We have a LAN with 20 machines on it. They are connected to a switch which also connects to our primary Westell DSL modem for internet access.
The modem does the NATing and is the DHCP server, (so that our internal addresses are 192.168.1.*). We have a backup modem and phone line which is just like the primary except, of course, its IP address to the outside world is different.
The plan is that if the primary line fails, we just move the cable that connects the modem and switch from the primary to the backup modem. It's crude and not exactly real-time but sufficient for our needs.
The question is, when the switch-over is made, what happens to the DHCP assigned addresses? The machines already have addresses, but the backup modem doesn't know about them. So what happens? Will confusion reign on the LAN?
Best Answer
Will confusion reign on the LAN?
Generally, no. The machines that have addresses will still have them, until their lease expires. I'm not sure how smart the Westel DHCPd is, but most will verify an address is not in use before assigning it. You can avoid any possible collisions by setting each router to use a different range of addresses: router 1 -- 192.168.1.100-149, router 2 -- 192.168.1.150-199. When new machines come online or old leases expire, they'll get an address from the currently active router.
(Just remember to not set any host to 100-199.)