how to combine etherchannel and stp

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Jun 16th, 2009
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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?



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Istvan_Rabai Tue, 06/16/2009 - 05:37
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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.



Giuseppe Larosa Tue, 06/16/2009 - 05:41
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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


martin.alt Tue, 06/16/2009 - 12:43
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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.




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