IPIP-tunnel with 1500 MTU

Thu Jan 11 22:52:16 UTC 2001

Thus spake "Mikael Abrahamsson" <swmike at swm.pp.se>
> (config-if)#tunnel mode gre ip
> (config-if)#mtu 1500
> % Interface Tunnel1 does not support adjustable maximum datagram size

router(config)#in tun0
router(config-if)#mtu 1500
% Interface Tunnel0 does not support adjustable maximum datagram size
router(config-if)#ip mtu 1500
%Warning: MTU set 1500 is greater than default 1476, fragments will
happen

> 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?

Nope.  With fragmentation of the GRE packet, your tunnel MTU can be
greater than your real MTU.

S

     |          |         Stephen Sprunk, K5SSS, CCIE #3723
    :|:        :|:        Network Design Consultant, GSOLE
   :|||:      :|||:       New office: RCDN2 in Richardson, TX
.:|||||||:..:|||||||:.    Email: ssprunk at cisco.com