WRT54GL - Default Route In Quagga Doesn't Work If Specified Using Interface Rather Than Address

I have the following zebra.conf on my router box "A":

hostname nuclear-router
password password
enable password password
interface eth0
ip address 192.168.2.1/24
multicast

[code]...

eth0 is connected to a switch and contains the 192.168.2.0/24 range, and A is connected to my WRT54GL "B" (with IP 192.168.1.1, containing 192.168.1.0/24 range, DHCP serves IPs above .100) wirelessly. B is connected directly to my DSL modem.

The problem is that when I specify the default route using the interface name, I can't connect to the internet from A or any hosts behind it - I'm always hit with a "no route to host" error. Name lookup and pinging any local host (even in a different range) or the DSL modem works fine - it's only when going beyond the modem that things stop working. However, when I use the IP of B as the gateway, it works fine. I noticed that route output on A when the default route was specified using the interface alone had only an asterisk in the gateway column. I was under the impression that these approaches should be identical in practice, so though I got it working, I'd like to know what I'm misunderstanding (and/or misconfiguring). Why didn't the default route work when specified using just the interface?

All routers and the modem too have RIP (version 2) enabled, and of course the password isn't really the word "password".