Implementation
The N+1 backup process consists of active/standby negotiation, active/standby switchover, and active/standby switchback.
Active/standby negotiation
Inactive/standby negotiation, an AP selects an active AC and a standby AC from multiple ACs, and then sets up a primary CAPWAP tunnel with the action but does not set up a CAPWAP tunnel with the standby AC.
In the N+1 backup networking, the procedure for setting up a CAPWAP tunnel between an AP and an AC is similar to that of establishing a common CAPWAP tunnel. For details, see "CAPWAP Tunnel Establishment" in AP Online Process. The only difference lies in the Discovery phase. After the AP discovers ACs, it selects the AC with the highest priority as the active AC and connects to the AC.
In the Discovery phase, an AP sends a Discovery Request packet to find available ACs. After receiving the packet, the ACs return a Discovery Response packet containing the N+1 backup flag, AC priority, load, and IP address. Based on the information contained in the Discovery Response packet, the AP selects an active AC to set up a CAPWAP link. The AP selects the active AC based on the following rules:
1. Check primary ACs on the AP. If there is only one primary AC, the AP selects it as the active AC. If there are multiple primary ACs, the AP selects the AC with the lowest load as the active AC. If the loads are the same, the AP selects the AC with the smallest IP address as the active AC.
2. If there is no primary AC, check backup ACs. If there is only one backup AC, the AP selects this AC as the active AC. If there are multiple backup ACs, the AP selects the AC with the lowest load as the active AC. If the loads are the same, the AP selects the AC with the smallest IP address as the active AC.
3. If no alternate AC is available, the AP compares AC priorities and selects the AC with the highest priority as the primary AC. A smaller priority value indicates a higher priority.
4. If the AC priorities are the same, the AP selects the AC with the lowest load as the active AC.
5. When the loads are the same, the AP compares the ACs' IP addresses and selects the AC with the smaller IP address as the active AC.
Global priority: AC priority configured for all APs
Individual priority: AC priority configured for a single AP or APs in a specified AP group
When receiving a Discovery Request packet from an AP, the AC checks whether an individual priority has been specified for the AP. If not, the AC replies with a Discovery Response packet carrying the global priority. If so, the AC replies with a Discovery Response packet carrying the individual priority. Configure proper priorities on the active and standby ACs to control access of APs on the two ACs.
In the Discovery phase, AP_1 sends a Discovery Request packet to all ACs.
Each AC returns a Discovery Response packet containing its own priorities. Before replying, the AC first checks whether an AP-specific priority has been configured. If so, the AC returns the AP-specific priority. If not, the AC returns the global priority. As shown in Figure 1, when AC_1 receives the Discovery Response packet from AP_1, AC_1 returns the AP_1-specific priority 3 to AP_1. There is no individual priority for AP_1 on AC_2 and AC_3, so AC_2 returns global priority 6 and AC_3 returns global priority 5.
3. AP_1 compares AC priorities in the Discovery Response packets and selects AC_1, which has the highest priority, as the active AC to send an association request.
If AC_1 or the CAPWAP link between AC_1 and AP_1 fails, and no standby AC is designated, AP_1 sends new Discovery Request packets to obtain priorities of the remaining ACs. AC_2 returns global priority 6 and AC_3 returns global priority 5. AP_1 compares AC priorities and selects AC_3 with a higher priority as the standby AC to send an association request.Active/standby switchover
If the IP address of the standby AC is configured on the active AC, the AP sets up a CAPWAP tunnel with the standby AC directly.
If the IP address of the standby AC is not configured on the active AC, the AP broadcasts Discovery Request packets to discover ACs and selects the standby AC to establish a CAPWAP link.
After the CAPWAP tunnel is established, the standby AC delivers configurations to the AP again. To ensure that active and standby ACs deliver the same WLAN service configurations to an AP, perform the same configurations on both ACs. In an active/standby switchover, the AP selects the standby AC to set up a CAPWAP tunnel and get online, and the standby AC delivers configurations to the AP.
The number of online APs supported by the standby AC cannot be smaller than the number of online APs on any of the active ACs.
Assume that the standby AC supports 500 online APs. If an active AC that has 600 online APs becomes faulty, only 500 APs can go online on the standby AC. The remaining 100 APs are forced to go offline, unable to provide services for STAs.
The total number of online APs on all active ACs cannot exceed the configurable number of APs on the standby AC.
The number of APs supported on an AC refers to the maximum number of APs supported by the AC. Assume that the number of APs supported on the standby AC is 1000. If there are 300 online APs on AC_1 and 400 online APs on AC_2, a new active AC allows a maximum of 300 online APs. That is because APs on all active ACs must also be added on the standby AC and have corresponding services configured on the standby AC. When an active AC fails, the standby AC can maintain services for APs.
If multiple ACs become faulty concurrently, not all APs managed by these ACs can go online on the standby AC after the active/standby switchover. In Figure 2, there are 300 online APs (from AP_1 to AP_300) on AC_1 and 400 online APs (AP_301 to AP_700) on AC_2. AC_3 works as the standby AC and allows a maximum of 500 online APs.
Figure 2 Active/standby switchover
If AC_1 becomes faulty, 300 APs (AP_1 to AP_300) perform an active/standby switchover and get online on AC_3. After AC_1 recovers, the APs switch back to AC_1.
After AC_1 recovers from the fault, AC_2 becomes faulty. 400 APs (AP_301 to AP_700) perform an active/standby switchover and get online on AC_3. After AC_2 recovers, the APs switch back to AC_2.
If AC_1 and AC_2 become faulty concurrently, the top 500 APs that associate with AC_3 can implement an active/standby switchover and get online on AC_3. The rest APs cannot go online on AC_3 and services on these APs are interrupted.
Active/standby switchback
After an AP sets up a CAPWAP tunnel with the standby AC, the AP obtains the IP address of its active AC from the standby AC and sends Primary Discovery Request packets at regular intervals to detect the active AC status. After the active AC recovers, it returns a reply packet carrying the AC priority to the AP. When the AP receives the reply packet from the active AC, the AP learns that the active AC recovers and the active AC priority contained in the packet are higher than the priority of its currently connected AC. If the switchback function is enabled, an active/standby switchback is triggered. To prevent frequent switchovers caused by network flapping, the ACs perform an active/standby switchback after 20 heartbeat intervals. As shown in Figure 3, the AP disconnects from the current AC and sets up a new CAPWAP tunnel with the active AC. At the same time, the AP transfers STA data to the original active AC to release resources on the standby AC. The standby AC then continues to provide backup services. During an active/standby switchback, the AP re-establishes a CAPWAP tunnel with the active AC to get online, and the active AC delivers configurations to the AP.
If a primary or backup AC is selected as the active AC, the active AC returns a reply packet to the AP after it recovers. The AP then learns that the active AC has recovered from the reply packet. If the switchback function is enabled, an active/standby switchback is triggered.
Figure 3 Active/standby switchback
Configuring N+1 Backup
Prerequisites
Configure basic WLAN services on the active and standby ACs. For details, see WLAN Service Configuration Guide.
More than two ACs may exist on the N+1 backup network. Each AP has only one active AC and one standby AC planned. You only need to create the same AP system profile on the active and standby ACs, and specify active and standby ACs as the primary and backup ACs respectively in the AP system profile.
It is recommended that different AP system profiles be created on different active ACs. Otherwise, the standby AC cannot identify AP system profile configurations, causing incorrect configurations.
Perform the following configurations on both the active and standby ACs. You can refer to the link:https://support.huawei.com/hedex/hdx.do?docid=EDOC1100154875&id=EN-US_TASK_0205823278&lang=en
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