Re: [LARTC] Multipath, 2.4.16 + Julian's patches.

[Christoph Simon <ciccio@kiosknet.com.br>]

On Fri, 14 Dec 2001 10:52:28 -0500
Adrian Chung <adrian@enfusion-group.com> wrote:

> Actually, after I wrote my last couple of messages, I was thinking
> about it more, and I think it's a misunderstanding that I have.  Your
> HOWTO is mainly geared towards load balancing, and failover when one
> of the links goes down.
> 
> I'd like to do that, but I'd like to mainly use one link, and only
> fall back (fail over) to the second link when the first goes down.

You are right, this is a quite different situation. It worked for me
like that before installing Julian's patches. I had no multipath route,
but explicit routes to the gateways and never more than one default
route. Then I had a daemon running which ping'ed the gateways and
would delete the default route to put the other. But this is loosing
the bandwidth of a line which is being payed, so I was unhappy with
that solution. Unfortunately, I don't know which is the best routing
setup for your case. Maybe somebody else on this list can give you a
hint.

> My two links are through different ISP's, and I want to be able to use
> both ISP's news, and proxy servers.  In order to do this, I need to
> make sure that when accessing ISP1's news server, I always connect via
> the same link.
> 
> Load balancing defeats this, by sometimes routing to ISP1's news
> server via ISP2, and then I get denied access because I don't have a
> source IP on ISP1's network.

You should be able to add an explicit host route in table main. That
is hit before the multipath route is queried. You do know the IP
of that ISP's news server, do you?

> 1) is there a way to only use the second link if and when the first
> goes down, instead of load balancing via both links all the time?

As I said above, the contrary was the aim of my setup. I would imagine
that the solution is not using a multipath route but just two default
routes with different priority. If the first is working, it should be
used always; if it goes down, the second would be used. And if the
first comes up again, thanks to Julian's patches, it should be used
again. But note, that for this last step to work, I think you still
need to ping/arping the gateways, such that there is a chance to
detect that the failing link is up again.

> 2) is it possible to implement source-based policy routing but fall
> back to the second link when the preferred gateway is down?

I think it is, using two default routes with different priority.

> Does any of this make sense?

Unless you have to pay for traffic on one link and not on the other,
I'm not sure why you would want to leave a link unused. Beside that, I
think it does make sense.


--
Christoph Simon
ciccio@kiosknet.com.br
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