RE: [nsp] ICMP Unreachable and CEF

From: Douglas M. Todd, Jr. (
Date: Thu Jan 24 2002 - 15:33:17 EST

Hash: SHA1

Hi all:

Can't get access to bug: CSCdj55180.

However here is some thoughts.

According to Cisco Documentation null is the bit bucket and will
generate an unreachable for any packet routed to that interface. The
only exception is if we have the no unreachables statement on the

doing a show cef drop might help with what is causing the packets to
be discarded.

One difference between the GSR and the 7500/2600 etc is that the GSR
uses dcef by default. This might cause the difference in behavior
between the 7500 VIP and GSR. If you have a second generation VIP
you could try using dcef and see if the unreachables go away.

I would run sh ip cef adj null and see how many null adjacency you
have this would cause some problems. The CEF/FIB table should be an
exact copy of your active routing table. I would make sure that the
routing table and the cef table are the same.
Any packet destine to the null adj is dropped and I would assume
that an unreachable is sent.

Good general doc on cef:

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Douglas M. Todd, Jr.
Network Engineering
Partners Health Care
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>-----Original Message-----
>From: Elijah Kagan []
>Sent: Thursday, January 24, 2002 4:22 AM
>Subject: [nsp] ICMP Unreachable and CEF
>I always thought that when a router forwards packets to Null0 it
>also generates ICMP Unreachable message to indicate this event. It
>seems that
>this is not the case on routers running CEF. I checked this on
>several platforms: 7500, 7200 and 3600. Whenever CEF is turned off I
>see those unreachables pouring in, with CEF on - nothing.
>Is this the expected behavior? Can anyone expand on this issue?
>-- elijah
>P.S. I am running 12.0(x)S on 7200 and 7500.

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