Re: dhcpcd and multiple IPv6 gateways on one interface

[Roy Bixler <rcbixler%nyx.net@localhost>]

On Tue, Sep 12, 2017 at 09:48:01AM -0700, Andy Ruhl wrote:
> On Tue, Sep 12, 2017 at 9:28 AM, Roy Bixler <rcbixler%nyx.net@localhost> wrote:
> > Update: something changed on the network and I'm not getting the
> > syslog spam anymore.  The only evidence of the change I have is an
> > "arp info overwritten" message for the IPv4 default router.  So, I
> > suppose that the syslog spam might have been showing a real problem in
> > the router configuration.  Whatever it was, I still see multiple
> > default gateways for IPv6 in Linux, so I still have a question about
> > that.
> 
> It is the case that certain network devices (ahh Cisco) like to "be a
> router" as soon as you set up an IPv6 address on an interface. You
> have to do a bunch of stuff to turn off router advertisements if you
> just want an interface with an IPv6 address. This might be happening
> in your network.

I'm pretty sure that our network does not use Cisco as the router.  I
think that the admin. uses some kind of a Linux distribution which he
then sets up with an OSPF daemon and so forth.

> I'm not sure if there is a proper way for a host to handle this. Would
> be interested to see what the routing table on the Linux machine looks
> like. It would be interesting to know if it's trying to do
> reachability for something (don't know what) and decide which one
> works and put it higher in the table.
>
> That was an unresearched response, hopefully someone else knows more.

Fair enough.  I've tried to do a little research myself and my
understanding of Linux is that it sets up the multiple IPv6 default
routes on the same interface when it gets advertisements from
different routers which have the same preference.  Some folks may say
that's not recommended, but I could see using it if load balancing is
a consideration.  Otherwise, I would think it would be unnecessary.
Why not just set one router to have a higher preference?  Then I'd
imagine failover would occur if that router went down.  My only
experience with setting up a network is with a small SOHO LAN, so I
may easily be missing something.

The routing table looks like this:

% ip -f inet6 r
<line for route to local network>
fe80::/64 dev br0  proto kernel  metric 256
default via fe80::nnn:nnnn:nnnn:2e47 dev br0  proto ra  metric 1024 expires 1783sec hoplimit 64
default via fe80::nnn:nnnn:nnnn:2d5b dev br0  proto ra  metric 1024 expires 1783sec hoplimit 64
default via fe80::nnn:nnnn:nnnn:2df3 dev br0  proto ra  metric 1024 expires 1783sec hoplimit 64

So there are 3 default IPv6 routes on the same interface.

-- 
Roy Bixler <rcbixler%nyx.net@localhost>
"The fundamental principle of science, the definition almost, is this: the
sole test of the validity of any idea is experiment."
-- Richard P. Feynman