Etherchannel / Trunk fibre lengths

[Jonathan Axford]

Hi All,

An interesting little question for anyone out there who cares to have a go...

We have been discussing the use of Etherchannels (Cisco) Trunks (HP) and have come across an issue that we can't seem to find an answer for.

It may be a silly question, and i am sure it will still work, but what we want to know is:

When using Port channelling (Either Etherchannel or LaCP etc) is it necessary to have the same length fibres on each leg of the connection?

I.e Etherchannel consists of 2 interfaces. One connected via 500m+ fibre, the other via 200m Fibre...

Any thoughts on this???

[Thomas Joebstl]

I've been running a LACP trunk on a pair of single-mode fibre connections with ~8km difference in length using a pair of 2524's (later on some 26xx's) and some external media convertors.
Didnt notice any issues.
Just make sure you're using the proper media and convertors for your 500m+ connection since GBICs are limited to 550m on multimode fibre.

Regards,
Thomas

[Matt Hobbs]

Depending on the the SA/DA hash, the packets are sent down one link only at a time. Because of this I can't imagine any negative impact that would be caused by having different length connections.

Also considering that they're travelling at or close to the speed of light the difference in delay would be very very minimal.

Good question though. There was a recent bug fixed where the traceroute speeds reported on ProCurve switches gave results that were off by one decimal place - effectively it was reporting these packets to be travelling faster than the speed of light!

[Jonathan Axford]

Thanks for the input guys.

The reason i ask is because we need to set up some redundancy in the link between our 2 data centres.

We weren't sure whether to use trunks that used the same physical fibre or whether to split them out over diverse routes...

I think the plan now is to have 2 seperate trunks, each trunk using a different physical route and let STP block the longer of the 2.

Thanks again

Jonboy

[Jonathan Axford]

Closing this one now...