Refer to the exhibit.
What are three requirements for configuring Cisco Aironet access points (APs) in a WLAN
network which will allow for all wireless clients to work without service interruption while
roaming from access point to access point? (Choose three.)
A.
All access points should be configured with a unique IP subnet range.
B.
All access points should be configured with the same guest mode SSID.
C.
All access points should be configured with identical VLANs.
D.
All access points should be configured within the same IP subnet.
E.
All access points should be configured with identical SSIDs.
F.
All access points should be configured only with the native VLAN.
Explanation:
This question shows an example of layer 2 roaming. A L2 roam occurs when a WLAN client
moves from one access point to another within the same subnet. If the client moves to a new
access point on a different IP subnet, L3 roaming occurs after the L2 roam has completed.
Roaming is always a client station decision. The client station is responsible for detecting,
evaluating, and roaming to an alternative access point. Figure 3 Sequence of Events for L2
Roam illustrates a L2 roam.
Figure: Sequence of Events for L2 RoamThe arrows in the figure indicate the following events:
1. A client moves from access point A coverage area into access point B coverage area
(with both access points in the same subnet). As the client moves out of the range of access
point A, a roaming event (for example, maximum retries) is triggered.
2. The client scans all IEEE 802.11 channels for alternative access points. In this case, the
client discovers access point B and reauthenticates and reassociates to it. After associating
to the new access point B, if it is configured for 802.1X, the client begins IEEE 802.1X
authentication.
3. Access point B sends a null media access control (MAC) multicast, on the client’s virtual
local area network (VLAN), using the source address of the client. This updates the content
addressable memory (CAM) tables of the upstream switch and directs further LAN traffic for
the client to access point B and not access point A.
4. Using its own source address, access point B sends a MAC multicast, on the native
VLAN, telling access point A that access point B now has the client associated to it. Access
point A receives this multicast and removes the client MAC address from its association
table.
When a roaming event occurs, the client station scans each 802.11 channel.2 On each
channel the client station sends a probe, and waits for a probe responses or beacons from
access points on that channel. The probe responses and beacons received from access
points are discarded unless they have matching Service Set Identifier (SSID) and encryption
settings.
Reference:
http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a00
801c5223.html
