Alexander Gall gall at
Tue Feb 3 10:35:26 CET 2009

On Tue, 3 Feb 2009 08:02:59 +0100, Daniel Roesen <dr at> said:

> On Tue, Feb 03, 2009 at 01:11:59PM +1100, Geoff Huston wrote:
>> interface Tunnel0
>> no ip address
>> ipv6 address <something>
>> tunnel source
>> tunnel destination
>> tunnel mode ipv6ip
>> Tunnel0 is up, line protocol is up
>> Hardware is Tunnel
>> MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,
>> reliability 255/255, txload 1/255, rxload 1/255
>> 1514?
>> what a strange default selection!

> sh ip ro to find out egress interface for the tunnel. Then do a
> "sh ip int $EGRESS-IF | i MTU". I guess you will see 1538 there. Which
> might or might not be a misconfig itself.

No, I think Geoff did a "show interface tunnel" but that MTU is
irrelevant, I think.  What you say is true for the MTU from "show ipv6
interface tunnel".  That one will be the MTU of the egress interface
towards the tunnel destination minus the encapsulation overhead unless
you explicitely set the MTU in the configuration ("ipv6 mtu").  But
beware, if that value is the same as the default derived from the
egress interface, the "ipv6 mtu" statement does not appear in the
configuration and the MTU changes dynamically when the egress
interface changes (which can be a problem in certain cases).

Here is one of our tunnels:

swiCS4#sh int tun 100 | i transport|MTU
  MTU 1514 bytes, BW 100 Kbit, DLY 50000 usec, 
  Tunnel protocol/transport IPv6/IP, key disabled, sequencing disabled
swiCS4#sh run int tun 100 | i ipv6 mtu
swiCS4#sh ipv6 int tun 100 | i MTU
  MTU is 9172 bytes

And the MTU on the egress towards the endpoint (20 bytes encapsulation

swiCS4#sh int gig 3/4 | i MTU
  MTU 9192 bytes, BW 1000000 Kbit, DLY 10 usec, 


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