How to setup a redundant internet connection using 2 routers

[gknyc]

I have 2 seperate internet connections. One through cable modem which has a dynamic IP and the other one a static IP based DSL connection. They are at the moment connected independently through their individual Linksys WRT54G wireless routers. I also have a LinkSys 8 port switch where both routers are connecting into.
I would like to run my web and mail servers through the static IP based DSL and everything else through the other router due to performance. The PC that hosts my Exchange server also happens to be my domain controller. So, it is important that clients on the cable modem side can see the domain controller on the other router.
I have 192.168.0.1 range on cable modem side. My DSL modem uses 192.168.2.1 and the router connected to it uses 192.168.1.1. This does not work at the moment.
I need to be able to resolve cross router addresses and provide the DSL based gateway as a failover to the cable based one. I think I can accomplish this as long as clients can see both subnets by entering 2 IP addresses for the default gateway, first to be the cable and the second to be the dsl.
I would appreciate any help on configuring these routers so that clients on either one can see the clients on the other side.

 

[lui3]

Can you place both networks behind the routers on the same subnet?

You can use another router in place of the switch and specify specific routes in its routing table to load balance and failover.

If you do specifiy 2 default gateways on the hosts make sure that one has a higher metric than the other ( the one with the higher metric will be less preferred). The router will choose the lower metric if load balancing is not enabled.

With the mail server if the static ip address that is registered in dns for mail delivery becomes non responsive then the mail will cease to be delivered. Even if the mail server could send mail out the cable modem side it would become a different ip address once it was natd and left the cable modem. Hence any mail servers wanting to send back to the mail server would use the static ip address that was on the dsl modem not the one on the outside of the cable modem side.

This solution would work fine for the clients but web and mail would be broken in the event of a failure.

Handing out dhcp on the mail server would be an easier solution since the either router could fail(both even) and still your LAN clients could use local network resources. You could back up dhcp on the web server by spliting the dhcp scope across both so that in the event of a failure of one of the servers dhcp would still work however if your domain controller fails clients can't login in to network resources anyways. Depends how you are setup.

I would start by standardizing the subnet behind both routers.

Say give the dsl wireless router 192.168.1.2/24 address and the cable modem 192.168.1.1/24 (the main gateway). Then use static ips in the servers to point the gateways at 192.168.1.2 first and then 192.168.1.1 making sure that one has a higher metric that is not preferred. Then on the dhcp server handout dhcp options to point all clients at the 192.168.1.1 gateway for internet access.

A router would do the same thing here just a little cleaner. You might also want to lookup virtual gateway capabilities in the linksys routers. I am not sure if its availble. Cisco has it. Its called HSRP. It allows two gateways to share the same gateway address and when one fails the other takes over. policy routing controls what traffic goes where in the failure and non failure events. Really nice stuff. Open standard version is called VRRP.

Good luck.

Lui3
CCNP,CCDA,A+/Net+
Cisco Wireless Specialization