Some Packet Drops in ping when using higher MTU size

[Abo.Anas]

I am facing omitted ping results when I ping with MTU size higher than 1000.

We have 4 pairs of C6509 VSS chassis connected at the distribution layer with a c6509 VSS chassis as the aggregator. We have configured MTU 4000 on all uplinks (L3 interfaces) on both sides. All 4 VSS pairs are identical in hardware and IOS versions and pretty much similar configurations. 

On 2 distributions when I ping any L3 interface (mpls L3 PO, SVIs, sub-interfaces, P2P) with MTU size higher than 1000, there are packets drop and it increases when increasing MTU size. Like on 1000 30% pack drops, on 2000 40% and on 3000 50% packet drops..

Packets drop even while pinging its own interface IP (mpls L3 PO, SVIs, sub-interfaces, P2P)!!!

I have already verified mtu size in "show interface" commands on all related interfaces and there is nothing in log file to look around, no any hardware failure notification etc..

The other two pairs do not have any mtu issue which has the same configs. Please advise how can I troubleshoot this issue? 

[balaji.bandi · Accepted solution]

Do you have any QoS configured - can you post the configuration, version and interface information to look further.

[Abo.Anas]

Thankyou balaji for the prompt response.

I can post interface status by tomorrow but I have attached some available information in the attached file. I apologize for the omitted output in the file.

After your reply, I noticed that I have copp-policy different on these 2 problematic switches than the other working fine (instead of "exceed-action transmit". its "exceed-action drop"). Is this the reason? I'll check "sh policy-map control-plane" output tomorrow...

policy-map copp-policy
class copp-undesirable
police cir 32000 bc 4000 be 40000
conform-action transmit
exceed-action drop
class coppclass-bgp
police cir 4000000 bc 400000 be 400000
conform-action transmit
exceed-action drop
class coppclass-filemanagement
police cir 6000000 bc 60000 be 60000
conform-action transmit
exceed-action drop
class coppclass-management
police cir 500000 bc 5000 be 5000
conform-action transmit
exceed-action drop
class coppclass-monitoring
police cir 900000 bc 9000 be 9000
conform-action transmit
exceed-action drop
class coppclass-critical-app
police cir 900000 bc 9000 be 9000
conform-action transmit
exceed-action drop
class copp-catch-all-IP
police cir 1000000 bc 10000 be 10000
conform-action transmit
exceed-action drop
class class-default
police cir 500000 bc 5000 be 5000
conform-action transmit
exceed-action drop
!

Thanks. 

[Abo.Anas · Accepted solution]

Issue resolved by replacing "exceed-action drop" with "exceed-action transmit" in copp policies. Network performance is dramatically improved, no more video break or ping drops with higher mtu...

Thanks balaji for the qos clue.

[balaji.bandi]

Thank and it was usefull, most of the time we see the issue because of QoS, glad all resolved now.