[17609] in North American Network Operators' Group
Re: MTU problems with GRE tunnels
daemon@ATHENA.MIT.EDU (Jose Manuel de Arce)
Sun Jun 7 18:42:54 1998
Date: Fri, 06 Jun 1997 01:37:18 +0200
To: philip bridge <bridge@ip-plus.net>, nanog@merit.edu
From: Jose Manuel de Arce <josema@ibernet.es>
In-Reply-To: <3.0.3.32.19980605095358.006ebd4c@mailhost.ip-plus.net>
Hello
We had similar problems and we used 1524 bytes mtu's in serials interfaces
in the cisco config:
# int ser 0
# mtu 1524
I am also interested in extra info regarding this gre problem, but for a
quick fix try playing with mtu size over serials interfaces.
Kind regards
Josema
At 09:53 AM 6/5/98 +0100, philip bridge wrote:
>I'm experiencing problems with fragmentation due to Cisco GRE tunnel
>overhead: the way I understand it, the MTU if a GRE tunnel will always be
>less than the MTU of the underlying IP cloud (in our case 1500 bytes) due
>to the IP encapsulation overhead. So 1500 byte packets attempting to
>traverse the tunnel will be fragmented, or dropped if the DF bit is set, in
>which case an ICMP message is send back to the originating host
