Feature Guidelines and Limitations
Note the following guidelines and limitations before enabling this feature in your high availability deployment:
n Only APs that support 802.11n and 802.11ac can support client state synchronization.
n The client state synchronization and standby switch over-subscription features are mutually incompatible
and cannot be enabled simultaneously. If your deployment uses the standby switch over-subscription
feature, the feature must be disabled before enabling state synchronization.
High Availability Inter-Switch Heartbeats
The high availability inter-switch heartbeat feature allows for faster AP failover from an active switch to a
standby switch, especially in situations where the active switch reboots or loses connectivity to the network.
The inter-switch heartbeat feature works independently from the AP mechanism that sends heartbeats from
the AP to the switch. If enabled, the inter-switch heartbeat feature supersedes the AP's heartbeat to its switch.
As a result, if a standby switch detects missed inter-switch heartbeats from the active switch, it triggers its
standby APs to failover to the standby switch, even if those APshave not detected any missed heartbeats
between the APs and their active switch. Use this feature with caution in deployments where the active and
standby switches are separated over high-latency WAN links.
When this feature is enabled, the standby switch starts sending regular heartbeats to an AP's active switch as
soon as the AP has an UP status on the standby switch. The standby switch initially flags the active switch as
unreachable, but changes its status to reachable as soon as the active switch sends a heartbeat response. If
the active switch later becomes unreachable for the number of heartbeats defined by the heartbeat threshold
(default of 5 missed heartbeats), the standby switch immediately detects this error and informs the APs using
the standby switch to failover from the active switch to the standby switch. If, however, the standby switch
never receives an initial heartbeat response from the active switch, and therefore never marks the active switch
as initially reachable, the standby switch will not initiate a failover.
This feature is disabled by default. It can be used in conjunction with the high availability state synchronization
feature only in topologies that use a single active and standby switch, or a pair of dual-mode active switches
that act as standby switches for each other. High availability inter-switch heartbeats can be enabled and
configured in the high-availability group profile using the WebUI or Command-Line interface.
For more details on how to enable and configure inter-switch heartbeats, see Configuring High Availability on
page 638.
High Availability Extended Switch Capacity
The standby switch over-subscription feature allows a standby switch to support connections to standby APs
beyond the switch's original rated AP capacity. This feature is an enhancement to the high availability feature
introduced in AOS-W 6.3.0.0, which requires the standby switch to have an AP capacity equal to or greater than
the total AP capacity of all the active switches it supports.
The following section of this document gives and lists requirements and capacity limitations for this feature.
For more details on enabling the extended standby switch capacity, see Configuring High Availability on page
638.
Starting with AOS-W 6.4.0.0, OAW-40xx Series and OAW-4x50 Seriesswitches that acts as a standby switch can
oversubscribe to standby APs by up to four times that switch's rated AP capacity, as long as the tunnels
consumed by the standby APs do not exceed the maximum tunnel capacity for that standby switch.
AOS-W 6.5.3.x | User Guide Increasing Network Uptime Through Redundancy and VRRP | 637
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