Etherchannel or multiple L3 links??

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Feb 1st, 2010
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Hi All- I'm trying to figure out which setup would provide more even load balancing across multiple links. Let's say i have a core and distribution switch connecting via multiple links and run OSPF between them. Would it be better to create an Etherchannel and run OSPF on this aggregated link or configure individual L3 links and let OSPF load balance across. Any PRO or Cons with either aside from wasting few extra /30s for l3 links

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Jon Marshall Mon, 02/01/2010 - 11:41
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singh.andy wrote:


Hi All- I'm trying to figure out which setup would provide more even load balancing across multiple links. Let's say i have a core and distribution switch connecting via multiple links and run OSPF between them. Would it be better to create an Etherchannel and run OSPF on this aggregated link or configure individual L3 links and let OSPF load balance across. Any PRO or Cons with either aside from wasting few extra /30s for l3 links


Depends on whether you are talking about L2 or L3 etherchannel. If L2 personally i would go with routed links because this keeps STP out of the core which i believe is a good thing to do. If you needed to span vlans across the core then obviously thay wouldn't work but you should be able to avoid.


If L3 etherchannel not so clear cut. Etherchannel always uses the same link for the same src/dst pair which can lead to very uneven use of the links. I don't believe that per-destination load-balancing shares the same restriction. So a lot depends on your traffic distribution.


Jon

sachinraja Mon, 02/01/2010 - 11:44
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Hi Andy


If it was a Layer 3 swtich with end to end links, i would consider etherchannels.. When bundling with etherchannels, you would basically have a single routing adjacency with your peer..when any l3 link is unstable, etherchannel technology makes sure the packets are routed thro other link, instead of resetting routing adjacencies with peer. with pure l3, ospf/eigrp neighborship is reset individually.. apart from this, etherchannels load balancing mechanism can be used to a good extent... as you said before, we also save on some /30 IPs which can be used for other purposes... just make sure your IOS does not have any open bugs related to etherchannels/PAGP etc..


in any case, if you have to manage links individually, or to analyze each link seperately, then direct L3 links are a good option.. even with etherchannels, you can see the bandwidth utilization of seperate links, but on the network management system, it is easy to map, if it is a direct L3.. so, its all upto each company's standard...


Hope this helps.. all the best


Raj

singh.andy Mon, 02/01/2010 - 11:53
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thanks..This will be between juniper and cisco..potential for some link aggregation incompatibilties..also juniper doesn't let you configure which algorigthen to use for load balacing on an aggregated link. L3 links might be a better choice here to aviod any etherchannel issues between different vendors..

sachinraja Mon, 02/01/2010 - 11:54
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Yes.. L3 link is the way to go then.. We thought it is a cisco-cisco L3 link, and etherchannel works like a charm.. If you already knew of issues between cisco-juniper, you should take the safest approach.. ie thro L3...


Raj

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