[c-nsp] EIGRP - Retry Limit Exceeded

[Rodney Dunn]

On Tue, Feb 13, 2007 at 04:06:49PM -0000, Steve Wright wrote:
> Hi all,
> 
> I'm currently trying to diagnose a problem that I hit when performing on IOS upgrade on one of our routers last night.
> 
> We run EIGRP as our IGP between the routers in the network, and I started receiving an error of
> %DUAL-5-NBRCHANGE: IP-EIGRP(0) 6368: Neighbor x.x.x.1 (GigabitEthernet0/0) is down: retry limit exceeded
> 
> Gi0/0 has an MTU set of 1900, and the routers it was establishing a neighborship too (but then producing the above error) has an MTU 
> of 1500.
> 
> Previously, the neighborhips had been up for a substantial amount of time in this configuration. Adjusting the MTU on Gi0/0 back to 
> 1500 corrected the issue.

Retry limit exceeded means you sent a unicast packet that is supposed to be
ack'd (ie: an update) and you didn't get an ACK back. That means either the
update didn't get to the other side or the ack was los. Most likely the
update didn't make it to the other side and get processed.

> 
> I'm believe the issue is with regard to the updates/ replies being sent out at 1900 and the receiving router dropping the packet.
>

Yep.

 
> Is their any workaround such as within OSPF 'ip ospf mtu-ignore'?

MTU ignore is just a neighbor MTU check override. If your links can't
take packets off the wire and process them with different MTU sizes
that command will not help you.

> 
> The devices in question are a 7200 NPE-G1 running 12.3.22 and a 7301 running 12.4.11(T1)  (I've labbed this with 2 x 7301's and 
> haven't been able to recreate this though :(  )

A sniffer trace would be good.

We did have an issue where EIGRP doesn't honor interface IP MTU setting
which was fixed via:

CSCsc72090
Externally found severe defect: Resolved (R)
EIGRP doesn't honor interface IP MTU when sending packets

What code did you upgrade from/to and which router did you upgrade?
The one reporting the retry limit exceeded or the router on the other side?

I wonder if you upgraded to code that has this fix and now the router
really does send packets of the MTU that you changed it to and the
receiving router or device in the middle can't process packets with that
MTU size for some reason.

Rodney

> 
> Any thoughts or advice much appreciated.
> 
> Thanks,
> Steve 
> 
> 
> 
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