Allied Telesis AT-S60 - Manual (Page 217)

Chapter 12: STP, RSTP, and MSTP
Section II: Local and Telnet Management 216
This example illustrates Allied Telesyn’s implementation of MSTP. It
shows that a tagged port cannot be a member of VLANs that belong to
different MSTIs. That is why each MSTI in the example has its own tagged
link.
MSTI Guidelines
Here are several guidelines to keep in mind about MSTIs:
❑ 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.
❑ 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 resided in different spanning tree instances.
❑ A router or Layer 3 network device is required to forward traffic
between different VLANs.
VLAN and MSTI
Associations
Part of the task to configuring MSTP involves assigning VLANs to
spanning tree instances. The mapping of VLANs to MSTIs is called
associations. A VLAN, either port-based or tagged, can belong to only
one instance at a time, but an instance can contain any number of
VLANs.
Multiple
Spanning Tree
Regions
Another important concept of MSTP is regions. A MSTP region is defined
as a group of bridges that share exactly the same MSTI characteristics.
Those characteristics are:
❑ Configuration name
❑ Revision number
❑ VLANs
❑ VLAN to MSTI ID associations