Nortel Networks 350 manual Spanning Tree Considerations, Path Cost Arbitration Example

BayStack 350 10/100/1000 Series Switches

Spanning Tree Considerations

The spanning tree Path Cost parameter is recalculated based on the aggregate bandwidth of the trunk. For example, Figure 1-39 shows a four port trunk (T1) with two port members operating at 100 Mb/s and the other two port members operating 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). If a second three port trunk (T2) is configured with an aggregate bandwidth of 210 Mb/s, with a comparable Path Cost of 4, the switch software chooses the trunk with the larger bandwidth (T1) to determine the most efficient path.

S1
100 Mb/s	100 Mb/s
100 Mb/s	100 Mb/s	Path Cost T2 = 4
Path Cost T1 = 4
10 Mb/s	10 Mb/s
10 Mb/s
T1	T2
Aggregate Bandwidth	Aggregate Bandwidth
220 Mb/s	210 Mb/s
S2
		BS35029A

Figure 1-39. Path Cost Arbitration Example

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