[radvd-devel-l] Multiple IPv6 Routers

Iain J. Watson lists at ij-watson.co.uk
Tue Nov 30 17:03:41 EST 2010

On Fri, Nov 26, 2010 at 11:26 AM, Pekka Savola <pekkas at netcore.fi> wrote:
> On Fri, 26 Nov 2010, Iain J. Watson wrote:
>> Also, when I hard power-off one of the routers to simulate a failure I
>> can no longer ping in either direction. If I run radvdump on either of
>> the test servers I see nothing being received. If I then restart radvd
>> on the remaining router I become able to ping again.
> Some thoughts:

All thoughts welcome, thanks Pekka.

>  1) please note that your client OS may not support high/low preferences.

This may have been the cause of the problems. I noticed a difference
in how Ubuntu and Debian behaved when I killed one router. For some
reason Ubuntu lost both routes from 'ip -6 show routes'. I swapped out
the ubuntu edge server for a Debian one and failover between the
routers now happens.

>  2) the amount of time it takes to remove a dead default gw is governed by
> AdvDefaultLifetime which you have not set.

This defaults to 3 * MaxRtrAdvInterval so would have been 90 seconds.
I had been aware of that and waited more than 90 seconds to see no
failover. I have now tuned that down to 10 seconds as 90 seconds is
excessive for a failover.

>  3) AdvReachableTime might help but than again it might not. RFC4861 also
> specifies how the client should delete non-working neighbors or routers, but
> I have not tested this myself.

Now set to 1000 milliseconds. Not properly tested the effects yet.

> To get more accurate info on what is happening, you will need to look at the
> routing and neighbor table (/sbin/ip -6 r l, n l) and tcpdump -e (the MAC
> addresses used).
> Your remainder point about having to restart radvd on the remaining router
> is likely a combination of 1) and 2) but it could also be a bug (and more
> radvd debug log could be useful).

I still have to look into the reasons behind Ubuntu not behaving as
expected compared to my Debian OS. I take it my basic methodology is
correct though in how to best run redundant routers in an IPv6
environment. I find very little information on the internet on such

Thanks again for the help.

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