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How EIGRP calculates distance when a BGP route is redistribed into?

I have two equal capacity links from internal network to the outside carriers. BGP is running between my edge routers and the carrier routers (single link from each edge router to its carrier router). BGP is redistributed into EIGRP on the edge routers.

I'm using the same filtering on the edge routers for BGP to EIGRP redistribution. There is no routing difference from the edge routers to the internal network.

However I just noticed that traffic is not equally distributed between the two edge routers to the carriers. One link has obvious heavier traffic than the other. What would be the cause of this?

What BGP attributes would affect the EIGRP route selection when a BGP route is redistributed into EIGRP? How about redistributing into OSPF?

Thanks a lot

Gary

4 REPLIES

Re: How EIGRP calculates distance when a BGP route is redistribe

Gary

I think this is a problem (and a normal behaviour) due to the type of load balancing by default configured on each router (internal or edge). If you have a external route coming from two edge routers with equal cost and it gets installed in the routing table of an internal router. When this router routes packets using this particular route, it will use perdestination by default, which causes one of the two edge routers to be chosen as next-hop on a per-destination basis. This may cause unequal load distribution across the two routes. More than the routing protocol, i feel this is dependent on the type of load balancing configured on the outgoing interface of a router.

HTH

PS: Please remember to rate helpful replies!

Silver

Re: How EIGRP calculates distance when a BGP route is redistribe

Load balancing, especially in BGP implementations, needs to be looked at from both an incoming and outgoing perspective independently. Is this heavier load of traffic coming from the outside in (receive), or the inside out (transmit)? Your internal routers might be trying their best to load balance traffic leaving your AS. But what about the traffic entering your AS? Have you considered how the outside world sees the reachablility to your domain? Could it be that BGP routers are seeing only one carrier as the best path to your AS?

Take a look at this website:

http://www.traceroute.org

Here you will find links to internet route servers where you can query routes to your AS and see what various major providers see as the best path to your domain.

HTH. Please rate this post.

Thanks,

Brad

New Member

Re: How EIGRP calculates distance when a BGP route is redistribe

I looked at each direction (in and out). Most of the inbalance happens at the direction coming in from the carriers. Traffic going out is not balanced too, but in a much smaller scale.

I took one outside destination as an example and here is the "sh ip bgp ..." output on both edge routers:

rtdiftc01#sh ip bgp 165.30.199.1

BGP routing table entry for 165.30.199.1/32, version 90786

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Not advertised to any peer

65098 13979 65123 65115

rtdiftc02#sh ip bgp 165.30.199.1

BGP routing table entry for 165.30.199.1/32, version 1417

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Not advertised to any peer

65099 65000 65122 65115

So there's equal number of AS paths to this destination from both edge routers. I'm not sure about the AS paths for a route coming in from outside. Have tried the traceroute.org, but not getting too meaningful results.

These are MPLS connections through carriers, so unlike through the Internet, the paths on both directions should be the same and reliable, I would suppose.

Am I thinking right? How to check the paths for routes coming in?

Thanks

Gary

Silver

Re: How EIGRP calculates distance when a BGP route is redistribe

If this is a private MPLS network then traceroute.org will not help at all. Have you looked at this traffic with a sniffer and assesed what IP address(s) the bulk of the traffic is destined to in your network? From there you can check with your providers to trace how this traffic is entering your network.

Also, any network diagrams you can post illustrating your observations would help.

HTH. Please rate all helpful posts.

-Brad

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