North American Network Operators Group

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RE: IPIP-tunnel with 1500 MTU

  • From: Mikael Abrahamsson
  • Date: Thu Jan 11 14:59:40 2001

On Thu, 11 Jan 2001, Bulger, Tim wrote:

> There is an 'ip mtu' option in the tunnel interface config on Cisco routers
> that you can set to 1500.. This will cause the tunnel packets themselves to
> be ip fragmented outbound on the router, hence shifting the fragmentation
> away from your internal network packets and over to the tunnel packets.  I
> know that this works with GRE and IP/IP on Cisco routers running 12.x code.

(config-if)#tunnel mode gre ip
(config-if)#mtu 1500
% Interface Tunnel1 does not support adjustable maximum datagram size

I believe this is only true if you have serial links that support MTU >
1500 all the way between the two routers doing the tunneling, right?

Mikael Abrahamsson    email: [email protected]