How to Setup DD-WRT Roaming using one 1 SSID and multiple APs
Roaming access is actually much easier then you may think. If you are installing additional Access Points (APs) to cover a wider area with Wi-Fi, it is possible to allow clients to roam freely between them. The method that DD-WRT Engineers recommend is to use the same SSID and Security settings on each access point.
Use a different channel on each AP. e.g. if you are in the US and installed two access points, use channels #1 and #11. Or if three access points, then use channels #1, #6, and #11 (setting the channels at least 5 apart should help keep interference between APs to a minimum). If you have a residential gateway with wireless turned on, and just one AP, then the same applies: each gets a different channel.
LAN to LAN uplink
To complete the link between the two routers, connect a LAN port on the central router, to a LAN port on Linksys router (to be used as your WAP). You may need a crossover cable to do this, although many modern routers have an automatic polarity sensing. To test this, connect a standard Ethernet cable between the two routers. If the LAN light comes on, the router has automatically switched the polarity and a crossover cable is not required. *NOTE* – Almost every router you will find will be auto sensing.
When using multiple Access Points, each one should be connected by LAN to LAN uplink as described above. They can even be attached to different switches within the same organization.
I got this information directly from DD-WRT’s website here