Spanning Tree Considerations For Multilink Trunks; Figure 1-56. Path Cost Arbitration Example - Nortel BayStack 450-24T Using Manual

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 1-56 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
Path Cost T1 = 4
10 Mb/s
10 Mb/s
Aggregate Bandwidth
220 Mb/s
S2
Figure 1-56. Path Cost Arbitration Example
309978-D Rev 01
T1
BayStack 450 10/100/1000 Series Switches
100 Mb/s
100 Mb/s
10 Mb/s
T2
Aggregate Bandwidth
210 Mb/s
Path Cost T2 = 4
BS45029A
1-93