[nsp] CEF routing bug?

[Ed Ravin]

On Thu, Jan 30, 2003 at 10:11:29AM +0100, Ilker Temir wrote:
> If you lose the route from your IGP, the network will be directly connected.
> And CEF will create /32 adjacency based on ARP, as long as this spesific
> adjacency exists it will be used even if the route is relearned from IGP
> (since adjacency created from IGP will be less spesific).

This sounds like the explanation for one of our two problems
with CEF, where the route we are trying to override is indeed local
to the FastEthernet interface (and would thus have an ARP entry for
the IP address in question).

> If arp times out, corresponding /32 CEF adjacency will be removed and there
> will be no problem. However arp will never time out in this case (unless
> remote goes down).

Would clearing the Cisco's ARP table help?  I don't think we tried that yet.

> And to me, it is logical. Why would you want to remove an
> arp entry when the directly connected destination is still reachable ?

Because you want another machine to process the traffic for the
directly connected destination, like a Web cache or a load balancer?

Meanwhile, our other CEF problem has nothing to do with ARP.  It
looks a bit like this:

 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.

When the customer's DSL line is down, they dial in via ISDN, and
they use the same IP address so that they won't have to reconfigure
anything.  Router1 redistributes the dialup route via OSPF to
Router2.  If we have CEF turned on on Router2, it sends traffic
for that IP to the FastEthernet no matter what's in the routing table.
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.  Router2 is the 7513 with 12.0(21)S1.

Thanks,

	-- Ed

> ----- Original Message -----
> From: "Ed Ravin" <eravin at panix.com>
> To: <cisco-nsp at puck.nether.net>
> Sent: Thursday, January 30, 2003 12:33 AM
> Subject: [nsp] CEF routing bug?

> > I'm seeing similiar problems on two different routers - a 7206 with
> > 12.2(7c) and a 7513 with 12.0(21)S1.  On one router, routes received
> > via OSPF or RIP do not override CEF when the CEF route is on a
> > FastEthernet interface.  On the other router, the dynamic routes are
> > used when the router is restarted, but if the dynamic routing
> > protocol drops the route and CEF takes over, we can't lose the CEF
> > route without rebooting the router again.