enabling IPv6 on Cisco 6500 breaks IPv4 Internet connectivity.

Phil Mayers p.mayers at imperial.ac.uk
Fri Jun 8 10:57:29 CEST 2012

On 06/07/2012 11:44 PM, Jim Trotz wrote:
> By way of an update on the problem:
> We have been able to recreate the problem in a network lab environment.
> The router stops forwarding IPv4 traffic (ICMP still flows) whenever two
> IPv6 configuration statements are entered. Traffic resumes after some
> time (2-10 minutes).

This is all very odd.

This should not be happening on this platform, if what you've said is 
true - the box should be capable of forwarding IPv6 in VRFs, and if 
you're neither overflowing the TCAM of hammering the CPU with packets, 
then it is hard to imagine what is causing it.

It is particularly bizarre that ICMP continues to work; the 6500 doesn't 
forward ICMP any differently.

Have you:

  1. Used SPAN to look at what it hitting the CPU during an outage, as 
you enable it?

  2. Used "sh proc cpu sorted 1m" to confirm where the CPU is being 
spent during an outage?

  3. Ensured you have not enabled IPv6 uRPF (which is done in software 
on this platform) or any ACLs with logging statements?

  4. Ensured you don't have a routing loop?

TAC should be able to help you find the problem here; "no suggestions" 
is codeword for "I'm a dumb and lazy TAC engineer". Ask for your case to 
be escalated; hassle your account manager with the SR number. You should 
be aiming to get an engineer from backbone TAC on the phone with you, 
and let him screen-control a shell onto the box whilst you reproduce the 
outage, and then he should start poking at the innards of the box 
(probably with ELAM or similar).

If you can share your config (sanitised) on- or off-list, I can take a 
look; although we don't run full tables or EIGRP, we do run IPv6 in VRFs 
on this platform with no problems.


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