A gateway_icmp health monitor is configured on a pool.

A gateway_icmp monitor checks basic network reachability by sending ICMP echo requests (pings) to the pool member or its gateway. If the pool is markedDOWNwhile the server is confirmed to be online, the most likely cause is thatICMP traffic is being blocked.

Ahost-based firewall active on the server (Option C)can block ICMP echo requests or replies, preventing BIG-IP from receiving a successful response to the health check. This results in the monitor failing and the pool member being marked down, even though the server and application are otherwise functioning normally. This explanation is consistent with the scenario where other servers in the same subnet work correctly, indicating that routing and BIG-IP configuration are not the issue.

The other options are unrelated to ICMP monitoring. Logged-in users (Option A), missing patches (Option B), and stopped HTTP services (Option D) do not affect a gateway_icmp monitor. BIG-IP troubleshooting best practices recommend verifying ICMP reachability and firewall policies when diagnosing ICMP-based monitor failures.