Resolving AR1200 Sub‑interface Connectivity Issues by Enabling ARP Broadcast

Problem description: After configuring a sub‑interface on an AR1200 router for internal network access, the internal PC could not reach the internet; pinging the router’s next hop from the PC failed.

Investigation steps:

1. Ping the next‑hop address directly from the router; it succeeded, confirming external connectivity. 2. Enabled ip soft-forward enhance enable on the device; pinging the next hop using the sub‑interface gateway succeeded, suggesting the issue lies between the PC and the router. 3. The PC could ping its own gateway and the external address but not the next hop. 4. While pinging the next hop from the PC, the NAT session table showed the PC’s address had been NAT‑translated, indicating the ping packet reached the egress interface, so the PC likely did not receive the reply.

Customer configuration examined:

interface GigabitEthernet0/0/1
 ip address 192.168.0.1 255.255.255.0
!
interface GigabitEthernet0/0/1.30
 dot1q termination vid 30
 ip address 192.168.30.1 255.255.255.0
 dhcp select interface

The internal sub‑interface was configured without the arp broadcast enable command. In this state the system discards IP packets on that sub‑interface, effectively creating a black‑hole route.

Root cause: In firmware versions prior to V2R5, sub‑interfaces do not enable arp broadcast enable by default, causing ARP reply packets to be dropped while ARP requests are still forwarded. From version R5 onward, the command is enabled by default.

Solution: Add the arp broadcast enable command under the sub‑interface configuration. After applying this command, the PC could successfully ping the next hop and access the internet.

Recommendations and summary: When working with sub‑interfaces, first verify whether a termination sub‑interface is configured, then check the device version and ensure arp broadcast enable is set if required. This prevents black‑hole routing and restores normal communication.

Note: Without arp broadcast enable , the system discards the IP packet, treating the sub‑interface route as a black hole. When the command is present, the router constructs a tagged ARP broadcast packet and forwards it from the sub‑interface.