Redundant network router setup?

[Modulok]

QUESTION: Is there a way to setup a redundant router, such that I can
offload traffic from the primary router to another machine, without
breaking TCP sessions?

BACKGROUND: I have a FreeBSD machine acting as a gateway, running
natd(8) through ipfw(8). When maintenance time comes,  the server is
taken offline, causing every client who was dependent on it
connectionless until I'm done monkeying around. I could hook up
another server with an identical configuration and then just jack the
cable from one machine to the other. This disconnects all clients
momentarily, until they re-initiate their TCP connections. If we
excuse it as a "computer glitch" no one gets too angry. Needless to
say, there must be a better solution for high availability. Maybe this
happens at the switch level? I don't know. Looking for pointers on how
to set something like this up.

Pointers, Ideas?
-Modulok-