We have two 6500 with two 20-G uplink (using etherchannel to combine two 10-G interface) to two 3750E which are stacked together. Currently one of the 6500 is the root bridge. We are testing failure scenario when one of the 10-G interface in the port-channel on the 6500 or 3750E (in the forward state)go down, which make the etherchannel to become 10-G only, we would like the spanning tree to block this 10-G link and make the 20-G link (the other etherchannel) forwarding.
When we test shutting down one 10-G link in the forwarding etherchannel, the path cost go from 1 to 2, but the port is still in forwarding.
Any suggestion that we can make the 20-G etherchannel to become forwarding while the 10-G will be blocked automatically?
I would advise to use spanning-tree path cost long mode
As Roman said, in case of failure of a link in the etherchannel, the root will remain root.
> Election root bridge
root Bridge ID
If the link between the 2 x 6509 is also 20Gig and 1 uplink fails in the uplink between root and 3750, then the port of the 3750 will be blocked towards the root. This means that the traffic will go via backup root over the crosslink towards the root
I assume your link between two 6500's is cost of 1. When one of your uplink's port cost changes from 1 to 2, you now have two equal cost paths:
1. Stack - 10G (cost 2) - Root 6509 = Path Cost 2
2. Stack - 20G (cost 1) - Backup Root 6509 - 20G (cost 1) = Path Cost 2
In an equal cost path scenario, the bridge (3750 stack) will make a decision based on the lowest bridge ID of the neighbor. In your case, Root bridge 6509 has lower BID than Backup Root 6509, therefore it will continue to forward on the 10G uplink. One way you could solve this is manually increase port cost of all 10G interfaces on the stack from 2 to a higher value.