cloned route gets wrong mtu

[Toni Mueller <support-openbsd-misc_(_at_)_oeko_(_dot_)_net>]

Hello,

I just stumbled across a problem where a directly connected host gets a
wrong MTU in his route entry in an OpenBSD 3.7 box.

Network diagram:

 openbsd .1 ---------- linux .2

The two hosts are connected via Fast Ethernet which has a nominal MTU
of 1500. The entry for the linux box in the OpenBSD's routing table
says that the MTU is only 1428. This wasn't always the case, but
occurred "suddenly" - without any known human interference. Manually
deleting the route returned operation back to normal (the routing entry
now shows a '-' in the MTU column).


If you have an idea about how and why such things happen, I'd very much
like to know.


Thank you!


Best,
--Toni++