Etherchannel across 2 x L2 circuits but each circuit uses a different ISP - Is this possible / recommended?

[twhittle1]

Hello,

I currently have 2 sites with 2 x 1Gbps connections between them. The connections are layer2 - LAN extension services, however they are provided by 2 different ISPs for resiliency.

My question is, can I make these two links into a L2 Etherchannel? And would there be any negative implications of doing this?

When I think about it I the latency will be different depending on which path the traffic take, I.E. one ISP might have a higher latency than the other. This may impact applications which use UDP or cannot tolerate variation i latency, however I think we can get around this by using source / dest IP based load balancing, so all traffic between two IP addresses will always take the same path. Is this a valid assumption?

I can't think of any other issues with doing this. Can anyone help? Is this a valid thing to do?

Many Thanks,

Tom Whittle

[Joseph W. Doherty]

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Posting

My question is, can I make these two links into a L2 Etherchannel?

99.999% no.

And would there be any negative implications of doing this?

If it could, unlikely to offer any real benefit. What do you think Etherchannel will do for you?

however I think we can get around this by using source / dest IP based load balancing, so all traffic between two IP addresses will always take the same path. Is this a valid assumption?

That depends on the hashing algorithm, but generally a single flow will only use one link and normally use the same one, every time, every thing else being the same.

Is this a valid thing to do?

Generally no because the bundled links need to be on the same device.

[twhittle1]

Hi Joseph,

Many Thanks for responding.

You mention that it's not possible to do this but I don't understand why? Can you elaborate?

There is only one device at each end connecting to both circuits and the circuits are seen just as a layer 2 connection between the sites. In my head I cannot see why this wouldn't work but I may well be missing something.

We are hoping to take advantage of the extra bandwidth. I appreciate 2 x 1Gbps in an Etherchannel is not 2Gbps link but it will offer a degree of extra bandwidth.

I agree with your points about a single flow always using the same link, but different flows could use different links so I think there is definitely a benefit to be had I just have never done this before and am keen not to play about on a live network without doing some research first :)

Many Thanks,

Tom

[Joseph W. Doherty]

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Posting

The reason it normally would not work, you normally need hardware to tightly bind multiple links into one logical L2 link.  The moment you mentioned two different ISPs, you're likely going to be unable to bind the ISPs' side onto one device.

Yes, Etherchannel offers additional aggregate bandwidth, but so does using multiple links with L3.  If you're just looking to utilize extra bandwidth, Etherchannel doesn't ofter much, if anything, over an L3 approach.

If what I say is true, if you're wondering why use Etherchannel at all, well it has some other advantages over L3 multiple path.  Often, link failover is "better", as it doesn't disturb a L3 topology with a link status change.  Generally it always done in hardware, while L3 multiple path forwarding might be done in software.  It doesn't "add" to the L3 topology or L3 maintenance load.

Today, if your equipment supports it, PfR can dynamically chose the best performing link to a destination and/or dynamically load balance links, all at L3.

[mfurnival · Accepted solution]

I don't see why this would not work if both links terminate on 1 device at either end - the devices don't care what goes on inbetween.

I would question why you want to do this though? Is there a specific requirement to extend the layer 2 boundary between these two sites. If not I would make them routed links and use equal cost load sharing to utilize both links.

[twhittle1]

Hi Both,

Thanks for your responses. I've been asking internally in tandem and we are doing this elsewhere so it is possible but there are number of considerations:

link speed, duplex and interface type.

As you say Layer 3 interfaces may be a better way to do but we do have the need to extend the L2 network across sites so this is what we have.

Thanks again for your responses.

regards,

Tom

[Joseph W. Doherty]

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Posting

- See more at: https://supportforums.cisco.com/discussion/12486551/etherchannel-across-2-x-l2-circuits-each-circuit-uses-different-isp-possible#sthash.89WEWXSy.dpuf

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

- See more at: https://supportforums.cisco.com/discussion/12486551/etherchannel-across-2-x-l2-circuits-each-circuit-uses-different-isp-possible#sthash.89WEWXSy.dpuf

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

"I've been asking internally in tandem and we are doing this elsewhere so it is possible ..."

With different ISPs terminating the other side of the same Etherchannel?

Ah, unless you mean, you have two different MetroE service providers that are providing logical L2 links between your sites, and its your site equipment, same device each side, that is terminating the Etherchannel.

If that's what you're doing, yes that can work.  The only thing you need to watch for, some MetroE services will filter special L2 control frames.  So for example, LACP might have issues, such as it won't dynamically build the Etherchannel, but if so, manual mode should work.

I thought you wanted to terminate Etherchannel with two different ISPs, themselves.

I've been asking internally in tandem and we are doing this elsewhere so it is possible - See more at: https://supportforums.cisco.com/discussion/12486551/etherchannel-across-2-x-l2-circuits-each-circuit-uses-different-isp-possible#sthash.89WEWXSy.dpuf

[twhittle1]

Ah sorry I think I've miss used terminology and misled you. 

Thanks again for your replies, it's been an interest learning piece.

Regards,

Tom

[Joseph W. Doherty · Accepted solution]

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The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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Posting

Not a problem.  Your usage of ISP (Internet Service Provider) is what really threw me.  I didn't pick up on the significance of "LAN extension services".

Now that we got that straight, not only can you make two such links into an Etherchannel, it can make sense to do so.  (For reasons I've already posted.)

Regarding your other concerns, Etherchannel doesn't split a flow's frames across links, so that should avoid almost all problems.  Choice of hashing algorithm only impacts how well Etherchannel distributes flows between the links.

[Richard Clayton]

I had this the other day as a customer requirement, the two circuits were VMB and Vodafone.  The Vodafone circuit blocked LACP so that was the deciding factor to not go ahead.  I ended up configuring them as active standby with spanning tree.  Interestingly VMB block BPDU's by default but they will forward them by request, basically they have a tick box on the GUI management of their NTU that they need to tick and apply, enabling this does cause an outage on the VMB NTU.

[twhittle1]

Hi Richard,

Thanks for the comment, it's always really useful to get actual real world experience.

Cheers,

Tom