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

9-5
Fast Layer 2 Roaming and Layer 3 Mobility
Overview
same redundancy group. When a user authenticates to one module, that
module uses the redundancy group communications to transmit the user’s
credentials to all modules in the group. (You can set up encryption to protect
the credentials.) The other modules cache the credentials so that they are
ready to be sent to the RADIUS server should the user later roam to one of
these modules.
Note that the redundancy group solution does not enable the Web-Auth WLAN
to include any more RPs than the single module solution: a redundancy group,
just like a single module, has a 48 RP limit. The only reason to have multiple
Wireless Edge Services xl Modules support the WLAN would be to add
capacity to the system.
Layer 3 Mobility
A station can roam seamlessly between two RPs adopted by the same Wireless
Edge Services xl Module (as long as both RPs support the WLAN). Likewise,
a station can roam seamlessly between two RPs adopted by different modules
as long as those RPs support the same WLAN. However, two modules that do
not support the same virtual LANs (VLANs) complicate the roaming process:
the station’s IP address is no longer valid, so it loses its active sessions.
Wireless Edge Services xl Modules use Layer 3 mobility to solve this problem.
You must enable Layer 3 mobility to support roaming between modules that
have these characteristics:
■ The modules support the same WLAN (or WLANs).
■ Each module places traffic from that WLAN in a different subnetwork.
For Layer 3 mobility to function correctly, the different subnetworks must
use different VLAN IDs.
Figure 9-1 illustrates a network that requires Layer 3 mobility. 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.