Re: [LARTC] route with different metric

[Christoph Simon <datageo@terra.com.br>]

On Wed, 26 Sep 2001 15:10:35 +0200
"Christian Lox" <lox@eq-media.de> wrote:

> Hi everyone!
> 
> Sorry if this is a FAQ but I did not find a searchable archive.
> 
> What I am planning to do:
> 2 different connections to 2 different ISPs.
> One is handled by an ISDN card, one via ethernet to their router
> (DSL).
> We want to use the DSL line; only if this fails we need the ISDN
> line.
> I played around with the metrics but could not get it to work.

I'm trying to find a solution for this very situation since a long
time, but wasn't able to do so. What is working for me now is a small
daemon I wrote which essentially replaces ipup and ipdown.

Initially it reads the standard /etc/network/interfaces files, paying
attention to an additional parameter of priority which is commented
out. Then, the interfaces are set up the normal way, but the routes
are set with mask /32 just to reach the gateway and no default
route. Then, every 60 seconds, a single ping is sent to the gateway of
each of the external interfaces, marking it either up or down. After
that, the priority value is checked and the program chooses that with
the highest priority. If that is also marked active, nothing else is
done, but if it wasn't active, the default route is put to that
interface and the firewall script is reinitialized. Of course, if the
active interface becomes down, the default route is removed.

This works fine, but it's a real pitty that I can't use both
interfaces in a balanced way as this would effectively duplicate my
bandwidth while both are up. Another (small) problem with this
approach is, that switching interfaces automatically stops operators
from realizing that a line is not working for to take corrective
action. So I decided to send an email in such case, which of course
fails if both interfaces are down because I'm geografically far from
these networks. But then, operators do realize that something is
wrong.

I wrote this program some days ago and it's really a dirty hack, so I
doubt the code would be more useful than this description.

I don't have experience with Cisco routers, but was told that load
balancing to two or more independent external links is possible with
them. So I guess, somehow it should also be possible with Linux. If
somebody could shed light on this, I would be really greatful.

--
Christoph Simon
datageo@terra.com.br
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