Link Switchover or Revertive Switchover Occurs

Common Causes

• The active AC fails. Traffic is switched from the active AC to the standby AC.
• After the active AC recovers, traffic is switched back from the standby AC to the active AC.
• If the active and standby ACs have the same priority but the active AC has a larger IP address, traffic is switched to the standby AC.
• The AC that starts first has a lower priority. Traffic is switched to the AC that starts later. The AC that starts later must be the standby AC and must have the dual-link backup function enabled.
• The AC that starts later has the same priority as the AC that starts first but has a smaller IP address. Traffic is switched to the AC that starts later. The AC that starts later must be the standby AC and must have the dual-link backup function enabled.

Troubleshooting Flowchart

Troubleshooting Procedure

1. Check whether the link connected to the active AC is faulty.
   1. Check whether the AC is faulty and cannot be logged in.
   2. Log in to the device and perform the ping operation to check whether the link between the AC and AP works properly.
      If the following information is displayed, the link between the AC and AP is faulty.

      <AC> ping xx.xx.xx.xx               
      PING xx.xx.xx.xx: 56  data bytes, press CTRL_C to break         
      Request time out                                                            
      Request time out                                                            
      Request time out                                                            
      Request time out                                                            
      Request time out                                                            
                                                                                      
      --- xx.xx.xx.xx ping statistics ---                                          
      5 packet(s) transmitted                                                     
      0 packet(s) received                                                        
      100.00% packet loss     

   Conclusion: An active/standby link switchover occurs when the active AC becomes faulty or the link between the AC and AP is faulty.

2. Check whether the faulty AC recovers.
   1. Observe the AC status and check whether the active AC recovers from the fault.
   2. Log in to the AC that recovers from a fault.
      • Run the display ap id ap-id command in the user view to check the IP address of the standby AC configured based on the AP.

        In the command output, if the Protect ac IP address field displays as the IP address of the AC that the AP switches to, the AC is the original active AC. If the Protect ac IP address field displays as the IP address of the other AC, go to 3.

        [AC6605-wlan-view] display ap id 0
        AP 0 detail information:
        ----------------------------------------------------------------------
        AP type ID: 28
        AP type: AP7110DN-AGN
        AP profile ID: 0
        AP region ID: 0
        Sysname: ap-0
        Keep-Service: Disable
        AP access priority mode: Priority Access 5GHz Radio
        State: fault
        AP mac address: dcd2-fcf3-b180
        AP sn: 210235555610D1000003
        AP static IP address: -
        AP performance-statistic cycle: 15 minutes
        AP System Network: -
        AP lineport total number: 0
        AP high temperature threshold(°C):  60
        AP low temperature threshold(°C):  -10
        AC priority: 0
        Protect ac IP address: -    
        AP lineport mode: root
        ...
        ...

      • If the Protect ac IP address field is blank, run the display wlan ac protect command in the WLAN view to check the IP address of the standby AC configured globally.

        In the command output, if the Protect AC field displays as the IP address of the AC that the AP switches to, the AC is the original active AC.

        [AC6605-wlan-view] display wlan ac protect
        ------------------------------------------------------------
        Protect state   : enable
        Protect AC       : xx.xx.xx.xx
        Priority         : x
        Protect restore : enable
        ------------------------------------------------------------

   Conclusion: After the active AC recovers from a fault, traffic is switched back to the active AC.

3. Check whether the active and standby ACs have the same priority.
   1. Log in to the active and standby ACs respectively and check their priorities.
      • Run the display ap id ap-id command in the user view to check the AC's priority configured based on the AP.

        If AC priority is not blank, the AC priority is configured based on the AP.

        [AC6605-wlan-view] display ap id 0
        AP 0 detail information:
        ----------------------------------------------------------------------
        AP type ID: 28
        AP type: AP7110DN-AGN
        AP profile ID: 0
        AP region ID: 0
        Sysname: ap-0
        Keep-Service: Disable
        AP access priority mode: Priority Access 5GHz Radio
        State: fault
        AP mac address: dcd2-fcf3-b180
        AP sn: 210235555610D1000003
        AP static IP address: -
        AP performance-statistic cycle: 15 minutes
        AP System Network: -
        AP lineport total number: 0
        AP high temperature threshold(°C):  60
        AP low temperature threshold(°C):  -10
        AC priority: 0       
        Protect ac IP address: -
        AP lineport mode: root
        ...
        ...

