Packets from net 10 (no, not the lyrics)

Kevin Oberman oberman at
Tue Sep 23 15:22:35 UTC 1997

> Date: Tue, 23 Sep 1997 10:45:19 -0400
> From: "Randall S. Benn" <rbenn at>

> I think you'll find that your router's CPU will be happier if you just dump
> the 1918 networks to the bit bucket on your border routers with a static
> route via interface Null0:
>    ip route null0
>    ip route null0
>    etc.
> Considering resource utilization on the router, it is cheaper to do a
> routing table look-up than it is to do ACLs.  Also, when you're doing
> outbound filtering on the router, you have to do a routing table lookup
> first before you can do outbound filtering.  Save a step and just do the
> routing table lookup.

I don't think so. The static routes will require processing every
packet destined for the and nets, but you will
still have the bad route. The CPU will have to deal with any traffic
for and any interior routers will forward packets since you
have a route.

On the other hand, a filter on the BGP session will block the route
from being accepted and only require CPU action once...when it is
announced. You have no route to these nets and can't propagate the
routes since you don't have them.

Andrew clearly has the correct approach.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634

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