Failed iBGP

Answered Question
Jul 19th, 2010

All,

Over the weekend, I attempted to convert us from a single router to a redundant router with hsrp. This worked fine, but in the process I wanted to change the currently configured ebgp config between our switch and routers to ibgp.

The problem was that the switch was receiving all of the routes from RtrA, but only 2 routes from RtrB. All of the routers were peered and sessions were established. I can't reproduce this is GNS, so I'm wondering what could've happened. I'm going to eventually have to revisit this because this is the design that I want. Any ideas why this could've happened? I've attached a diagram of how it was laid out. I was always under the impression that a fully meshed ibgp peering would pass all routes between ibgp peers. In my bgp table on the switch, I only showed the routes from RtrA and 2 from B that were external to this AS. I should've had about 180 subnets.

Thanks,

John

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Correct Answer by milan.kulik about 6 years 4 months ago

Hi John,

more details would be necessary about the missing prefixes.

What I can imagine is:

Let's say both RtrA and RtrB are receiving the same prefixes from eBGP.

RtrA is sending all prefixes to RtrB via iBGP.

RtrB has something in the configuration (weight, local preference), which makes the prefixes received from RtrA more preferred than the same prefixes received from eBGP.

In that case, RtrB would NOT advertise these prefixes to the 3750 switch (as only best prefixes are advertised but prefixes received by iBGP are not advertised to another iBGP neighbor).

Wasn't that your case?

BR,

Milan

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Correct Answer
milan.kulik Mon, 07/19/2010 - 08:01

Hi John,

more details would be necessary about the missing prefixes.

What I can imagine is:

Let's say both RtrA and RtrB are receiving the same prefixes from eBGP.

RtrA is sending all prefixes to RtrB via iBGP.

RtrB has something in the configuration (weight, local preference), which makes the prefixes received from RtrA more preferred than the same prefixes received from eBGP.

In that case, RtrB would NOT advertise these prefixes to the 3750 switch (as only best prefixes are advertised but prefixes received by iBGP are not advertised to another iBGP neighbor).

Wasn't that your case?

BR,

Milan

John Blakley Mon, 07/19/2010 - 08:44

You sent me in the right direction. On RtrB, I had forgotten to set my weight higher for the DR router. That led me to get the routes from RtrA and kept me from getting the routes on my switch, or even showing a route from RtrB. I was able to reproduce that, and hopefully it's back to the drawing board again this weekend.

John

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