneously. For example, a port can be in the MSTP
blocking state for one MSTI and the forwarding state for
another spanning tree instance.

A network can contain any number of regions, and a
region can contain any number of AT-GS950/48 switches.
 The AT-GS950/48 switch can belong to only one region at
a time.
 A region can contain any number of VLANs.
 All of the bridges in a region must have the same
configuration name, revision level, VLANs, and VLAN to
MSTI associations.

An MSTI cannot span multiple regions.

Each MSTI must have a regional root for locating loops in
the instance. MSTIs can share the same regional root or
have different roots. A regional root is determined by the
MSTI priority value and a bridge’s MAC address.
 The regional root of an MSTI must be in the same region
as the MSTI.
 The CIST must have a regional root for communicating
with other regions and single-instance spanning trees.
 MSTP is compatible with STP and RSTP.
A port transmits CIST information even when it is associated with another
MSTI ID. However, in determining network loops, MSTI takes precedence
over CIST. (This is explained more in “Associating VLANs to MSTIs” on
page 387).