Nortel Networks 1000BASE-LX, 1000BASE-SX manual Spanning tree considerations for MultiLink Trunks

82Chapter 2 Network configuration

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

S1	BayStack 420-24T Switch	BayStack
		420
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	BayStack 420-24T Switch	BayStack
		420
			10013EA
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