North American Network Operators Group
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Re: MTU problems with GRE tunnels

  • From: Jose Manuel de Arce
  • Date: Sun Jun 07 18:47:19 1998 
	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