LLC broadcast frame was generated from SZ

Summary

After upgrading SmartZone from R3.6.2 to R5.2.1+, the customer observed high CPU utilization on the switch and loop detection messages. Packet captures revealed LLC broadcast frames originating from SmartZone, causing MAC address flapping.

Question

What is this LLC broadcast frame and why is it generated by SmartZone?

Customer Environment

Switch: ICX 7450 Controller: SZ104 (2-node cluster) Configuration: Tunneled WLANs SmartZone Version: 5.2.2.x

Symptoms

Loop detection syslog messages:

LoopDetect [meta sequenceId=10750] BOMLOOP DETECTION: port 1/3/4 detect Loop, Ignoring shut down event in shutdown-disable mode
0013.9200.0000 moved from interface 1/3/4 to interface 2/3/4

High CPU utilization on the switch.
Packet captures show LLC frames from SZ nodes with MAC 00:13:92:00:00:00.

Root Cause

Starting from R5.0, the Data Plane (DP) sends L2UF (Layer 2 Unicast Flood) frames when a UE roams or disconnects from DP. IP phones in power-save mode remain silent for >900 seconds. When an IP phone fails over from DP1 to DP2, DP2 does not immediately know the UE is connected. To resolve this, AP sends L2UF frames to DP2 to declare UE roaming. DP2 updates its host table and bridges the L2UF. If DP1 and DP2 share the same subnet, receiving L2UF from the core allows DP1 to remove stale UE entries. Impact: LLC frames with source MAC 00:13:92:00:00:00 are broadcast from SZ nodes. Switch sees the same MAC on multiple ports (connected to different DPs), causing MAC flapping and loop detection events.

Troubleshooting Steps

Review switch logs for loop detection and MAC flapping events.

---
LoopDetect [meta sequenceId=10750] BOMLOOP DETECTION: port 1/3/4 detect Loop, Ignoring shut down event in shutdown-disable mode

0013.9200.0000 moved from interface 1/3/4 to interface 2/3/4
---

Capture packets on SZ-connected ports; confirm LLC frames with source MAC 00:13:92:00:00:00.

Frames came from different switch ports that connected to different DPs, so it caused MAC address flapping on the switch, and these were learned from the client VLANs configured on the tunneled WLAN

Verify VLAN configuration for tunneled WLANs and compare the same with the above logs

Workaround

This behavior cannot be disabled on SZ.
Configure a static MAC entry on the switch for 00:13:92:00:00:00 to prevent MAC flapping by dropping these frames:

mac-address-table static 00:13:92:00:00:00 vlan <VLAN-ID> drop

This prevents forwarding of frames to this MAC without impacting system functionality.

Resolution

This is by design for L2UF messaging to maintain UE session consistency across DPs.

Article Number:
000011557

Updated:
December 19, 2025 07:31 AM (5 months ago)

Tags:
Troubleshooting, SmartCell Gateway

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