ARP dead window: why the fix is Proxy ARP, not DHCP
"Connected but no internet. DHCP lease valid. IP correct. DNS responds. The first packets from the gateway drop silently. This is not a DHCP problem." ·
The ARP dead window is the gap between when a client associates and when the gateway has a valid ARP cache entry for that client. During this window, traffic the gateway tries to forward drops at Layer 2 — no MAC address to put in the Ethernet frame.
The symptom looks like DHCP failure. Engineers check: lease valid, IP assigned, gateway correct. They increase DHCP lease time, speed up responses, check for exhaustion. None of it helps because DHCP is not involved in the dead window at all.
Finding it in PCAP
EAPOL M4 completion followed by first GARP. Measure delta. Then check if the first outbound client traffic gets a response. If gateway-to-client frames fail in this window, dead window confirmed.
Android connectivity probe. Unanswered = Android marks network as no-internet and routes via LTE. This is the dead window on Android devices.
What Proxy ARP does
The AP intercepts ARP Requests and responds on behalf of associated clients using the client last-known MAC. The gateway gets a valid MAC immediately, regardless of whether the client has completed GARP. Dead window eliminated. Enabled per-SSID on all major WLC platforms. No downside in an infrastructure BSS.