stability issue between 2951 router and 2960 switch

[nicolas.cousin1]

Hi,

I'm observing a very strange stability issue between my C2951 router and my C2960 switch.

The switch is connected to the router via a fiber. When trying to ping the switch from the router, we have a very important packet loss ratio and the RTD can vary between 1ms and more than 1500ms. BUT, when pinging the router from the switch, it's perfectly stable: no packet loss and RTD between 1 and 9ms. Another thing: we have a host located on the LAN behind the switch, that uses the router's WAN access, and it's working perfectly. As if only ICMP trafic was impacted by this stability issue.

Also to conclude, we observed that, when we activate the CDP, the stability issue disappears! When we deactivate the CDP and it keeps working fine for a few weeks until the problem reappears...

The network experts in my company can't explain this... Do you have any idea ?

BR,

Nicolas

[Madhukrishnan Gopinathan Nair]

Do you see intermittent cpu spiking on any of these?

Madhu

[nicolas.cousin1]

No CPU or memory spikes on the router/switch.

Maybe interesting to mention that we have another 2960 switch connected to this router. And we don't observe any issue with it.

[Madhukrishnan Gopinathan Nair]

Please share the output of

sh tcp brief  from the switch.

[nicolas.cousin1]

I've had to remove the IP addresses for confidentiality reasons.

TCB       Local Address           Foreign Address        (state)
0230005C  XX.XX.XX.XX.23         XX.XX.XX.XX.61302    ESTAB

I assume it's my telnet session.

[Madhukrishnan Gopinathan Nair]

Hi,

Could you please check if you have any flapping routes on the router.

You can check sh ip route to see if any of them flapping. 

The reason i am thinking if the router is busy dealing with packets wich has priority, it may respond to slower to less priority icmp.

Also on the other hand check any of the switch port has input queue drop.

Use show interface to check.

Thanks,

Madhu.