Configuring MTU on VPC

[dbuckley77]

We have a pair of Nexus 9ks and a pair of Nexus 3Ks that we are configuring two VPCs between.  There is one link between each 9K to each 3K that makes up the two different VPCs  The port-channel interfaces and physical interfaces on the 9Ks have an mtu of 9000 while there is no mtu vlaue shown on the port-channel interface on the 3K and the member physical interfaces are set to 1500. 

Does the MTU value on the 3K ohysical interfaces and port-channelinterface need to match for the VPC's to work correctly?  I don't actually see a way to configure it on the port-channel interface on the 3K switches.

[balaji.bandi]

Sometime the configuration per global or per interface basis

If you looking to use Jumbo frames, not set global configuration -  then prefer to setup manually on each interfaces required to meet the requirement, so it work as expected.

[M02@rt37]

Hello @dbuckley77 

it's generally recommended to ensure consistency in the MTU settings across the links participating in the vPC. Mismatched MTU settings can lead to issues with proper communication and may result in unexpected behavior.

https://www.cisco.com/c/en/us/support/docs/switches/nexus-9000-series-switches/118994-config-nexus-00.html

[MHM Cisco World]

https://www.cisco.com/c/en/us/support/docs/switches/nexus-9000-series-switches/118994-config-nexus-00.html

Check this guide how config mtu in different nexus platforms 

[dbuckley77]

When I run the command show queuing interface on the physical interfaces of the 3k it says the mtu is 9000 (9000) configured but when I run sh int eth it says the mtu is 1500.  What is the reason for the difference and which value is the true mtu on the interface?

[MHM Cisco World]

policy-map type network-qos jumbo
  class type network-qos class-default
          mtu 9000
system qos
  service-policy type network-qos jumbo

This need to set mtu in N3K 

[dbuckley77]

That's how to configure it but how do I verify what's actually set.  The command in the guide provided earlier says to use the sh queuing interface x/x.  I have run that on a number of interfaces and they all say HW MTU: 9000 (9000 configured) in the output

However when I do a sh int x/x it shows the mtu on the interfaces as 1500.

Before I start messing with the configs on the 3ks I want to verify what is actually set as the current mtu.  Is the sh queuing interface x/x command the definitive way to do that?

[MHM Cisco World]

module-1# show hardware internal mac port 33 qos configuration | beg mtu

Queue and Check this also.

But to be honest' mtu in nexus is complex and headache some time.

Read guide we share and apply command.

Good luck 

MHM

[MHM Cisco World]

Can I know the exact n3k platform you have?

Note:- some per qos and other per interface

[Reza Sharifi]

The MTU has 2 parts, one is layer 2 and the other one is layer 3. If you want to see jumbo MTU on the layer-3 interface, you need to configure the second part. See below: 

Layer-2
policy-map type network-qos jumbo
  class type network-qos class-default
    mtu 9216


Layer-3
system qos
  service-policy type network-qos jumbo