eBGP / iBGP Behavior question

Unanswered Question
Jan 29th, 2009

Hello,

I currently have a iBGP full mesh of 6 routers in my internal network. My 2 edge routers have 1 upstream ISP connection each.

My question is, if I learn a prefix, for example 7.0.0.0/9 from each of my eBGP peers, the edge routers that have those peers on them will each have their own route for it from the local eBGP peer, but 1 of them will not have the route from the other eBGP peer, is that normal behavior?

Taking that further, the iBGP only speakers in the middle will only have the best path, is that normal also?

I am just trying to nail down how the bgp table should look on my 2 edge routers and my internal iBGP only routers for a specific prefix, and assuming nothing special is configured, it would be the same for all other prefixes, depending on which eBGP peer was the better option for certain prefixes.

Thank you

I have this problem too.
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rais Thu, 01/29/2009 - 07:37

It is normal. On your edge [or other routers], you will only see the route that is the best.

Thanks.

Istvan_Rabai Thu, 01/29/2009 - 09:39

Hi Jason,

On the edge routers you should see the 7.0.0.0/9 prefix 2 times:

- one as an external route from the ISP

- one as an internal route from the other edge router

On the internal IBGP peers you will see only the best routes from each edge router because of this rule:

BGP advertises only those routes that it uses.

Those prefixes not selected into the routing table on the edge routers will not be advertised to IBGP peers.

Cheers:

Istvan

Jasonch518_2 Thu, 01/29/2009 - 09:52

lstvan,

The iBGP peers only having the best option for a destination makes perfect sense to me, and they do have that.

The eBGP edge routers are the ones I believe I am confused on. On one of the edge routers, we will call it router A, it shows the router to 7.0.0.0/9 from its local peer, but not the route from the other edge router's peer, router B, the other edge router, has the route from it's local peer, and the one from the other edge router.

Now that I am thinking about it, it must be that it has both because the local one on edge B is not the best option, so it will learn the one from edge A, but edge A has the best option locally, so edge B did not announce his option to edge A over iBGP because it is not installed in the routing table.

Going further thinking about the rule you mentioned, that would make sense, seem right to you?

If it worked the way where both of the edge routers had both routers, then scalability would be an issue, considering if you had say 5 full internet bgp tables on all of your edge routers, you would run into memory problems etc. That would go back to what you just said, a edge ebgp router should only have double paths if its local ebgp path is not as good as the one it learned through ibgp, from something like AS_Path etc.

Thanks for your thoughts, I think I am back on the right page now.

Istvan_Rabai Thu, 01/29/2009 - 10:01

You're welcome Jason.

I suggest you to make configs on BGP to see what really happens.

All we're talking about is just theory until you see it in a router.

Istvan

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