Allied Telesis AT-S60 - Manual (Page 268)

Chapter 16: Multiple Spanning Tree
Section II: Advanced Features 268
MSTP with STP and RSTP
MSTP is fully compatible with STP and RSTP. If a port on an AT-8400
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
since 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, and adds a few new ones:
❑ An AT-8400 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 port on the switch can belong to only one spanning tree
instance at a time. This means that a port cannot be a tagged and
untagged member of VLANs that belong to different MSTIs. For
example, if Port 1 on a line card is an untagged port in one VLAN
and a tagged port in three other VLANs, all four VLANs must be
assigned to the same MSTI. This rule is required because a port can
be either blocking or forwarding. A port cannot perform both
functions simultaneously, which could occur if it was a member of
VLANs that reside in different spanning tree instances.
❑ 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-8400 Series switches.
❑ An AT-8400 Series switch can belong to only one region at a time.
❑ A region can contain any number of VLANs.