Multiple default routes, same subnet

[Dale Dellutri]

On Fri, Nov 9, 2012 at 9:39 AM, Fernando Lozano <fernando at lozano.eti.br> wrote:
> 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.

Some routers will do this for you, in either a "load-balanced" or contingency
configuration.  For example, the Cisco RV042.

Some of the router distros, Clear OS, Vyatta, m0n0wall, and the shorewall
package can help with multiple ISP configurations.

For shorewall:
  http://www.shorewall.net -> Documentation -> Multiple Internet Connections
from a Single Firewall.

> 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.
>
>
> []s, Fernando Lozano
>
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-- 
Dale Dellutri