      • If AC priority is blank, run the display wlan ac protect command in the WLAN view.

        In the following information, x in the Priority field indicates the AC's priority.

        [AC6605-wlan-view] display wlan ac protect
        ------------------------------------------------------------
        Protect state   : enable
        Protect AC       : xx.xx.xx.xx
        Priority         : x
        Protect restore : enable
        ------------------------------------------------------------

   2. If the two ACs have the same priority, check their IP addresses. In the WLAN view, run the display wlan ac source interface command to check the AC's source interface. Enter the view of the source interface and run the display this command.
      • In the following information, xx.xx.xx.xx in the ip address field is the AC's IP address.

        [AC6605-wlan-view] display wlan ac source interface
        wlan ac source interface vlanifxxxx
        [AC6605-wlan-view] interface vlanif xxxx
        [AC6605-Vlanifxxxx] display this
        #
        interface Vlanifxxxx
        ip address xx.xx.xx.xx 255.255.255.0
        #
        return

   Conclusion: If the active and standby ACs have the same priority, the AP switches to the AC with a smaller IP address.

4. Check whether two ACs start successively and have different priorities.
   1. Check whether the standby AC starts later and has a higher priority than the active AC.

      For details on how to check the AC's priority, see 3.

   2. If the standby AC starts later than the active AC but two ACs have the same priority, go to 5.

   Conclusion: If the standby AC starts later and has a higher priority than the active AC, a link switchover is triggered.

5. Check whether two ACs start successively and have the same priority.
   1. Check whether the standby AC starts later but has the same priority as the active AC.

      For details on how to check the AC's priority, see 3.

   2. If the active and standby ACs have the same priority, check their IP addresses. If the ACs have different priorities, go to step 5. Run the display wlan ac source interface command in the WLAN view to enter the view of the AC's source interface, and then run the display this command.

   Conclusion: If the standby AC starts later, has the same priority as the active AC, but has a smaller IP address, a link switchover is triggered.

   If the fault persists, contact technical support personnel.

Troubleshooting Cases

The active link works properly but link switchover is still triggered.

Networking Description

The AP sets up links with AC1 and AC2 through the Switch. AC1 is the active AC while AC2 is the standby AC.

Fault Analysis

• The link between the AP and AC1 works properly and the two ACs are all started before the AP goes online. Normally, a link switchover does not occur.
• The two ACs have the same priority.
• AC2's IP address is smaller than AC1's.

Procedure

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

   If neither of the receding situations occurs, go to the next step.

2. Check AC priority.

   If the two ACs have the same priority, go to step 3.

3. Check the IP addresses of the two ACs.

   AC2's IP address is smaller than AC1's. The AP switches links based on the revertive switchover algorithm. It first compares the two ACs' IP addresses and then switches traffic to the AC with a smaller IP address. Since AC2's IP address is smaller than AC1's, the AP performs a revertive switchover.

Summary

• This fault occurs because the algorithm to choose an active AC is inconsistent with the revertive switchover algorithm.

  In old versions, the AP selects the active AC based on the following sequence: priority, load, and AC's IP address. That is, the AP preferentially goes online on the AC with a higher priority. If the two ACs have the same priority, the AP selects the AC with a smaller load. If the AC load is also the same, the AP connects to the AC with a smaller IP address.

  The AP determines whether to perform a revertive switchover based on AC priority and IP address (AC priority > IP address).

• Based on the algorithm to choose an active AC, the AP selects AC1 as the active AC to go online. Since AC2 has a smaller IP address, the AP switches traffic back to AC2 according to the revertive switchover algorithm. Consequently, the fault occurs.

  The inconsistency in the two algorithms has been modified. Currently, the AP performs a revertive switchover only according to the AC priority.