Following is a snippet from the Campus VRD available at
http://www.arubanetworks.com/vrd/CampusWNetworksVRD/wwhelp/wwhimpl/js/html/wwhelp.htm
To determine which pool to put the user into, the user MAC address is run through a hash algorithm. The
output of this algorithm places the user into one of the VLANs in the pool and ensures that the user is always
placed into the same pool during a roaming event. As the user associates with the next AP, the address is
hashed. The user is again placed into the same VLAN on the new AP, because the hash algorithm generates the
same output as before. The user can continue to use their existing IP address with no break in their user
sessions.
A single VLAN or a VLAN pool can be named by the administrator. The VLAN names are global, but the VLAN IDs
associated with those names are local to the controller. The VLAN names are configured globally in the master
controller and are synchronized to the local controllers. The VLAN IDs that are associated to a particular VLAN
name are defined in the local controllers and can vary across the controllers.
The example network uses 10 VLANs (VLAN 150-159) split into these two pools:
pool-7 is used by the employee and application VAPs in the AP group that uses the virtual IP (VIP) of
Virtual Router Redundancy Protocol (VRRP) instance 7 as the local management switch (LMS) IP.
pool-8 is used by the employee and application VAPs in the AP group that uses the VIP of VRRP instance
8 as the LMS IP.
N O T E
The hashing algorithm does not place users into the available pool of VLANs in a
round-robin method. Ten clients that join a WLAN are not load balanced equally
among the VLANs. Instead, the distribution is based on the output of the hash. One
VLAN might have more users than the others. For example, consider 150 clients that
join a WLAN with just two VLANs in the pool and with 80 addresses per VLAN available
for clients. Based on the output of the hashing algorithm, 80 clients are placed in one
VLAN and 70 in the other. When the 151st client joins, the output of the hash might
place the client in the VLAN whose scope of 80 addresses has already exhausted. The
result is that the client cannot obtain an IP. To avoid such a rare situation, the network
administrator should design pools with sufficient number of user VLANs and DHCP
scopes to accommodate the user density.