Cisco ASA null0 route question

[bjohnson1224]

We have our headquarters in Japan and they have issued 10.240.0.0/13 subnet for use in the USA.  They are reporting that unused IP's on this subnet are causing loops on their core switch. They are asking us to put a null0 route statement for that subnet into our Cisco ASA 5525 which handles all L3 routing in one of our data centers.

route null0 10.240.0.0 255.248.0.0

My concern is that this will drop the legitimate traffic for this subnet that does need to talk to Japan.

We do route subnets from the 10.240.0.0/13 that are being used by business units across North America out OSPF and across our MPLS.  My question is will this work for the IP's not being used in this subnet to be denied but still allow the legitimate traffic to Japan that is advertised in our OSPF?

Any clarification on this would be greatly appreciated.

Thank you,

B.Johnson

[MHM Cisco World]

I send message check it

[Marius Gunnerud]

Routing works on a longest prefix match.  So you could route a whole /13 to null so long as you have more specific longer prefix routes defined for the relevant traffic.