Allied Telesis AT-S62 - Manual (Page 512)

Chapter 23: Multiple Spanning Tree Protocol
512 Section IV: Spanning Tree Protocols
MSTI Guidelines
Here are several guidelines to keep in mind about MSTIs:
ˆ An AT-8500 Series can support up to 16 spanning tree instances,
including the CIST, at a time.
ˆ A MSTI can contain any number of VLANs.
ˆ A VLAN can belong to only one MSTI at a time.
ˆ 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.
ˆ 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 are used to designate a port as
an edge port or a point-to-point port.
The second group can be applied independently on a port for each MSTI
where the port is a member. One of the parameters is the internal path
cost. This parameter specifies the port’s operating cost if it is connected to
a bridge that is a part of the same MSTP region. You can give a port a
different internal path cost for each MSTI where it is a member. This group
also has a parameter for setting port priority, used as a tie breaker when