Cisco 3850 - Per Interface MTU Size

[danielashleysmith]

Hi,

I'm building a network and have just been told by somebody that for some particular equipment we need to use jumbo frames. We're using 4x Cisco 3850 switches stacked, I've googled some example commands but they do not appear to be supported by our switches.

Does anybody know what commands I can use to change the MTU size on our 3850 switches? Ideally we would like to change the MTU size per interface, or per VLAN.

If anybody has any advice I'd appreciate it.

Thanks

[Seb Rupik]

Hi there,

first you need to change the global ethernet payload size:

!
system mtu 9000
!

This will require the stack to be reloaded. Jumbo frames will be enabled on all interfaces.

If you have any SVIs configured you will also need to change the mtu value there:

!
int vlan100
  ip mtu 9000
!

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/int_hw_components/configuration_guide/b_int_3se_3850_cg/b_int_3se_3850_cg_chapter_0110.html#concept_39E508CDB4DB4C14A44C9B592E2E132E

cheers,

Seb.

[danielashleysmith]

Ok thanks Seb.

Can enlarging the MTU size affect devices still running at 1500? Or does using the 'system mtu 9000' command simply increase the total size of MTU the switch can handle?

We only have a few devices that require jumbo frames, I don't want to affect anything else.

Thank again

[Seb Rupik]

Increasing the MTU size to 9000 will have no effect on those devices still sending at 1500.

cheers,

Seb.

[danielashleysmith]

Great, thanks :)

[Seb Rupik]

Please mark this question with an accepted solution.

cheers,

Seb.

[Joseph W. Doherty]

It can, if you have 1500 and jumbo Ethernet hosts connected on the same L2 jumbo enabled topology, the latter can send jumbos to the former, which the former will be unable to accept.
If the 1500 and jumbo Ethernet hosts are on separated by a L3 interface, then if the latter sends jumbos to the former, a L3 interface should fragment the packets. I.e. communication between hosts should work, but if can be inefficient if PMTUD isn't in effect and fragmentation continues.