option 212 for 6RD

Mikael Abrahamsson swmike at swm.pp.se
Sat Jan 19 11:45:26 CET 2013

On Fri, 18 Jan 2013, Daniel Roesen wrote:

> Also you might break communication between statically configured systems 
> not listening to RAs (and thus assuming MTU=1500) with other devices 
> listening to the volatile MTU advertised by the CPE router. Suddenly you 
> can't print anymore because of MTU mismatches in your LAN. Fun!

Strictly speaking, what I'm talking about is the equivalent of:

mtu 1514 ! default
ipv6 mtu 1400

on a cisco router. This is done all the time, like having "mtu 4470" "ipv4 
mtu 1500" "ipv6 mtu 1500" "clns mtu 1497" on an mpls enabled interface. 
Having per-protocol MTU is done all the time on a lot of platforms.

Also, having "ipv6 mtu 1400" doesn't mean the MRU goes down to 1400.

Be strict with what you send, be liberal with what you receive.

Also, the references to the RFCs, that talks about MTU advertised in RA. I 
see no reason why a host system can't have a per-route MTU and treat each 
RA received and the connected routes created from it, have a different 

I'm going to test this someday, I don't see a reason for it not to work.

Mikael Abrahamsson    email: swmike at swm.pp.se

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