[c-nsp] Incarnations of 0.0.0.0

[Sascha Pollok]

Hello William,

> 2) Routing to 0.0.0.0 does not do what you may think it does. That is
> because CEF maintains a receive entry for 0.0.0.0/32
>
> ex:
> ip route 0.0.0.0 0.0.0.0 172.16.1.1
> ip route 10.0.0.0 255.255.255.0 0.0.0.0
>
> This will cause all traffic except the 10.0.0.0/24 prefix to be routed
> to 172.16.1.1. When it comes time for 10.0.0.0/24 to be routed, you
> just pointed the traffic back at the router. This is because of the
> receive entry in the CEF table. This behavior originates from RFC5735
> (formerly RFC3330) which makes 0.0.0.0/32 represent "[...] this host
> on this network."[1]

It does not seem like Cisco is always treating it like this.
I just tried this on an SXI3 running box that gives an "invalid next hop"
so far so good.

However, a Cisco 2811 running 12.4(23) where I configured something like
this many years ago is still running and does indeed route 10.0.0.0
(or actually a /32 in my case) recursively to the active 0.0.0.0 route.
Still the 0.0.0.0/32 is in the CEF table as receive. No explanation
so far. And I'd rather not look for one.

Cheers
Sascha