BGP Query

Unanswered Question
Aug 12th, 2008

Hi Guys,

Being a little new to 'production' BGP i have a scenario at my company that confuses me. We have 5 ibgp peers and 2 ebgp peers at two different NAPs.

The ibgp peers advertise routes to either other using a mix of network statements, null0 routes and redistribution.

Our ebgp speakers get their routes form the ibgp peers. Now, the ebgp speakers have a number of peering relationships each. However, they do not appear to advertise any routes. They do not have network statments, null0 routes or otherwise. All the have is a few peer groups and distribute lists/prefix lists.

My question is, how do the external peers get updates from our ebgp speakers if the network statement is not present under the BGP config on each ebgp speaker?

Thanks

A confused stephen :)

I have this problem too.
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Richard Burts Tue, 08/12/2008 - 08:42

Stephen

Perhaps there are aspects of your situation that I do not yet understand well. But it seems to me that based on your description your IBGP routers are injecting routes into BGP (advertising into your AS) using a combination of network statements, null0 routes, and redistribution. So your routes are injected into BGP. Your EBGP speakers are learning these routes from their internal peers and are then advertising those routes to their external peers.

If your EBGP speakers are learning your BGP routes from the internal speakers then there is not a need for the external speakers to also have the network statements or redistribution. Once the routes are injected into your BGP AS then your external speakers will learn them and will advertise them to external peers.

HTH

Rick

stephen.stack Tue, 08/12/2008 - 09:11

OK, this makes sense. And what you are describing tallys with my setup. With that said, i need to filter inbound and outbound updates. I assume that this is done at the AS edge i.e. at my ebgp speakers and not at my ibgp speakers

Regards

Stephen

lamav Tue, 08/12/2008 - 10:12

Stephen:

If I may insinuate myself for a moment, I would like to remind you that synchronization must be enabled if eBGP is depending on iBGP for routes to prefixes that it will then advertise itself.

HTH

Victor

Richard Burts Tue, 08/12/2008 - 11:02

Stephen

Yes, in the way that it normally is done the filtering of inbound and outbound updates is done at the EBGP speakers on the edge of the AS. It makes less sense to try to filter BGP updates on routers that are doing just IBGP.

One important aspect of filtering outbound updates for most customers running BGP to multiple providers (multi homed customers) is to filter the routing update and to prevent advertising to one external peer routes that you learned from a different external peer. Another way of saying this is that you filter outbound so that you only advertise routes that originate within your AS.

HTH

Rick

stephen.stack Tue, 08/19/2008 - 05:51

Hi Guys,

Just to let you knwo that i now understand why and how ebgp advertises route to peers. I also have carried out some filtering on inbound and outbound routes from a new peers very successfully.

Thanks for your help

Stephen

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