Case Study: In AC VRRP HSB Scenarios, APs Go Offline Unexpectedly Due to a Loop on the Network

Symptom

As shown in the following figure, two ACs work in VRRP HSB mode. An AP cannot go online after going offline unexpectedly.

Relevant Alarms and Logs

None

Cause Analysis

Cause 1: The HSB link transparently transmits packets of the WLAN service VLAN and AP management VLAN.

Cause 2: STP configured on the AC does not take effect, and the blocked port still forwards traffic.

Procedure

1. Ping the S7706 and AirEngine 9700-M from each other. Packet loss occurs.
2. View logs in the log buffer on the switch to check whether MAC address flapping occurs.
   1. Run the display logbuffer command. The command output shows that MAC address flapping occurred on gi1/0/23 and gi1/0/24.
   2. Run the display mac-address flapping record command to check MAC address flapping records on the switch.

   After the preceding steps are performed, it is confirmed that a loop exists on the network. Then, locate the cause of the loop.

3. Check the port interconnection status, port configuration, and STP status in MAC address flapping records.
   1. Run the display current-configuration interface gi y/y/y command to check the interface configuration and VLAN transparent transmission information.
   2. Run the display interface gi y/y/y command. The command output shows that the number of broadcast and multicast packets on the interface where MAC address flapping occurs increases greatly.
   3. Run the display lldp neighbor brief | interface gi y/y/y command to check the neighbor relationship between interconnected devices.

4. Check STP blocked interfaces.

   Run the display stp brief command to check the STP status and the port blocking and forwarding status.

5. On the master and backup ACs, locate the cause of the loop by referring to 3 and 4.

   The check results are as follows:

   Interfaces connecting the master and backup ACs transparently transmit packets of the AP management VLAN, WLAN service VLAN, and VLANs connecting the AC to the switch.

   When STP is enabled on the master AC, gi0/0/1 on the AC is blocked by STP. The interface connected to the S7706 can still learn the MAC address of the WLAN service VLAN.

6. Only allow packages in the HSB VLAN to pass on the HSB interconnection interface between the master and backup ACs. Do not allow packets in the HSB VLAN to pass on the interconnection interface between the AC and switch.