I have a /18 in RIB but I would like advertise smaller subnet in BGPfor specific reason to diff ISP etc. Is there a way to do it without creating a null route or static route. Is some way in policy statement I can do this advertisement by matching against the bigger subnet or something?

  • 1
    Do these smaller subnets exist in your IGP? Is there any specific reason why you don't want to use a nullroute?
    – Teun Vink
    Nov 23 '18 at 14:32
  • I have a /16 actually on a whole so it is too many /24s to have a null route.
    – user88975
    Nov 23 '18 at 16:56
  • And they are not in IGP as well. All Traffic just goes to firewall...
    – user88975
    Nov 23 '18 at 17:08
  • I am not sure I get this right. So with a static or null there is no way is that right ?
    – user88975
    Nov 24 '18 at 3:23
  • As I said in my answer, a route needs to exist to be able to advertise it. Either because it's in your IGP, or because you add statics or nullroutes. I fail to see why adding those routes is such a challenge though.
    – Teun Vink
    Nov 24 '18 at 12:07

The typical way to solve this would be to create static routes or nullroutes which you then advertise to your upstream ISP's. You need to have a route for them in your routing table to be able to announce the prefix to your peers.

However, you don't need to deaggregate your /16 into /24's. You can use larger networks (/21's for example), or you can choose to announce the /16 as the overlapping aggregated prefix and select a few /24's for which you want to advertise a different preference.

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