CAT3850 - Can I configure a different MTU on an SVI to the System MTU?

[d-fillmore]

Hi, I need to change the System MTU on a 3850 to the maximum value - 9198. I will have a client facing SVI also set to 9198, but then I need a service provider facing SVI to be 9000, as the service provider MTU is set to 9000 and we are running OSPF with them. 

Can I set the MTU on the SP facing SVIs to a value that's different to the System MTU as long as it's a lower value than the System MTU and accept fragmentation if I need to route between both interfaces?

Many Thanks in advance, Dom

[pieterh]

MTU =  Max. Transmission Unit. so other "TU"'s are possible, but you need some mechanism that either

- detects the maximum usable packet in the path and uses a lower packet size on this connection

- fragments too large packets.

if fragmentation is not your concern, then I guess you should be all-right

[Cristian Matei · Accepted solution]

Hi,

    Yes, technically speaking it has to work, though never done this exact setup on a 3850. Put the system MTU to the maximum value and play with lower or equal values per SVI.

Regards,

Cristian Matei.

[d-fillmore]

Thanks, I guess the most important question is, If I set the SVI MTU to 9000, and want to form an OSPF adjacency with something running 9000 as well, will the device use the System MTU when forming the OSPF adjacency, or the MTU that's configured on the SVI?

Unfortunately I don't think think this is something I can lab so looking for anyone who's done it before

[Reza Sharifi]

Hi,

On the 3850 series switches, the MTU setting is global, which means when you change it, it is for the entire switch. 

HTH

[d-fillmore]

For L2 yes, but is different MTU for L3 SVIs supported?