[radvd-devel-l] Changing MTU with radvd (AdvLinkMTU in radvd.conf)

Pekka Savola pekkas at netcore.fi
Mon Mar 31 03:48:19 EDT 2008

On Mon, 31 Mar 2008, David Shwatrz wrote:
> Any idea why the MTU was not changed from 1500 to 1495 on that host?

You're looking at the wrong place.  Neighbor Discovery -derived MTU is 
updated in a sysctl such as net.ipv6.conf.eth0.mtu -- it isn't 
reflected in 'ifconfig -a' that (generic) MTU because that affects v4 
as well.

However, if you did tcpdumps of traffic, you should see that v6 
connections use the lower MTU.  Otherwise the client OS MTU 
calculation has a bug.

I agree that this is not very intuitive place to look for the MTU 
information but there isn't much I can do about it.. :-)

Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

More information about the radvd-devel-l mailing list