RE: IPIP-tunnel with 1500 MTU

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1

(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?

2

Thus spake "Mikael Abrahamsson" <swmike@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
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