Allied Telesis AT-S63 - Manual (Page 288)

Chapter 17: MSTP
288 Section II: Advanced Features
MSTP with STP and RSTP
MSTP is fully compatible with STP and RSTP. If a port on an AT-9400
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 to be 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 contains a few new ones:
❑ An AT-9400 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 is an untagged port on 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. Aport cannot perform both functions
sumultaneously, which could occur if it was a member of VLANs
that resided 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-9400 Series switches.
❑ An AT-9400 Series switch can belong to only one region at a time.
❑ A region can contain any number of VLANs.