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Maximum MTU of Routerboard Ethernet?

Posted: Mon Apr 07, 2008 3:10 pm
by oscarBravo
Hi - what's the maximum MTU that the Ethernet ports on Routerboards are capable of?

As I understand it, Atheros radios can handle an MTU of 1550. I'd like to run an entire link (Ethernet -> Atheros -> Atheros -> Ethernet) at an MTU greater than 1500, as I'm running an EoIP tunnel over it, and I want to avoid fragmentation of tunnel packets.

Re: Maximum MTU of Routerboard Ethernet?

Posted: Mon Apr 14, 2008 6:53 am
by JJCinAZ
You can increase the MTU of the atheros cards to large values (I think it was something like a 3400 byte limit) but the Ethernet interfaces are limited to 1500 byte MTU's. You can also use the NStream framer policy to combine the two fragments of the EoIP GRE packets on the air automatically. Finally, use a mangle on the EoIP TCP-SYN packets to reduce the MSS to 1300 or so to avoid fragmentation in the GRE tunnel. You can't affect the UDP size like that, but the TCP MSS adjust helps a lot.