Hi Patrick, On Thu, 28 Sep 2000, Patrick W. Gilmore wrote:

At 02:49 PM 9/28/2000 +0100, James A. T. Rice wrote:

...

ip verify unicast reverse-exists

i.e. only accept the packet on this interface if there is a route back to the source, *not necessarily on the same interface*.. This should be safe to use on all interfaces and could use the existing CEF FIB, and might catch a lot of spoofed packets on a good day.

That would only stop Bogons on most core routers (full tables, right?).

Yes, about the only place it wouldnt be appropriate would be on an ISP's peering router which only had peers and customers routes on, i.e. for putting in NAPs. If the router has a default route, its useless, no harm, just no use. So agreed, the router would have to have full tables for this to help.

...

ip verify unicast destination-advertised

This would check the destination address on any packet coming into an interface, and drop it if a route to that destination WASNT advertised out of that interface - /ideal/ for NAPs & IX's. Couldnt use the existing cef tables, cisco would need to write an advertised-table for each interface. Again this should be safe to use on almost any interface.

Hrmmm.... That would be nice....

But there are other ways to do this. They may or may not be useful / applicable in your environment, but it can be done without this feature.

Like the solution just mentioned above of having a peering router with only peers / customers prefixes ? Apart from that, I can't see a way of doing it, apart from using access lists, but those obviously dont automagically update when a customer starts announcing a new route to you. What other ways do you have of doing it ? Regards James