Allied Telesis AT-S62 - Manual (Page 484)

Chapter 24: Multiple Spanning Tree Protocol
Section IV: Spanning Tree Protocols 484
MSTI Guidelines
Here are several guidelines to keep in mind about MSTIs:
❑ An AT-8500 Series 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 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. For example, a port can be in the MSTP
blocking state for one MSTI and the forwarding state for another
spanning tree instance, simultaneously.
❑ 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.
Ports in Multiple
MSTIs
The AT-8500 Series switch allows a port to be a member of more than
one MSTI at a time. This can happen if a port is a tagged member of one
or more VLANs and the VLANs are assigned to different MSTIs. If this
occurs, it is possible that a port might be required to operate in different
spanning tree states simultaneously, depending on the requirements of
the MSTIs. For example, a port that is a member of two VLANs assigned
to two different MSTIs might be operating in the forwarding state for
one MSTI and in the blocking state for the other.
When you configure a port’s MSTI parameter settings you will notice
that the parameters are divided into two groups. The first group is
referred to as generic parameters. These are set just once on a port,
regardless of the number of MSTIs where a port happens to be a
member. One of these parameters is the external path cost, which sets
the operating cost of the port in situations where it is connected to a
device that is outside its region. A port can have only one external path
cost even if it belongs to multiple MSTIs. Other generic parameters
designate whether the port is an edge port or a point-to-point port.