Case Study: A Dual-Link Switchover Occurs When the Active Link Is Normal

Symptom

As shown in the following figure, an AP sets up active and standby links with AC1 and AC2, respectively, through a switch. A dual-link switchover occurs when the active link is normal.

Relevant Alarms and Logs

None

Cause Analysis

• If the active link is not faulty and the two ACs are started, the situation that an AP goes online on one AC and the other AC is started does not occur. In this case, the active/standby link switchover is abnormal.
• Check the priorities of the two ACs. The two ACs have the same priority.
• Check the IP addresses of the two ACs. It is found that the IP address of AC2 is smaller than the IP address of AC1.

Procedure

1. Check whether AC1 is faulty or AC2 recovers from a fault.

   According to the check result, the preceding two situations do not occur.

2. Check whether the priorities of the two ACs are the same.

   According to the check result, the two ACs have the same priority.

3. Check the IP addresses of the two ACs.

   According to the check result, the IP address of AC2 is lower than that of AC1. According to the revertive switching algorithm on the AP side, if the priorities of ACs are the same, the AP compares the IP addresses of the two ACs. If the IP address of the standby AC is lower than that of the active AC, the AP performs a revertive switching.

Suggestion and Summary

• The root cause of this problem is that the algorithm used by the AP to select the active AC is different from the revertive switchover algorithm.

  In the new version, an AP selects an active AC in the following sequence: priority, load, and AC IP address. That is, the AP selects the AC with the highest priority to go online. If the ACs have the same load, the AP selects the AC with the largest load to go online. If the ACs have the same load, the AP selects the AC with the smallest IP address to go online.

  The condition for triggering a revertive switchover is as follows: AC priority > AC IP address.

• According to the active AC selection algorithm, the AP selects AC1 as the active AC to go online. According to the revertive switchover algorithm, AC2 has a smaller IP address than AC1. Therefore, the AP considers that a link switchover is required.

  If the revertive switching algorithm is different from the algorithm used by the AP to select the active AC, the AP has been modified. Currently, the revertive switching algorithm considers only AC priorities.