STP Port Cost and Etherchannel

[dfelter]

How does CatOS modify the STP port cost when using Etherchannel? In my particular situation I have a 4006 (with Uplinkfast configured) connected to a 6509 (which is the STP root bridge) via 4 GbE ports. On the 4006, the port cost for each link in the channel is 1140. With a single GbE link between the 4006 and 6509 the cost is 3004 (as expected). How is the 1440 derived?

[bribrown]

the path cost is calculated as the rounded sum of the individual port costs

(C1, ..., Cn) belonging to the logical port, divided by the number of ports to the 1.7th:

PathCost= (C1+...Cn) / (n^1.7)

So in your case you have:

pathcost = (3004 + 3004 + 3004 + 3004) / (4^1.7)

= 12016 / 10.556

= 1138

rounded up = 1140

[stheman]

Could you elaborate on the beahviour of the channel in a single port failure scenario. For instance, if you have used the "set channel cost" command to set the STP cost for the channel, will this value remain static even if one of the ports within the channel has failed or does it dynamically change and potentially cause a topology change?

[dfelter]

With a port failure, the port cost does not change for any of the ports in the channel. To verify this behavior, I simply disconnected one of the cables in this scenario. The command displayed the same port cost for each of the 4 ports in the channel. Thus, the port cost for a port channel is only changed when the channel is created.