To guarantee that an account session ID is unique across router reboots, we added the following config to our LNS routers:
routert#conf t
router(config)#radius-server unique-ident 1
router(config)#end
router#wr m
If we ever replace a router, we need to increment the unique-ident number to ensure that the new router does not re-use the same session-id's as the old one. This is because we use nas:sessionid as the unique key to identify a session.