Case Study: An RU Goes Offline Unexpectedly Due to a Loop on the Downlink Wired Interface of the RU

Symptom

The S12700 functions as the AC to manage central APs and RUs. The RUs go offline unexpectedly because a loop occurs on the downlink wired interfaces of the RUs.

Relevant Alarms and Logs

None

Cause Analysis

The downstream interfaces of RUs in dormitories are connected to the hub, and a loop is formed in the downstream direction.

Procedure

1. Run the display ap offline-record all command. The command output shows that the AP goes offline due to heartbeat timeout (echo timeout).
2. Ping the RU management address from the S12712. Packet loss occurs.
3. Check information in the log buffer on the S12712. Address flapping occurs in the VLAN of the RU's wired interface.
   1. Run the display logbuffer command. The command output shows that MAC address flapping occurs in the VLAN where the RU-preferred interface resides.
   2. Run the display mac-address flapping record command to check MAC address flapping records. Most records show that MAC address flapping occurs between XG3/0/20 and other interfaces on the S12712.

   According to the preceding steps, a loop occurs on the wired interface of the RU. You need to locate the RU where the loop occurs.

4. Determine the interface where MAC address flapping occurs based on MAC address flapping records, and check the neighbor and configuration of the interface.
   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 increases greatly.
   3. Run the display lldp neighbor brief | interface gi y/y/y command to check the neighbor relationship between interconnected devices.

5. Log in to the central AP and perform the preceding step. An exception occurs on the gi0/0/1 interface on the AD9431DN-24X, indicating that a loop occurs on the RU connected to this interface.
6. Shut down gi 0/0/1 on the AD9431DN-24X, locate the faulty RU, and remove the loop. Then enable gi0/0/1 on the AD9431DN-24X.

Suggestion and Summary

To prevent loops on the AP's downlink interface, enable STP on the AP's downlink interface and configure the interface to shut down when a loop is detected. The configuration is as follows:

[AC-wlan-view] wired-port-profile name test
[AC-wlan-wired-port-test] stp enable
[AC-wlan-wired-port-test] stp auto-shutdown enable
[AC-wlan-wired-port-test] user-isolate l2
[AC-wlan-wired-port-test] quit
[AC-wlan-view] ap-id 0                //Bind the wired interface profile test to an AP or AP group.
[AC-wlan-ap-0] wired-port-profile test gigabitethernet X   //Reference the ACL on the interface connecting the AP to the PC. Do not reference the ACL on the interface connecting the AP to the PC.