I happened across an interesting scenario this morning while I was doing configuring jumbo MTU of 9216 on the Nexus 5500 switches in our lab.
I wanted to enable jumbo frames and with Nexus 5500s you have to do this with a QoS policy map. Here are the steps:
- Create a new policy map of type network-qos
- Add the default network-qos class type of class-default
- Configure the MTU to 9216
- Add the new policy map to system qos
More about this in the Cisco Nexus 5500 QoS Configuration Guide – QoS Configuration Guide
As soon as I added the new network-qos policy map to the system-qos I checked the configuration using the command “show queuing interface e1/20”
As you can see from the output image below the MTU on the interface is now 9216. I then noticed that there was no longer a qos-group 1 enabled on the interface. I was concerned about this because qos-group 1 is for FCoE traffic and we are using FCoE mulithop from our UCS Fabric Interconnects.
I check the ipqos using the command “show run ipqos” and noticed that the fcoe-default-nq-policy policy map was no longer enabled on the system qos.
This made now made since and I didn’t think about it when I applied the new jumbo-MTU policy. System qos can only have a single policy from each type applied to it at any given time.
To get around this I created a new policy-map named fcoe-with-jumbo-mtu. Here is the configuration for it.
Once I applied the new policy map to the system qos the output of “show queuing interface e1/20” looked correct.