Allied Telesis AT-GS950/48, User Manual

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AT-GS950/48 Web Interface User Guide
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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 MSTIs. 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 MSTIs in which 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 MSTIs, 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.