The Port Lockdown feature on a BIG-IP Self IP address is a security control that specifies which network ports and protocols are permitted. Setting this feature to “Allow None” configures a strict default-deny policy, blocking all traffic to and from that Self IP address.

High Availability (HA) functionality, including network failover and state mirroring, depends on communication over specific ports (e.g., UDP 1026, TCP 4353). If the Self IP used for HA communication is set to “Allow None,” this essential traffic will be blocked. As a result, each BIG-IP unit will be unable to receive heartbeat packets from its peer, assume the peer is offline, and attempt to become the active unit. This leads to an active-active state, causing IP address conflicts and service outages.

B. Blocking HA communications would cause the peer to be reported as offline or unknown, not "online ready."

C. The "Allow None" setting explicitly blocks all ports. The "Allow Default" setting, not "Allow None," is what permits default HA ports like 1026.

1. F5 Networks Knowledge Base, K17333: Overview of port lockdown behavior.

Section: "Port lockdown settings"

Content: This article explicitly states, "Allow None: The BIG-IP system allows no traffic to the self IP address." It further warns, "F5 recommends that you do not use the Allow None setting for self IP addresses that you use for network failover or BIG-IP GTM iQuery traffic." This directly supports that the setting blocks HA traffic.

2. F5 Networks Knowledge Base, K2397: Ports required for BIG-IP features.

Section: "BIG-IP LTM"

Content: This document lists the specific ports required for HA. For "Network failover," it specifies UDP port 1026. For "Device service clustering (DSC)," it specifies TCP port 4353. Setting Port Lockdown to "Allow None" would block these required ports.

3. F5 BIG-IP TMOS: Implementations, Version 17.1.

Chapter: "Securing Self IP Addresses" -> Section: "Understanding port lockdown"

Content: The documentation describes Port Lockdown as a mechanism to secure the BIG-IP system from unwanted connection attempts. It details the "Allow None" option as the most restrictive setting, which disallows any service on any port, thereby preventing the communication necessary for HA functions if applied to the relevant Self IP.