Jumbo Frame Issue

[sarpareashish]

Hi All,

In a scenario where router has MTU set to 1500 on both LAN and WAN interface. If any server from LAN sends a jumbo frame (9000 byte) then will that frame be sent outside or drop?

[Reza Sharifi]

Hi,

If the LAN and WAN interfaces are layer-3, then the packet will get fragmented to smaller size (1500) and forwarded.

HTH

[sarpareashish]

Hi Reza,

Thanks for the reply. What if we have layer 2 switch before the router. Can you please explain me the packet/frame flow in this case.

Thanks in advance. 

[Joseph W. Doherty]

A L2 switch should not forward a frame larger than the port's configured MTU.  (I.e. switch should drop the frame.)

A L2 switch, receiving a frame larger than the port's configured MTU might be unable to physically accept the frame, and if unable to accept the frame, the frame will be dropped.

[sarpareashish]

Thanks Joseph.

That means if we have switch between our server and router then jumbo frame will be dropped. How can we make it work then?

[Reza Sharifi]

Hi,

If you have a L-2 switch between the router and the server, you can simply enable jumbo frame on that switch or you would have to redesign the network and put the router before the switch (not sure if this is possible because of limited port density on the router). 

HTH

[Joseph W. Doherty]

As Reza says although not all switches support jumbos, and since jumbos isn't a standard, you might find different devices don't support the same max jumbo (and if they don't, you would likely need to insure the device with the smaller jumbo support isn't exceeded).

Also keep in mind routers generally don't automatically support jumbos either, and again, as there's no jumbo standard, you can bump into the different max jumbo size problem too.