Definition
Hot-standby backup (HSB) is a feature that improves network reliability through redundancy between two devices. After the two devices determine the master and backup roles, the master device forwards service packets and the backup device monitors status of the master device. The master device periodically sends its status information and data to be backed up to the backup device. When the master device fails, the backup device takes over the services immediately.
Purpose
On a WLAN, an AC can manage several hundreds of APs. If the AC becomes faulty, services of all the APs that associate with the AC are interrupted. The reliability of ACs greatly affects HA of the network.
The AC hot-standby backup (HSB) function can solve this problem. This function has two modes: HSB+VRRP and HSB+dual-link backup. HSB supports batch backup and real-time backup between the two access devices. Before link switching, the standby device synchronizes information from the active device. When the active device fails, service traffic is immediately switched to the standby device without interrupting services. This improves connection availability. Dual-link backup or VRRP can fast detect whether the active AC is faulty so that the standby AC can become the new active AC in a timely manner. This function ensures user service continuity.
Backup Modes
The HSB solution provides two networking modes: active/standby mode and load balancing mode.
Active/Standby Mode(Using VRRP hot standby)
As shown in Figure 1, AC1 and AC2 form a VRRP group. AC1 is the master device and AC2 is the backup device. When AC1 is working normally, it processes all services and transmits session information to AC2 through the HSB channel. AC2 does not process services and only backs up session information.
When AC1 fails, AC2 starts to process services, as shown in Figure 2. Because session information is backed up on AC2, new sessions can be set up without interrupting the current session. This improves network reliability.
When the original master device (AC1) recovers, it becomes the master in preemption mode. In non-preemption mode, AC1 retains in Backup state.
Load Balancing Mode(Using Dual-link hot standby)
As shown in Figure 3, the AP establishes CAPWAP tunnels with two ACs and differentiates the active and standby ACs based on the priorities of the CAPWAP packets delivered by the two ACs. A hot-standby backup (HSB) tunnel is established between the two ACs. AC1 is the master device for AP1 and AC2 is the backup device. AC1 processes all service traffic from AP1 and transmits session information to AC2 through the HSB channel. AC2 does not process service traffic from AP1 and only backs up session information. For AP2, AC2 is the master device and AC1 is the backup device. AC2 processes all service traffic from AP2 and transmits session information to AC1 through the HSB channel. AC1 does not process service traffic from AP2 and only backs up session information.
On the network, AC1 forwards service traffic from AP1 and AC2 forwards service traffic from AP2. In this way, service traffic is load balanced on the network.
When AC1 fails, service traffic from AP1 is automatically switched to AC2 to ensure nonstop service forwarding, as shown in Figure 4. Service traffic from AP2 is still forwarded by AC2.
When the original master device (AC1) recovers, service traffic can be switched back to the master device or retained on the backup device, depending on the configuration.
