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(usagi-users 04098) Re: Default Router Preference - RFC4191, default route switching

Hi Arnaud,

On Tue, Jun 29, 2010 at 03:49:03PM +0200, Arnaud Ebalard wrote:
> Hi,
> Sorry for the delay.
No worries, I was glad that anyone answered at all :D.
> Linus LÃssing <linus.luessing@xxxxxx> writes:
> > I've lately been trying the Default Route Preference on a 2.6.27
> > kernel, compiling a custom kernel with this option enabled.
> I remember discussion on the topic on netdev leading to the following
> commit which was added just after 2.6.27 was published if I am not
> mistaken: 
Argh, missed by one (kernel version), I guess :). 
> > For the test setup, in the beginning the host with RFC4191 enabled is
> > using the a router-box with a medium preference prefix / default route
> > for the first connection for a netcat6 connection. Then I added
> > another router with a different subnet and a higher preference to the
> > same local link. The old netcat6 session still used the medium
> > preference prefix / default route as expected and did not break the
> > tcp sessions therefore. Then I started another netcat6: It correctly
> > used the new prefix with a higher preference. However, it still uses
> > the same default route, although I had explicitly set a different
> > AdvRoutePreference for the default route on this second
> > router. Shouldn't the new netcat connection then use the new default
> > route instead (while the old one sticks to the old default route)? 
> I think it should.
Cool! I now did some new tests with a custom-build
kernel. It seems to work a little better, in that it now also
switched the default route. The new netcat session goes over the
right prefix and to the right router with the high preference.
However, there still seems something wrong: The old netcat
connection still uses the old prefix (which is good, otherwise I'd
lose the connection, of course), but new packets take the
high-preference default route immediately - which is going to fail
if the gateway routers / the ISPs behind them do strict subnet checking.

And just for clarification, a route ::/0 with the same preference
is probably superfluous, right (though I tested both with the same

The gateway is running a 2.6.32 kernel by the way.

Thanks for your reply Arnaud.

Cheers, Linus