I've been playing with my ER-X while travelling, and I decided to plug it on to my hotel cabled network (and stick my Apple Airport Express on the LAN side) so I could use OpenVPN on the ERX to keep myself connected to the office without starting OpenVPN on my Thinkpad or Mac (yes, I travel with a lot of gear).
Getting the config up and OpenVPN up was no problem, the ER-X did one thing wrong that caused me some initial head scratching: when it first boots it sets all 5 ports as a switch, then it takes another few seconds for my config to load.
My config splits off eth0 as a WAN port, and leaves the other as a vswitch interface. The config also takes care of firewalling, NAT and OpenVPN and those parts work.
The part that does not work well is that the time used from apparently being a 5 port switch and up to my config being loaded is when the Airport (or anything else plugged into the ERX'es LAN ports) decides that it's will ask for a DHCP address and gets the wrong adresses.
Instead of getting a tidy 10.0.0.x from the LAN side of my ERX (with my config active) it ends up in that short time span to get DHCP that is at that time bridged through what is in my config a WAN port (but still a LAN port untill my config loads) and voila, they get weird IPs all over the place, depending on your hotel's DHCP.
Basically the ERX shouldn't start forwarding packets untill the config in /config/config.boot is loaded, or you get unpredictable results.
-A