Wireless/Redundant Edge Services xl Module Management and Configuration Guide WS.02.xx and greater

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Introduction
Layer 2 and Layer 3 Roaming Between RPs and Modules
Roaming Between RPs on Different Wireless Edge
Services xl Modules at Layer 3
Roaming always occurs within a WLAN—that is, a station can roam only to
another RP if that RP supports the same SSID. Otherwise, the station does not
roam; it connects to a new network.
For the roaming described in the previous sections, the roaming station’s
traffic arrives in the same VLAN when it is bridged into the Ethernet network.
This allows the station to keep the same IP address and maintain active
sessions. Roaming between two wireless cells but within the same VLAN on
the wired side is called radio frequency (RF), or Layer 2, roaming.
Whenever possible, you would configure all of your modules to support the
same VLAN for the same WLAN, and all roams would be at Layer 2. However,
two Wireless Edge Services xl Modules separated by a router must forward
wireless traffic into different VLANs, complicating the roaming process. When
a station roams to an RP adopted by a different module, the station’s IP address
is no longer valid, and the station loses its active sessions.
Wireless Edge Services xl Modules use network, or Layer 3, roaming to solve
this problem. Figure 1-26 illustrates a network that requires Layer 3 roaming.
The module on the left places wireless stations in WLAN A in VLAN 1 while
the module on the right places stations in WLAN A in VLAN 20.