Multiple gateway networking question

Question

We are a small company owned by a big company. We already have our own network/gateway/ISP which works fine, but our parent wants to install their own T1 line to give us access to their private network, which contains things we'll need (which we want) but also restricts and slows down our Internet access (which we don't want). I'm told there should be a way of switching automatically between the two gateways without having to manually change router addresses (ie. default would be our network, but it would switch to parent's network if the particular request cannot be fulfilled via ours). Any ideas on how to set something like this up? Sorry... I'm a bit of a networking noob.

Answer 1

The easiest way depends on how the T1 and your existing ISP connection terminate in your network.
Ideally both connections should connect to the same router - then you point all your machines to use this router as their default gateway and the router takes care of sending traffic to your ISP/the T1 based on destination IP.
If you don't have a single router that both connections terminate on then you'll have two routers in your network. At this point you have two options:
One is to set one router as your default router and set a static route on that router (if it supports it) so that traffic for the main company's network is sent over to the other network:
Host --> Router1 --> ISP
                |
                |
                V
               T1 router --> head office
In this model you only need to make one change on the default router and you're done.
If that doesn't work for some reason (e.g. your ISP router doesn't support static routes) then you can setup static routes on each host so that each host sends the traffic to the appropriate router:
Host --> (default route) --> ISP router --> internet
  |
  v
 (static route) -->T1 router --> head office
The disadvantage here is that you need to make a configuration change on each machine, typically along the lines of adding a static route:
sudo route add -net <subnet of head office> <IP of T1 router>
This tells your machine to send all traffic for <subnet of head office) to <IP of T1 router>
Either way you need to have some understanding of the network infrastructure - IP address/subnets, router addresses, etc.