On Tuesday 10 June 2008, Deepak Jain wrote: > In the "old" days, null was handled by CPU (software > switched), so lots of us old-timers got into the habit of > using loopback instead of null. On a modern platform it > should make no operational difference provided you have > everything you need set up properly. (null routing the > /16, for example, might be bad if you ever actually try > to route that whole prefix instead of just subs -- as it > would be specific and static).
How often is this the case, though? If a /16 were routed to a downstream customer, then it'd point to an interface and/or remote IP address. One could "subnet" a /16 on an interface and assign addresses out of that with a 16-bit long netmask, but aside from NAT'ed environments, I haven't quite seen this in production. We null route our aggregates (/19's, /18's, e.t.c.), and originate them on our route reflectors for onward announcement to customers, upstreams and peers. We tag them with a next-hop that sends them to the Null0 interface on edge/peering routers, should any of these destinations attempt to send traffic to directly to them. Mark.
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