3Com PathBuilder S200 Series Owner's Manual page 96

Spanning Tree Protocol Entity (STPE)
Consider the
Nature and
Expected Number
of SVCs
92
A further consideration for setting path cost is the nature and expected number of
SVCs that the bridge link uses to achieve its connectivity and adjust the value of
incremental path cost accordingly. For example, consider the topology shown in
Figure 45.
Node B
X25-2
Bridge 2
Node C
Figure 45. Bridge Links Within Network
Bridge 2 is linked to bridge 1 by two separate links. The link through Node B must
traverse two separate SVC hops while the direct link has only a single hop. If the
speed of the lines are all equal, then the cost for the bridge link through Node B
should be higher than the cost for the direct link.
On the other hand, if the amount of traffic (due to sources other than bridging traffic)
causes added delay for the direct route, or if the direct route has a lower speed, it
might be better to use the direct route as a backup. Therefore one would set the cost
for the direct route higher than for the Node B route. For the multipoint line
connecting bridges 2, 3, and 4, the cost for each link should be increased in
proportion to the amount of bandwidth-sharing involved. This also accounts for the
fact that a slave node such as Node D or Node E must wait to be polled before it can
pass data to its master, thus adding some extra delay.
Bridge 1
X25-1
Bridge 3
Node D
Node A
X25-3
MX25
Bridge 4
Node E
Bridging