DHCP Snooping Database Corruption on FortiSwitch causing Client IP Dropouts
A timing race condition in the FortiLink switch-controller DHCP snooping synchronization causes client MAC-to-IP binding entries to be dropped from FortiSwitch hardware tables, triggering Dynamic ARP Inspection (DAI) drops for legitimate campus workstations.
Technical Root Cause Analysis
In enterprise campus environments combining FortiGate security firewalls with managed FortiSwitch infrastructure via FortiLink, DHCP snooping protects access VLANs against rogue DHCP servers. When a client workstation receives a DHCPACK from an authorized DHCP server, the snooping engine records the client's MAC address, IP address, VLAN ID, and physical port into a centralized binding database on FortiGate and mirrors it to the local switch hardware TCAM.
In FortiOS versions 7.2.4 through 7.2.7, rapid DHCP renew bursts or roaming events (such as laptop docking stations reconnecting or Wi-Fi to Ethernet transitions) trigger an asynchronous race condition within the switch_mgr daemon. If two binding updates for the same MAC address arrive simultaneously over FortiLink, the database sync thread purges the existing entry before committing the updated entry. When the switch hardware TCAM detects missing entries for an active access port, Dynamic ARP Inspection (DAI) and IP Source Guard (IPSG) classify all subsequent ARP queries from that client as unauthorized spoofing, dropping traffic at the switch port level.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Mitigation Strategy |
|---|---|---|---|
| FortiOS 7.2 | 7.2.4 – 7.2.7 | 7.2.8+ | Disable DAI temporarily or flush snooping DB |
| FortiOS 7.4 | 7.4.0 – 7.4.2 | 7.4.3+ | Upgrade to 7.4.3+ for atomic sync fix |
| FortiOS 7.0 | Not Affected | 7.0.x Branch Clean | Legacy synchronous IPC unaffected |
Platform Impact: Affects FortiGate units managing campus FortiSwitch models (FS-124E/F, FS-148E/F, FS-424E, FS-448E) where DHCP snooping and Dynamic ARP Inspection are enabled on access VLAN interfaces.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands to check FortiLink switch synchronization status, dump current DHCP snooping tables, and identify missing client bindings:
Diagnostic Commands
# 1. Dump current FortiLink DHCP snooping binding table
diagnose switch-controller dump dhcp-snooping
# 2. Check FortiLink synchronization state across all managed switches
execute switch-controller get-sync-status
# 3. Verify switch controller MAC address cache for a specific switch
diagnose switch-controller switch-info mac-cache all
# 4. Check system crashlog for switch-controller synchronization errors
diagnose debug crashlog read | grep -i "switch-controller"Remediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the Ferrite interactive platform:
Frequently Asked Questions
A timing race condition in switch_mgr during rapid DHCP renewal or roaming bursts causes binding table entries to be dropped, triggering DAI to block valid client ARP traffic.
The client's operating system successfully obtained an IP via DHCP, but because the switch hardware table dropped the binding, Dynamic ARP Inspection silently drops the client's ARP packets to the gateway.
Disabling Dynamic ARP Inspection (DAI) on the affected VLAN or flushing the DHCP snooping table restores client connectivity within seconds without restarting hardware.