[33461] in North American Network Operators' Group
Re: IPIP-tunnel with 1500 MTU
daemon@ATHENA.MIT.EDU (Stephen Sprunk)
Thu Jan 11 18:00:18 2001
Message-ID: <012101c07c21$3b8f83c0$26132ca1@glock>
From: "Stephen Sprunk" <ssprunk@cisco.com>
To: "Mikael Abrahamsson" <swmike@swm.pp.se>, <nanog@merit.edu>
Date: Thu, 11 Jan 2001 16:52:16 -0600
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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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