Hoping for some help again..

The setup is as following [ISP-router] -> RB2011uias-in -> two Nanostations M2 in bridge mode that work as access points..

ISP-router has an MTU setting of 1492 (connected in eth1)

RB2011 and NSM2 (eth3 and eth4) have 1500 and additionally RB2011 L2MTU 1598

Should all 1500's (eth1, eth3, eth4) be changed to ISPs 1492 ? (and what about L2MTU)

Thank you

All MTUs on same IP subnet should be the same. If ISP modem has MTU 1492 on LAN interface, then that's the value on all other LAN devices. Note, that MTU on ISP modem WAN interface can be different. If RB is routing between ISP modem and nanos, then again MTU does not have to be the same on both sides.

L2MTU can be as high as it gets, but should not be lower than MTU+ethernet overhead, normally that's MTU+14 (if VLAN is used, it's +18).

All MTUs on same IP subnet should be the same. If ISP modem has MTU 1492 on LAN interface, then that's the value on all other LAN devices. Note, that MTU on ISP modem WAN interface can be different. If RB is routing between ISP modem and nanos, then again MTU does not have to be the same on both sides.

L2MTU can be as high as it gets, but should not be lower than MTU+ethernet overhead, normally that's MTU+14 (if VLAN is used, it's +18).

Thank you!

Hi,

I also need help with MTU...

I have two Mtik devices:



What MTU must be on L2TP and EoIP...

Bridges on both sides are 1500.

Traffic through EoIP is multicast (IPTV) and it lags...
Thanks...

EoIP should have same MTU as other LAN devices are, most likely that's 1500. Since it comes with overhead and thus very often exceeds MTUs of underlying interfaces (e.g. PPPoE), it performs its own fragmentation/defragmentation, so at the end of the day it shouldn't be a problem other than performance hit.

With regards to multicasts over L2TP /EoIP: life sucks. Tunnels go via internet where all sorts of stuff happens. And multicasts don't like that. And there's nothing you can do about it.

use the ping with do-not-fragment , that will tell u a lot

If multicasts work somehow (albeit with pauses), then this is not the matter of MTU settings ... PMTUD doesn't work for multicast packets. If multicasts didn't work at all, then this could point towards MTUs.