lte1 mbim: network advertises lower mtu: 1428
I'm curious what other people do with these "network advertises lower mtu" messages with LTE MBIM modems. In the PPP days, I always adjusted the MTU to match the what the carrier documents, and PPP profile would add the "clamp-to-mtu" mangle rule. And if you didn't adjust the MTU, the upload performance was more variable/lower.
But with MBIM and LTE in V7, RouterOS doesn't do this – even though it actually seem to know the MTU might be lower. And the default MTU in V7 seems to be MTU 1500, while in older RouterOS they did use a lower MTU by default for PPP/LTE.
My question is MTU adjustment actually necessary? I haven't done extensive test with the default 1500... since I set MTU to match these "network advertise lower mtu" messages. I figured it's better to avoid fragmentation, and give PMTUD a chance to work if the MTU is constrained someplace in the LTE network.
But I don't see this as a recommendation in docs, nor any commentary in the forum (other than this post). Maybe any potential fragmentation isn't a problem. Or some other framing upstream in modem/LTE network "fixes" it, but dunno. Thus bump in this topic.
P.S. If only I saw these...
lte1 mbim: network advertises higher mtu: 1550
but the two main carriers in US advertise "lower mtu", generally 1430 or 1428... Thus the question.