ERROR: network-qos: MTU value different from class-default

[erinrjones]

Hi all,

I am trying to set a different mtu per VLAN on a (pair of) Nexus 3048.  I have found several attempts at this online (including, perhaps most concisely, Joshua Engels post here: https://supportforums.cisco.com/thread/2171880)

My problem is when I try set the mtu of a class, I get the error in the title.  A simple recreation is here:

(config)# class-map type network-qos JumboTest

(config-cmap-nq)# match qos-group 5

(config-cmap-nq)# ex

(config)# Policy-map type network-qos JumboTestPolicy

(config-pmap-nq)# class type network-qos JumboTest

(config-pmap-nq-c)# mtu 9000

ERROR: network-qos: MTU value different from class-default

(config-pmap-nq-c)#

I am fairly new getting this deeply into the switching environment.  I assume I am overlooking something simple.  I need to set the MTU on 2 of several VLANs on this switch.  All the cisco docs seem to set the mtu at class-default but I wasn't planning to enable jumbo frames for the rest of my traffic (side question: is there really any harm there?  I seem to find conflicting answers as to whether my network will explode if I just allow 9216 across my switches or not)

Thanks in advance

ej

[sysadmin2]

As far as I know, the Nexus 3k supports MTU as a default setting for all the ports, not for individual ports.

Define it in the class-default.

3k(config)# class-map type network-qos jumbo
3k(config-cmap-nq)# match qos-group 1
3k(config-cmap-nq)# exit
3k(config)# policy-map type network-qos jumbo
3k(config-pmap-nq)# class type network-qos class-default
3k(config-pmap-nq-c)# mtu 9126
Warning: This platform only supports class class-default configured MTU across all classes.
3k(config-pmap-nq-c)# system qos
3k(config-sys-qos)# service-policy type network-qos jumbo
3k(config-sys-qos)# end

and check it with:
show queuing interface ethernet 1/13