Jumbo MTU in Layer2 network

[rishad ahammed]

In our data center layer 2 network I want to enable Jumbo MTU in only 2 switches and all other switches will leave with default MTU. Does it make any negative impact on packet transfer between normal MTU switches and JUMBO enabled switch. How would layer 2 switches will handle the situation?

[johnd2310 · Accepted solution]

Hi,

When you enable jumbo frames on a switch, the end devices do not automatically start sending jumbo frames, you also configure the end device to use jumbo frames. If you are only enabling jumbo frames on the switches and not enabling jumbo frames on the end devices, then there is no problem. If you enable jumbo frames on the 2 switches AND enable jumbo frames on the end devices connected to the 2 switches, then you will have issues when the jumbo enabled end devices talk to devices on the other non-jumbo switches. A jumbo  enabled end device will send jumbo frames which a non-jumbo device cannot understand.

If you are going to use jumbo frames, you will need to ensure all devices(switches, routers, endpoints) on the layer 2 network support jumbo frames. The router connecting to the jumbo network will fragment any packets that are crossing to a non-jumbo network.

Thanks

John

[Joseph W. Doherty]

"The router connecting to the jumbo network will fragment any packets that are crossing to a non-jumbo network."

To expand a bit on John's answer, when he notes "router", this would also include L3 switches doing routing. I.e. fragmentation can happen at L3 hops, not L2 hops.

Also BTW, some devices support jumbo per interface. If doing L2, within the same L2 domain, you would want to insure all devices are configured for the same jumbo size. (As there is no jumbo size standard, you may need to run all your devices at the smallest maximum jumbo size supported [assuming all devices allow varied jumbo size setting].)

I understand on some Cisco switches, that support jumbo, but if it's not enabled, may accept a jumbo frame without throwing an oversized frame error. However, unless the transmission port is jumbo enabled (to support the received frame), the frame won't be re-transmitted.

Lastly, considering all the possible issues you can run into working with (non standard jumbo Ethernet), and that you only pick up about 2% data capacity, I often advise against it. If the issue is trying to improve effective transmission rate, assuming you're using something based on TCP, you might first insure RWIN, for the BDP (bandwidth delay product) is optimal.

[rishad ahammed]

Thank you very much for your detailed explanation. This is exactly what i was looking for