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82 Chapter 2 Network configuration
209418-A
Spanning tree considerations for MultiLink Trunks
The spanning tree Path Cost parameter is recalculated based on the aggregate
bandwidth of the trunk. For example, Figure 35 shows a four-port trunk (T1) with
two port members operating at 100 Mb/s and two at 10 Mb/s. Trunk T1 provides
an aggregate bandwidth of 220 Mb/s. The Path Cost for T1 is 4 (Path Cost = 1000/
LAN speed, in Mb/s). Another three-port trunk (T2) is configured with an
aggregate bandwidth of 210 Mb/s, with a comparable Path Cost of 4. When the
Path Cost calculations for both trunks are equal, the software chooses the trunk
with the larger aggregate bandwidth (T1) to determine the most efficient path.
Figure 35 Path Cost arbitration example
BayStack 420-24T Switch
BayStack 420-24T Switch
10013EA
T1 T2
100 Mb/s
100 Mb/s
10 Mb/s
100 Mb/s
100 Mb/s
10 Mb/s
10 Mb/s
Path Cost T1 = 4
S1
S2
Path Cost T2 = 4
Aggregate Bandwidth
220 Mb/s
Aggregate Bandwidth
210 Mb/s
BayStack
420
BayStack
420