BGP Static Route to Null0

CChN

Suppose you have two routes in your IP routing table you want to advertise via BGP:

192.168.1.0/24
192.168.1.1/32

So, according to the 'CCNP ROUTE 642-902 Portable Command Guide', you can advertise 192.168.0.0/16 and then create a static route for 192.168.0.0/16 destined to the null0 interface.

I understand why the static route needs to be created, but fail to see the usefulness of such a design.

Why advertise a larger block if we only have select destinations within that block? I know this example is an extreme case, but hard-coding each specific destination prefix seems like a safer thing to do to avoid dropping packets we have no routes for.

Right?

networker050184

With BGP you are usually just going to advertise your largest block up stream. It all needs to come to your AS anyway and your edge router will have the more specific routes. If there isn't a more specific route then it just goes to null there and doesn't waste any more processing in your network. So if you are assigned 192.168.0.0/16 thats all you need to advertise. A provider isn't going to accept anything smaller than a /24 anyway.

There will be some cases where you will want to break it up and advertise certain blocks here and there for traffic engineering, but thats getting into more complicated scenarios than ROUTE really covers.

Heero

CChN wrote: »

Suppose you have two routes in your IP routing table you want to advertise via BGP:

192.168.1.0/24
192.168.1.1/32

So, according to the 'CCNP ROUTE 642-902 Portable Command Guide', you can advertise 192.168.0.0/16 and then create a static route for 192.168.0.0/16 destined to the null0 interface.

I understand why the static route needs to be created, but fail to see the usefulness of such a design.

Why advertise a larger block if we only have select destinations within that block? I know this example is an extreme case, but hard-coding each specific destination prefix seems like a safer thing to do to avoid dropping packets we have no routes for.

Right?

When you advertise that larger block, it is assumed that you own the ENTIRE block, and not just the specific blocks. As such, the router doing the advertising of that larger block will need to have EVERY more specific network that falls within that supernet in it's routing table, or it will drop packets going to valid destinations. If you only have select destinations within the larger block, all traffic that matches the larger block but not the specific routes you have will be dropped.

It is really just a manual form of route aggregation, instead of using the aggregation method built into BGP.