The Cisco 5510 has a fixed IP address, but the Linksys BEFSX41 does not. I'm planning to use the Cisco/Linksys as a replacement for a pair of ZyWall 10 VPN routers. The ZyWall's allow me to specify (on the fixed IP end) the FQDN of the dynamic IP end, but it doesn't seem that the Cisco does.
I've read some posts that seem to describe similar configurations, but it's not clear how to implement them on the 5510.
The Cisco/Linksys solution was recommended by CDW as a replacement for the ZyWall's, but if it doesn't have this feature, it's pretty much useless.