Users Guide

622| Increasing Network Uptime Through Redundancy and VRRP Dell Networking W-Series ArubaOS 6.5.x| User Guide
Feature Requirements
This feature can be enabled on controllers in a master-local topology where centralized licensing is enabled on
the active and standby controllers, or on independent master controllers that are not using VRRP-based
redundancy. If centralized licensing is disabled, the standby AP over-subscription feature is also disabled.
Standby controller over-subscription and the high availability state synchronization features are mutually
incompatible and cannot be enabled simultaneously. If your deployment uses the state synchronization
feature, you must disable it before you enable standby controller over-subscription.
Standby Controller Capacity
The following table describes the AP over-subscription capacity maximum supported tunnels and the
controllers that support this feature.
To determine the number of standby tunnels consumed by APs on each active controller, multiply the number
of APs on the active controllers by the number of BSSIDs per AP. For example, consider a deployment with
four active W-7210controllers that each have 512 APs with 8 BSSIDs. The APs on each active controller
consume (512 * 8) tunnels, for a combined total of 16,384 tunnels. A single W-7210controller using the
standby controller over-subscription feature can act as the standby controller for all four active controllers in
this example because this topology is within the 4x rated AP capacity limit and maximum tunnel limit for the W-
7210controller model.
If the network administrator later changed all the APs in this deployment to support 10 BSSIDs, each active
controller would use (512 * 10) tunnels, for a combined total of 20,480 tunnels on the four active controllers.
The tunnels required by the APs on the active controllers would then exceed the maximum tunnel limit for the
standby controller, so the standby controller can no longer support all APs on the active controllers. Dynamic
changes to configuration (such as the addition of BSSIDs to any AP group) causes all the standby APs to
disconnect and reconnect back to the standby controller defined by their updated configuration
To view information about the numbers of currently associated APs and supported BSS tunnels, and the
remaining capacity for additional APs and BSStunnels, issue the CLIcommand show ha oversubscription
statistics.
AP Failover
If a standby controller reaches its AP over-subscription capacity or exceeds its maximum BSSID limit, the
standby controller drops any subsequent standby AP connections. A dropped AP attempts to reconnect to the
standby controller, but after it exceeds the maximum number of request retries, the AP informs the active
controller that it is unable to connect to the standby controller. The active controller then prompts the APto
create a standby tunnel to another standby controller, if one is configured.
If an active controller fails, the APs on the active controller failover to the standby controller. Once the standby
controller has reached its capacity for active APs, it terminates tunnels to any standby APs that the controller
can no longer serve. When these APs detect that there is no longer a heartbeat between the AP and the
standby controller, they notify their active controller that they can no longer connect to the standby. The
active controller then promptsthe APs to establish standby tunnels to another standby controller, if one is
configured.
Configuring High Availability
A controller using this feature can have one of three high availability roles: active, standby, or dual. An active
controller serves APs, but cannot act as a failover standby controller for any AP except those that it serves as
an active controller. A standby controller acts as a failover backup controller, but cannot be configured as the
primary controller for any AP. A dual controller can support both roles, acting as the active controller for one
set of APs, and a standby controller for another set of APs.