Nexus 7/5k - Input discard incrementing

[jay_7301]

Hi,

We have Nexus 5672 & 7706 in our environment. I have noticed on quite a few interfaces that the input discards are incrementing. 

I have ran the show hardware internal statistics module 1 pktflow dropped command and attached the output. I have done a comparison and listed the ones that are incrementing. I think my knowledge beyond here is limited to actually understand why we are seeing the discards.

Based on the attached output is anyone able to comment on this?

Thanks

[Sergiu.Daniluk]

Hi @jay_7301 

Based on the outputs shared it looks like you have an F3 linecard (Device: Flanker) and there are one or more egress ports which are congested. Why am I saying that? Because of these:

igr vq: l2 pkt drop count                     
igr vq: total pkts dropped                    

The F-series linecards are using a pure ingress VOQ-buffered architecture. VOQ = Virtual Output Queue.

The short explanation of VOQ: unicast traffic is buffered on ingress. If multiple ingress ports sent traffic to one single egress port, there might be over subscription of the egress port, resulting in input discards. 

The long explanation of VOQ: VOQs use the concept of credited and uncredited traffic. Unicast traffic is classified as credited traffic; broadcast, multicast, and unknown unicast traffic are classified as uncredited traffic. Uncredited traffic does not utilize VOQs, and traffic is queued on egress rather than ingress. If an ingress port has no credit to send traffic to an egress port, the ingress port buffers until it gets credit. Since the ingress port buffers are not deep, input drops might occur.

To find the congested egress port, this will help you:

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/nx-os-software/214503-troubleshoot-nexus-7000-f3-input-discar.html 

Stay safe!

Sergiu

[jay_7301]

Thank you for the quick reply. When doing an audit I can see the majority of our interfaces are showing input discards on the 7/5k's.

When you say:

If multiple ingress ports sent traffic to one single egress port, there might be over subscription of the egress port, resulting in input discards. 

This is quite normal in a network right? how do you get around this over subscription issue? do you have to police traffic?

Thanks

[Sergiu.Daniluk]

Hi @jay_7301 

I would not say normal, since in general, we take actions to prevent this.If you know that you have 2x 10G interfaces as ingress ports, both receiving 7Gbps let's say, and the egress port is 10G, you will have more input traffic than the output interface can handle.

One of the most desired approach is to have enough bandwidth on egress: if not already, convert the egress interface to a port-channel and add more interfaces to the bundle.

Cheers,

Sergiu