Nortel Networks 1000BASE-LX User Manual

Chapter 2 Network configuration

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The spanning tree Path Cost parameter is recalculated based on the aggregate 
bandwidth of the trunk. For example, 

 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

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