[c-nsp] VLSM

[Gert Doering]

Hi,

On Tue, Jan 11, 2005 at 08:49:53AM -0700, Mark Persiko wrote:
> If you have a hierarchical network topology with the gateway of last
> resort facing  one egress point at the core, then I've noticed that "ip
> classless" has the unfortunate side effect of sending all traffic out
> that egress pipe, whose destination is for subnets that aren't used
> within the network.  That is my condition right now and I am burning
> bandwidth, and seeking a solution!

Insert local null routes for network that you know you're authoritative
for.

> In other words, if you are using 172.16.1/24 and 172.16.2/24, and you
> have packets headed for 172.16.3/24, "ip classless" will make them head
> to the gateway of last resort instead of just dropping them.

ip route 172.16.0.0 255.255.0.0 null0

> One solution I've considered is explicit static routing for all valid
> subnets of 172.16 and then null routing anything else for 172.16/16
> itself.  What do you think?

Static routes for the subnets are not needed - more specific route wins
anyway, even if you have an overlapping null route.

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de