eBGP multipath

Unanswered Question
Dec 18th, 2008

Right now I'm using the maximum-paths command under my bgp router configuration to leverage both circuits we have to our Provider. However, the return or receive traffic is not being load balanced at all. Would using loopbacks and multihop to peer with the PE overcome this limitation of load balancing return traffic, or is the same issue apparent with the multihop loopback scenario?

thanks,

Bill

I have this problem too.
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Giuseppe Larosa Thu, 12/18/2008 - 09:31

Hello Bill,

probably your ISP is not using maximum-paths on its side.

The configuration of eBGP multihop with a loopback has the advantage that requires symmetrical configuration to work well but I would first ask them if they have maximum eibgp-multipaths under address-family ipv4 vrf vrf-name.

Hope to help

Giuseppe

WILLIAM STEGMAN Mon, 12/22/2008 - 12:43

The SP said the maximum-paths is in place under our vrf, and that the lopsided usage is probably a result of a few hosts creating flows that consume much of the bandwidth on one link. The flows are load balanced, but if a host downloads a large file over one of the 1.5 mb linksg, that flow won't be broken up. It will stay on the same link. He suggested perhaps using route maps to break up the traffic after identifying the top talkers. I guess I would need to use route maps with access lists and as path prepending. A little ugly. Anyone have any other suggestions?

thanks

Giuseppe Larosa Tue, 12/23/2008 - 03:51

Hello Bill,

if there are few high volume flows you can experience this kind of problems.

I think the provider collegue was thinking of doing PBR after having enabled ip accounting or using netflow to identify the top talkers.

BGP cannot help you in this because it works on IP subnets.

An alternative could be that of enabling per packet load sharing but be aware that there are side effects on some types of traffic like VoIP that are affected by out of order packets.

A second alternative to be considered is that of making a ppp multilink or FR multilink and leaving to these features the load sharing work.

To do this you need to work with the service provider and you will make a single L3 link with two member links.

Hope to help

Giuseppe

Jon Marshall Thu, 12/18/2008 - 09:36

Bill

No, i can't see how that would affect the return traffic path. Multi-hop is purely used when the interface used for the BGP peering is not directly connected.

If you want to influence how traffic is coming in you can look at either

1) MED - where you give a weight to the networks you are advetising out so you advertise alll networks out of both routers but split them in half in effect and on router1 give preference to first half and on router2 give preference to the second half. If one router/link goes down all traffic comes in the other link.

MED is only effective between you and your ISP ie. the preferences you set do not extend beyond the ISP

2) AS Path prepending. This allows you to give preference to networks beyond your local ISP.

Jon

Giuseppe Larosa Thu, 12/18/2008 - 10:03

Hello Jon,

I don't think so I think Bill is referring to a case where you build a single eBGP session on loopbacks and you have two static routes over two parallel links that provide reachability of the other router loopback.

If you do so on both sides you use recursion and you use both links according to CEF or other load-sharing.

My impression is that on the other side the current configuration is not using the correct version of bgp multipaths in af ipv4 VRF vrf-name

I used both options successfully in the past

Hope to help

Giuseppe

Jon Marshall Thu, 12/18/2008 - 10:06

Giuseppe

Ah okay, thanks for pointing that out. I'll leave this to you but i'll keep any eye on it as i might learn something new.

Thanks again.

Bill - apologies if i misunderstood your question.

Jon

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