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New Member

WAN etherchannel

I have 3750 stacks on both sites to terminate 2 x 100MB WAN links. I will configure etherchannel for the two links so that we have 200M bandwidth.

1) As the WAN link down will not indicate down to the switch port, will LACP help to detect the link failure? Or should I use other protocol?

2) I want to run layer 3 routing between the two sites. Will the layer 3 etherchannel working fine in this case?

Thanks a lot.


Re: WAN etherchannel

Since you have a Cisco switch on both sides I would recommend the following:

1. Keep the links at "auto-negotiate" to ensure the Layer-1 negotiation will detect a link failure.

2. Enable UDLD Aggressive mode on the links.  This uses a Layer-2 protocol, so even if the link appears ok if either switch can not send on the link, UDLD will disable the interface.

3. Since your using an etherchannel, I would recommend using LACP with the "channel-group x mode active" command on the interfaces.

You can read about UDLD here:

And Etherchannel modes, here:

I hope this helps.


New Member

Re: WAN etherchannel

Thanks Dan for your information.

The connection path is Switch-Modem-Link(100M)-Modem-Switch.

The switch is connected to the modem of the WAN link, I think the auto-negotiate may not detect problem from another switch. Also, Will the UDLD work in this situation? Switches are not connected directly.

Bye the way, both of your links points to UDLD documents.

Any other idea?


New Member

Re: WAN etherchannel

You could use BFD since you want to run routing and this will solve your link down detection issue but the downside is I think BFD can only be used with a Routing protocol. You could also use some other EEM scripts to ping the other sides ip address and after a certain amount of time probably bring down the line protocol though I have never tried this and am not certain if it works. Curious to know the reason why the modem is there if this is a ethernet handoff? Thx

New Member

Re: WAN etherchannel

Hi Vdadlaney,

The ISP provide the 100M link in this way. The modem belongs to ISP. My link is connected to the mode.

Thanks for your advise and may I clarify the following:

1) As I am using layer 3 etherchannel, I am afraid BFD only will detect the failure when both 100M links are down.

2) If only one of the link is down, if etherchannel cannot detect the link down, it will continue to follow packet to the failure link, which will loss data. Do you think BFD can handle this?

I really hope LACP will detect the link failure in layer 2 and forward packets to the good link only. Or do you have other idea?


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