A sub-domain is an ensemble formed
by a mobility controller and the mobility agents it directly
manages. An inter sub-domain mobility event implies
communication between two mobility controllers. These 2 mobility
controllers can be configured with the same mobility group value
and recognize each other. They will appear in each other's mobility list, or
they can be configured with different mobility group values, and
still recognize each other.
When the roaming event occurs across sub-domains
between MCs in the same mobility group, the 802.11r key
domain advertised by the new APs are the same. Additionally, the
client PMK is also transmitted to all MCs upon the client's
initial authentication. The new MC does
not need to force the client to reauthenticate, and the new
MC also knows which previous MC
was managing the wireless client mobility.
Figure 4. Inter Sub Domain Handoff
The following steps are involved in the inter sub domain handoff, when mobility controllers belong to the same mobility group:
- When a clients PMK was sent by the initial MA to all the MCs in the mobility group, the new MA already had already received the client PMK from its MC, and re-authentication is not required.
- The new MA was not notified previously of the station's presence on a neighboring MA inside a different SPG it transmits the mobile announce to the sub-domain's MC.
- On receiving the mobile announce message, the MC forwards the mobile announce to the MO, which performs a lookup in its database, and forwards the request to the MC that was previously providing service to the station.
- The previous MC, in turn, forwards the request to the MA that was previously providing service to the station.
- The old MA, shown in yellow color above, transmits a Handoff message directly to the new MA.
- The old MA must notify the other MAs
within its SPG of the fact that the station has left the
SPG using a Station Left message. This ensures that
if the station comes back to one of the MA , the MA is aware of the fact that the station is
no longer serviced by the old MA.
- Once the handoff is complete, the new MA transmits the Handoff Complete message in a reliable fashion to the new Mobility Controller.
- The new MA then transmits the Handoff Notification to all other MAs.
- The new MC then transmits the Handoff Complete to the old MC.