MTU problems with GRE tunnels (fwd)

Mon Jul 6 16:38:00 UTC 1998

> # it seems that there is a small but significant
> # amount of application traffic out there that has problems when traversing a
> # GRE tunnel with MTU < 1500. We've seen two problems:
> #  
> #  - 1500 byte packets with DF set. This is either application traffic, or MTU
> #  path discovery is broken, because the same packets get sent repeatedly

We see the same effect, if anyone wants a site to test against try one
of ours - news.bbc.co.uk (193.130.149.130)

This is a set of NT servers behind an RND Director, and we've found
you never get past MTU discovery (though oddly if you go straight to
an individual machine address, e.g. 193.130.149.131 it does work. The
only client verified is IE4 on Win95. We were working on the NT
stack as the probable cause (MTU black hole discovery defaults to
off)

Brandon