Nortel Networks 450 manual Spanning Tree Considerations for MultiLink Trunks

Using the BayStack 450 10/100/1000 Series Switch

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 1-47 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.

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
		BS45029A

Figure 1-47. Path Cost Arbitration Example

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