Allied Telesis AT-S62 - Manual (Page 489)

AT-S62 Menus Interface User’s Guide
Section IV: Spanning Tree Protocols 489
MSTP with STP
and RSTP
MSTP is fully compatible with STP and RSTP. If a port on an AT-8500
Series switch running MSTP receives STP BPDUs, the port sends only STP
BPDU packets. If a port receives RSTP BPDUs, the port sends MSTP BPDUs
because RSTP can process MSTP BPDUs.
A port connected to a bridge running STP or RSTP is considered a
boundary port of the MSTP region and the bridge as belonging to a
different region.
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, plus a few new ones:
❑ An AT-8500 Series switch can support up to 16 spanning tree
instances, including the CIST, at a time.
❑ A 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 15.
❑ The CIST ID is 0. You cannot change this value.
❑ A switch port can belong to more than one spanning tree instance
at a time. This allows you to assign a port as a 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 router or Layer 3 network device is required to forward traffic
between VLANs.
❑ A network can contain any number of regions and a region can
contain any number of AT-8500 Series switches.
❑ An AT-8500 Series 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.