Allied Telesis AT-S63 - Manual (Page 563)

AT-S63 Management Software Menus Interface User’s Guide
Section V: Spanning Tree Protocols 563
An MSTP region can be considered as a virtual bridge. The implication is
that other MSTP regions and STP and RSTP single-instance spanning
trees cannot discern the topology or constitution of a MSTP region. The
only bridge they are aware of is the regional root of the CIST instance.
Summary of
Guidelines
Careful planning is essential for the successful implementation of MSTP.
This section reviews all the rules and guidelines mentioned in earlier
sections, and contains a few new ones:
ˆ An AT-9400 Series switch can support up to 16 spanning tree
instances, including the CIST, at a time.
ˆ A MSTI can contain any number of VLANs.
ˆ A VLAN can belong to only one MSTI at a time.
ˆ An MSTI ID can be from 1 to 15.
ˆ The CIST ID is 0. You cannot change this value.
ˆ 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.
ˆ A router or Layer 3 network device is required to forward traffic
between VLANs.
ˆ A network can contain any number of regions and a region can contain
any number of AT-9400 Series switches.
ˆ An AT-9400 Series switch can belong to only one region at a time.
ˆ A region can contain any number of VLANs.
ˆ All of the bridges in a region must have the same configuration name,
revision level, VLANs, and VLAN to MSTI associations.
ˆ An MSTI cannot span multiple regions.
ˆ 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.
ˆ The regional root of a MSTI must be in the same region as the MSTI.
ˆ The CIST must have a regional root for communicating with other
regions and single-instance spanning trees.
ˆ MSTP is compatible with STP and RSTP.
ˆ 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 564.