Spanning tree considerations for multilink trunks, Figure 35, Path cost arbitration example – Nortel Networks 1000ASE-XD User Manual

Page 82: Spanning tree

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Chapter 2 Network configuration

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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

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