Allied Telesis AT-S63 - Manual (Page 256)

Chapter 23: Multiple Spanning Tree Protocol
256 Section V: Spanning Tree Protocols
Ports in Multiple MSTIs
A port can be a member of more than one MSTI at a time if it is a tagged
member of one or more VLANs assigned to different MSTI’s. In this
circumstance, a port might be have to operate in different spanning tree
states simultaneously, depending on the requirements of the MSTIs. For
example, a port that belongs to two different VLANs in two different MSTIs
might operate in the forwarding state in one MSTI and the blocking state in
the other.
A port’s MSTI parameter settings are divided into two groups. The first
group is referred to as generic parameters. These are set just once on a
port and apply to all the MSTI’s where the port is a member. One of these
parameters is the external path cost, which sets the operating cost of a
port connected to a device outside its region. A port, even if it belongs to
multiple MSTI’s, can have only one external path cost. ANother generic
parameter designates a port as an edge port or a point-to-point port.
The second group of port parameters can be set differently for each MSTI
in which a port is a member. One parameter, the internal path cost,
specifies the operating cost of a port when it is connected to a bridge in
the same MSTP region. The other parameter in this group sets the port
priority, which acts as a tie breaker when two or more ports have equal
costs to a regional root bridge.