Lower MTU with two 3650 switches?

[Marcus023]

I have two 3650 switches each connected to an encryption device.

The GRE Tunnel required for the encryption devices came up right away, but the encryption is still not happening.

The encryption devices cannot do an MTU over 1400, but I cannot set the 3650s lower than the default 1500.

"System MTU 1390" does not work in Global Config
"MTU 1390" does not work in Interface Config

Can the 3650 be set at an MTU under 1500?

[David Ruess · Accepted solution]

Hello,

You can try this article:

https://networkstoragesecurity.wordpress.com/l2-mtu/

Summary:

L2 MTUThere are 3 types of MTU that can be configured on a switch:
● Layer-2 MTU that affects 10 and 100 Mbps interfaces of a switch. Configured by system MTU {bytes} command in global config mode

● Layer-2 MTU that affects 1000 Mbps and higher speed interfaces of a switch. Configured by system MTU jumbo {bytes} command in global configuration mode

● Layer-3 MTU that affects SVIs and routed interfaces of a switch with IP addresses on them and originating or transit IP traffic that uses these interfaces as GW for routing between networks. Configured by system mtu routing {bytes} command in global config mode

I used this a couple times to figure out MTU on different devices and interface types.

-David

[Marcus023]

Much Appreciated 

[balaji.bandi]

MTU adjustment required on the devices doing GRE tunnel, 

I have two 3650 switches each connected to an encryption device.

what is that encryption devices you mentioned here, is the Switch in the path or behind Encryption device this was not clear ?

Personally you do not need to change MTU on the switches until it required.