mtu

[Daniel Roesen]

On Tue, Feb 03, 2009 at 01:11:59PM +1100, Geoff Huston wrote:
> interface Tunnel0
>  no ip address
>  ipv6 address <something>
>  tunnel source 10.0.0.1
>  tunnel destination 10.0.0.2
>  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 10.0.0.2 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.

IOS tries to be clever. See my other posting in the thread.

> I also don't understand the encapsulation behavior on tunnel ingress - but 
> maybe thats just me! i.e.when an IPv6 packet thats too big for a tunnel 
> gets wrapped in an IPv4 wrapper where IPv4 fragmentation is allowed, then 
> should the ingress router simply accept the packet, and fragment it?

Some discussion on the subject:
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a0080093f1f.shtml
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml


Best regards,
Daniel

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