Multiple default routes, same subnet

[lee]

Fernando Lozano <fernando at lozano.eti.br> writes:

> Hi,
>
> I have two internet links, from different ISPs for my office network,
> each one with it's own router (which is a linux PC) so I don't loose
> internet connectivity easily. One ISP has a much higher bandwitch, so
> usually all my computers use it as the default gateway. The other one
> is intended as a contingency link.
>
> Today I have to manually change the default gateway on all computers
> when the main link goes down. I want an automated way to do this.
>
> All how-to's I could find on google were such as:
> http://www.generationip.com/documentation/network-documentation/93-howto-setup-multiple-default-gateway-on-linux
>
> Where they use iproute2 so a single router/computer can switch between
> two internet links. While I can undersand this scenario, the single
> computer becomes a single point of failure. I don't want to setup a
> "cluster" using heartbeat, cluster suite and similars just to provide
> a router failover. It looks overkill.
>
> Other problem, all iproute2 samples I find thave those two gateways on
> different subnets and use the source address to route properly and
> avoid problems like "onion routing".
>
> I'd like to have two routers/computers on the same subnet, each one
> connected to it's own internet link, and have the network computers
> switch from the main one to the contingency one when needed.
>
> So a computer would have two default gateways on the same subnet.  I
> can't find how to change the iproute2 samples setup so the gateways
> can be on the same subnet and everything works. I allways end up
> having all traffic going though one link and when it fails I loose
> internet connectivity.

Each of your routers is a single point of failure at the moment because
both the internet connection and the router connected to it can go down
at the same time.

To avoid a single point of failure, you would need to connect each
router to each internet connection.  Add to that a way to remotely
configure which gateway to use on the computers on the LAN.  Then
establish communication between the routers so that they can agree upon
which of them configures itself as the gateway for the computers on the
LAN, considering that either of the routers and either of the internet
connections my be down.

Such a solution probably already exists ...


For a simple solution, you could set up each of the routers to be
connected to both internet connections and to be able to switch between
between them, and to act as a gateway.  In case the router goes down
(How often does that happen?), just manually plug the spare one in
instead.

The shorewall documentation describes a setup that might be useful for
you, see http://www.shorewall.net/MultiISP.html


-- 
Fedora 17