On 09/03/16 12:25 PM, Jonathan Thibault wrote: > Hello good people of netdev, > > When setting up gretap devices like so: > > ip link add mydev type gretap remote 1.1.1.1 local 2.2.2.2 nopmtudisc > > I'm observing two different behavior: > > - On system A, the MTU of 'mydev' is set to the MTU of the 'parent' > interface (currently 1600) minus 38. All other interfaces on that system > have a default MTU of 1500, only the parent was forced to 1600 to avoid > fragmentation. So 'mydev' accurately figures out that its MTU is 1562. > > - On system B, with the 'parent' interface MTU set to 1600 and all other > defaulting to 1500 (same situation as A), the MTU of 'mydev' gets set to > 1462. > > I'm trying to figure out which behavior is normal and what mechanism (if > any) causes the MTU to be set differently. In both cases the 'parent' > device has an MTU of 1600. The code in ip_gre.c does this: > > dev->mtu = ETH_DATA_LEN - t_hlen - 4; > > In this case, system B would have the expected behavior and I need some > way to explain what goes on with system A. > > Of course I can force the MTU on system B but I was rather pleased with > the 'magic' on system A. > > If anyone here familiar with this code can offer an explanation, it > would greatly ease my curiosity. > > Jonathan Replying to myself to leave a trace.
Turns out that the MTU of the gretap device is based on the MTU of the interface that has a route to the 'remote' address. In my specific case, it used the default route on system B and a static route on system A. The cause of the discrepancy was a missing route. Jonathan
