MTU value for GRE tunnels over LTE links

RouterOS General
1

Hi all,

I am running several RBM11G and LDFLTE6kit devices (all on ROS 7.20.8 long-term).

On all devices, I noticed that even thought the LTE interface MTU=1500, a GRE tunnel running over this interface sets its MTU to 1280

default-name="lte1" name="lte1" mtu=1500 apn-profiles=my_profile allow-roaming=no sms-read=no sms-protocol=auto network-mode=lte [...]

name="my-gre-tunnel" mtu=auto actual-mtu=1280 […] dscp=inherit clamp-tcp-mss=yes dont-fragment=no allow-fast-path=yes

How come? I would expect the MTU to be 1500-20-4=1476

Am I overlooking something?

Cheers

Denis

2

Sure the LTE have real MTU of 1500? Tested?
It's not necessarily 1500 right because you wrote it in the configuration...

3

pretty much, look at this:

/ping 9.9.9.9 interface=lte1  size=1500 do-not-fragment count=5
  SEQ HOST                                     SIZE TTL TIME       STATUS
    0 9.9.9.9                                  1500  59 39ms150us
    1 9.9.9.9                                  1500  59 36ms401us
    2 9.9.9.9                                  1500  59 35ms41us
    sent=3 received=3 packet-loss=0% min-rtt=35ms41us avg-rtt=36ms864us max-rtt=39ms150us

/ping 9.9.9.9 interface=lte1  size=1502 do-not-fragment count=5
  SEQ HOST                                     SIZE TTL TIME       STATUS
    0                                                              packet too large and cannot be fragmented
    1                                                              packet too large and cannot be fragmented
    2                                                              packet too large and cannot be fragmented
    sent=3 received=0 packet-loss=100%


so here we clearly see that packets up to 1500 get through when the DF-bit is set. Do you have another test in mind that I did not think of?
4

I just came across this in the 7.21.4 long-term changelogs:

*) lte - fixed user set MTU not applied to LTE interface;
*) lte - override the "auto" or 0 MTU in "interface" menu to 1500;

I will upgrade these LTE routers and see if the behaviour changes…