MTU configuration required for VXLAN deployment

[Sam N]

Hello All-

I am reviewing a new campus VXLAN deployment. The one thing always confuses me is the required MTU configuration across all the new switches.

Currently I see on all switches following configured "system mtu 9198". However at the same time since underlay is OSPF I also see interfaces running OSPF configure as "ip mtu 1500". I believe these are 2 different MTU's- the first one is the L2 MTU and the second on is L3 MTU.

For VXLAN underlay is this a normal configuration? Or along with L2 MTU even L3 MTU needs to be bumped up to jumbo MTU size?

This is one of the topics that always confuses me so any help or suggestion will be appreciated!

[M02@rt37]

Hello @Sam N 

You're facing 2 concept:

system mtu controls what can physicaly pass through the port _ important for VxLAN encapsulation.

Ip mtu controls what ospf or the routing plane will advertise and use...

[Sam N]

Thanks for the response.

So for VXLAN do both "mtu" and "ip mtu" need to be 9198 bytes? Or if "system mtu 9198" is configured it takes care of it?

I am trying to find if "ip mtu 1500" on OSPF interfaces will create a problem or not.

[Giuseppe Larosa]

Hello @Sam N ,

VXLAN packets in data plane are UDP packets in the underlay.

To avoid fragmentation that is possible but it can be a performance bottleneck I think ip mtu should be aligned with system mtu at least near to it ( 18 bytes less)

Hope to help

Giuseppe