cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
341
Views
0
Helpful
3
Replies

how to combine etherchannel and stp

martin.alt
Level 1
Level 1

Hello all,

I have a question regarding the combination of etherchannel and stp.

My goal is to have four 3560G Switches connected in a ring, with 2GBit/s links between the neighbouring switches. I have connected the switches by channeling two 1GBit/s links for each connection using LACP. Spanning Tree is also working fine, disabling one of the etherchannels to break the ring.

However, I really do need the 2Gbit/s bandwidth (or at least more than 1GBit/s). So if one physical link fails, I would like the whole etherchannel to fail, so that stp would detect a link failure and activate the backup link. But what happens is that if one physical link fails the etherchannel is just reduced to a capacity of 1GBit/s, but still has a link.

Any ideas how to configure the setup I described?

Thanks,

Martin

3 Replies 3

Istvan_Rabai
Level 7
Level 7

Hi Martin,

When one of the links fails within an Etherchannel, the spanning-tree cost of the Portchannel interface should increase automatically and the switchover should happen to the backup link.

If not, you can tune the STP port costs with the "spanning-tree vlan x cost" command under the Portchannel interfaces.

Cheers:

Istvan

Hello Istvan,

I was thinking of this but the topology is a ring so the STP cost increase can be not enough to switch to blocked link (likely going via more switch hops) it is a question of position of the fault and of the root bridge.

Hope to help

Giuseppe

Hello Istvan, hello Giuseppe,

thank you for your quick replies!

In fact you are both right - when a link goes down, the path costs do slightly increase (from 3 to 4) - which is enough for switches that are far away from the root, but does not make links block close to the root bridge.

So now my question is how I can influence the impact on the portchannel path costs that a single link failure has.

Thanks,

Martin

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community:

Innovations in Cisco Full Stack Observability - A new webinar from Cisco