Nortel Networks 24T manual Spanning Tree Considerations for MultiLink Trunks

Using the BayStack 410-24T 10BASE-T 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 and 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
		T1	T2
Path	100 Mb/s		100 Mb/s	Path
	100 Mb/s		100 Mb/s
Cost				Cost
T1 = 4	10 Mb/s		10 Mb/s	T2 = 4
	10 Mb/s		Aggregate Bandwidth
Aggregate Bandwidth			210 Mb/s
	220 Mb/s	S2
Key

10Mb/s

100Mb/s

BS41062A

Figure 1-47. Path Cost Arbitration Example

The switch can also detect trunk member ports that are physically misconfigured. For example, in Figure 1-48, trunk member ports 2, 4, and 6 of switch S1 are configured correctly to trunk member ports 7, 9, and 11 of switch S2. The Spanning Tree Port Configuration screen for each switch shows the port state field for each port in the Forwarding state.

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