BGP and OSPF Asymmetry

I have a network where I have traffic coming in from various AS No's into a Core AS call it AS 100.

At the bottom end I have 2 x links out of AS 100 into a Co-Lo using a private AS 65001. All remote AS's have to traverse AS 100 to reach my Co-Lo.

In my Co-Lo I have 4 x routers. The WAN element (2 x routers) connects AS65001 to AS100. On the inside of my WAN routers I have LAN connections to a pair of 6509's (one is active for all HSRP addresses). The connections between WAN and LAN devices are crossed over so each WAN router connects to each 6509. These connections are configured as /30 links. There is no iBGP internally just equal cost path OSPF links.

My problem is that OSPF is set to load share traffic back out to the remote locations. As traffic can come in down either WAN link, how can I get OSPF to route it back the same way.

I need a way to tag routes coming into AS100 differently based on my own scheme but route-maps don=92t allow this. How can I ensure that OSPF understands which BGP route my traffic has entered on, to allow it to route back the same way. If I can=92t find the answer I will just have to live with the probability of asymmetric traffic.



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Darren Green
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Maybe I don't have a full picture of what you're doing, but does the asymmetry matter? Are you transporting voice traffic? If I understand, you want the internal WAN links to send the return packets of a given flow over the same ingress pipe (from the co-lo's perspective). I'd say that if it is an issue of transporting traffic that would be sensitive to reordering, perhaps ensuring per-flow switching would be the way to go. Otherwise, I'm not sure worrying about the return trip is an issue unless you have some other management policy you need to adhere to, or as I just mentioned, packet reordering (non-TCP) is of concern

Sorry if I totally misunderstood the issue!

Good Luck!

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Thanks for the response.

You were spot on in your understanding. I had sort of got to the point that the asymmetric traffic would be OK, I suppose many networks have such an element anyway. Just wanted to avoid it if possible.

There is no voice at present but there is some Citrix which I believe will be OK in this type of environment. If voice comes along I will likely do some prepending and OSPF manipulation to ensure that the traffic routes in and out the way I want it.

Kind regards


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