3Com WX4400 Reference Manual page 65

The client uses the same authorization parameters for the new session
as for the old session. For example, changing the Encryption-Type or
VLAN-Name parameter might cause a new session to be recorded,
rather than a roam within the same session.
A disassociated session has a grace period of 5 seconds in which the
session history can be retrieved and forwarded. After 5 seconds, the
session is cleared, and its accounting is stopped. You cannot configure
the grace period.
If the client MAC address in a Mobility Domain is not found in 5 seconds,
the session is considered new.
The 802.1X reauthentication timeout has little impact on roaming. If the
timeout lapses, 802.1X processing is performed on the existing
association. Accounting and roaming history are not affected if the
reauthentication is successful, because the client is still associated with
the same MAP. If reauthentication fails, the session is cleared, and it is
not eligible for roaming. If the client associates to the same MAP, that is
recorded as a new session.
Roaming creates the following effects:
Remote Authentication Dial-In User Service (RADIUS) accounting is
treated as a continuation of an existing session, rather than a new
one.
You can view roaming history in the Monitor tab. See "Monitoring the
Network" on page 437.
The old session is cleared from the WX, even if the client did not
explicitly disassociate from the MAP and the 802.1X reauthentication
interval has not lapsed.
Defining a Mobility Domain 65