Last week I switched from ADSL to EuroDOCSIS cable. I couldn't just shut down the DSL connection as several services were still connecting to that IP. So I made my home router (Fedora 15) kind of multihomed.
Configuration
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I haven't even taken the time to seperate the ADSL and cable modems, I just hooked them up to the same network interface. Of course, they got their own IP range.
ADSL modem: 192.168.2.254/24
Cable modem: 192.168.1.1/24
Step 1: Add ip
ip addr add 192.168.1.2 dev eth0
Step 2: Routing
By default we send all outgoing packages to the default gateway at 192.168.2.254. Only packets that are answers to incoming packets on the 192.168.1.x network must be routed back to 192.168.1.1
ip rule add from 192.168.1.0/24 lookup 1001
ip route add default gw 192.168.1.1 table 1001
That's all. Of course, above setup only takes care of answering to incoming packets. If you want to load-balance or have a fail-over setup, I suggest you read the Linux Advanced Routing & Traffic Control HOWTO.
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