Q:
What are the scenarios in which preserve-vlan is supported?
How does the preserve-vlan knob on the virtual-ap profile work?
What are the scenarios in which the preserve-vlan knob on the virtual-ap can be enabled?
A:
The Even VLAN assignment aims to address the issue of uneven distribution of clients over the different VLANs in a pool. This algorithm will pick the least used VLAN in the pool in order to keep an even distribution of users in a VLAN pool.
Since the controller tends to maintain even distribution, there are more chances that a uses falls into a different VLAN whenever the users gets disconnected and re-connect back. To handle this scenario, the "preserve-vlan" option on the virtual-ap can be enabled so that the controller can try to retain the VLAN ID for the client by looking up the client MAC in the bridge table. If the entry is found, the controller will retain the VLAN for the client, i.e., if the user re-connects back within a short period of time before the bridge table entry ages out. If an entry is not found, the controller may allocate a new VLAN ID for the client.
Preserve-vlan feature can be used only with Even VLAN assignment and not with Hash assignment since hash results in a deterministic VLAN assignment for a client for a given VLAN pool.
Since preserve-vlan is a knob on the virtual-ap profile, it is supported only when the vlan pool is mapped on the virtual-ap profile. Preserve-vlan is not supported when the vlan pool is mapped on the user roles, user rules, VSA or a server derivation rules. In case the vlan pool is mapped on the virtual-ap profile and also on the user roles, user rules, VSA or a server derivation rules, preserve-vlan may not take effect since VAP based vlan assignment doesn't take precedence. (Note: If more than one VAP uses the same VLAN pool, preserve VLAN will not kick in.)