What happens to traffic/spanning-tree when a link in an etherchannel fails

Unanswered Question
Nov 16th, 2009

Could someone please explain what happens to traffic and spanning-tree when a link in an etherchannel fails. We use 2X 1GB links in an etherchannel on different switches in 3750 stack so use the "mode on" to assign ports to the etherchannel.

I would think if one of the links fails the traffic will need to be resent via the operational link.

Will spanning-tree need to reconverge?

Many Thanks

I have this problem too.
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hobbe Mon, 11/16/2009 - 02:54

I have a couple of etherchannel between different stacks.

The traffic that was on the wire will have to be resent, so if it was UDP or ICMP or so then it would just dissapear and the application would have to resend it.

if it was TCP the tcp protocol or in some instances the switch would resend over the other link. this is done between the switches so I do not se that there would be any change in the Spanning tree.

I normally test the links by simply pulling one of the Gbic, and max I loose is 1 icmp packet.

I have never seen any application hickups or anything like that.

Then I put it back and let the switches stabilize themselves and do the same with the other one in the etherchannel.

it have always been rock solid for me.

the only time i have had problems was when I manually added wrong vlans on the etherchannel.

It did not like that!

HTH

Mohamed Sobair Mon, 11/16/2009 - 02:55

Hi,

This is one of the benefit that Etherchannel provides. The redundancy purpose.

If single physical interface that member of the etherchannel fails, traffic will be redirected to the second physical link and Etherchannel is still Active. Spanning tree doesnt need to reconverge if the Etherchannel is up.

HTH

Mohamed

Peter Paluch Mon, 11/16/2009 - 03:16

Hi Mohamed,

Spanning tree doesnt need to reconverge if the Etherchannel is up.

Not entirely true. Removal of a physical link from an Etherchannel bundle will result in a different bandwidth of the entire bundle and thus in a different link cost of the Port-channel interface. If the Port-channel interface was the root port, the root path cost of the switch will increase. As a result, the spanning tree on that switch or on other switches may select a different root port - this depends on the topology of the network.

Best regards,

Peter

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