[nsp] CEF problem on 6509 (native)

Gert Doering gert at greenie.muc.de
Fri Nov 14 19:40:43 EST 2003


On Fri, Nov 14, 2003 at 01:59:57PM -0800, Siva Valliappan wrote:
> > Do you think Router2 sees ARP requests from 00a0.d2a4.5003 and, having
> > proxy-arp on my vlans on Router2 causes the problem i'm trying to fix?
> > Shouldn't the router learn arps only for directly connected ips?
> so the behavior you are describing is tickling an old memory on how CEF
> behaves under certain conditions.  i believe the router is receiving a
> grat. arp, which is causing it to override the route with some more
> specific.  yes one is a route, while the other is a ARP (and not a route),
> but i have worked on a couple of situations where this occurred.  i think
> it's arguable if it's a bug or a feature.

I have seen that as well, and I'd call this a major bug.

(I understand what happens, but in my opinion, the router MUST NOT
[never!] build a direct adjacency for a host that is not reachable
via a "connected" route, period.  The routing table specifies the way
packets flow.

> now when we have to do a packet forwarding decision we try to make the
> most precise match, so the /32 in your forwarding table is more precise
> then the /24, so we use the /32.
> you should open a TAC case to verify this, but this is what i believe is
> happening.

I have seen this behaviour in the past (some 12.0S release) but have
never been able to reproduce it :-(

Gert Doering
Mobile communications ... right now writing from * Berlin *

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