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

David Shwatrz dshwatrz at gmail.com
Mon Mar 31 03:24:10 EDT 2008


I had added to my /etc/radvd.conf file the following line:
AdvLinkMTU 1495

I started the service successfully.
I have radvd-1.0-5.fc8 on FC8.

I expected that the hosts on the LAN (which have 1500 on them)
will change the MTU to 1495.
It did not happen.
Shouldn't it be so ?
I also added in the kernel of one of the hosts a printk()
to make sure that rt6_mtu_change() , from ndisc_router_discovery in
net/ipv6/ndisc.c , is called. And indeed it is
called and the mtu parameter values is the same that
I put in the radvd.conf below, which is 1495. However, that host
does not change the MTU ! I ran "ifconfig -a" on that host and the MTU was
as before, namely 1500. I made sure that the eth0 interfcae got the prefix
from the radvd daemon and configured an IPV6 address based on it. This was
done correctly. I also see in a sniffer the MTU in the options of the
Router Advertisement message that this host gets.

Any idea why the MTU was not changed from 1500 to 1495 on that host?

(BTW, I made sure that /proc/sys/net/ipv6/conf/eth0/forwarding is 0
on that host).

here is my radvd.conf:

interface eth0
        AdvSendAdvert on;
				MaxRtrAdvInterval 30;
				AdvLinkMTU 1495;
				prefix 2004:db8:0:1::/64
        	AdvOnLink on;
          AdvAutonomous on;



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