Null0 with BGP

[4everlearning]

Let's say I have CEs connected to 2 different Networks (Network A and Network B), Network A and B are not connected to each others as far as i know.

The CEs connect to each network using ebgp.

Now is it possible to use null0 to advertise /24 to Network A, and then use null0 to advertise /29 (from the same subnet which was advertised to Network A) to Network B?

Would that create any issue?

Thanks,

[Naren K]

I do not see any issue as long as CE knows how to treat the return traffic from respective networks.

By adding two static entries to null0 for /24 and /29 you are creating two network entries for same subnet.

It’s ideal approach when you have to advertise limited number of IP's from said subnet to network B.

Happy to help

Rate if you find answer is helpful.

[milan.kulik]

Hi,

no issue with advertising this way.

(You will just need to add some filtering to assure only /24 is advertised to Network A nad only /29 is advertised to Network B.)

But does this really make a sense?

When the traffic would be sent from Network B to that /29 subnet and received by your router, it would be dropped (routed to Null), unless there is some even more specific route configured on your router.

Is that really what you want?

Wouldn't it make a better sense to configure some real next-hop for the static routes?

Best regards,

Milan

[4everlearning]

Thanks guys,

Milan,

why would the traffic from network B be dropped when received on the routers ? is it because of the /24 null0 configured on the router? 

and yes, there is more specific routes configured on the router. 

Thanks,

[milan.kulik]

Hi,

you are advertising /29 to Network B due to a static route to Null0 configured on you router.

So in a case your router receives a packet with that /29 destination sent from Network B, the router will drop it.

Unless there is some /30 or more specific route present on your router for the same destination.

Am I clear now?

Best regards,

Milan

[4everlearning]

My confusion here is that, same CEs connected to different networks. however, the CEs don't run MPLS or have VRFs configured. 

wouldn't both Null0 statements (/24 and /29) be in the same routing table on the CEs? and if so, would that create any problem? 

Thanks again, 

[milan.kulik]

Hi, I see no problem with two overlapping prefixes being routed to the same next-hop (even Null). Best regards, Milan