[nsp] CEF routing bug?

[Gert Doering]

Hi,

On Thu, Jan 30, 2003 at 11:45:09AM -0500, Ed Ravin wrote:
>  Dialup Customer with DSL address
>        |
>        |
>    +---------+             +---------+               +---------+
>    | Router1 |----T3-------| Router2 |-----FastE-----| Router3 |
>    +---------+             +---------+               +---------+
>                                                          |
>                                                          |
>                                                      DSL customer
> 
> Our DSL customers are attached to Router3, so we've got a static
> route on Router2 (with a poor metric so it can be overridden by a
> route in the IGP) to direct traffic to the FastEthernet.

I'd point the route from Router2 to *Router3* (and not onto the
FastEthernet).  No ARP, except for the base IP of Router3, and
no problem with overriding the route to some other destination.

Doing proxy arp, which is what you're doing (Router3 is answering
ARP requests for something that is not connected to the Ethernet)
will *always* cause problems and is, if unavoidable, a sure sign of a
bad network desing.

> If CEF is turned off, the routing table on Router2 is used as expected
> and traffic goes to Router1 for the IP in question.  There's no ARP
> anywhere in the equation here - the DSL customers are at least two
> hops away from Router2.  

If you route *to the Ethernet* (as opposed to "to Router3") then
Router2 is sending ARP queries out, and Router3 will (Cisco's default)
send out a proxy-arp reply, as it knows a way to the destination IP.

gert
-- 
Gert Doering
Mobile communications ... right now writing from * Amsterdam *