Last Modified: Oct 01, 2026
Affected Product(s):
BIG-IP TMOS
Known Affected Versions:
17.1.0, 17.1.0.1, 17.1.0.2, 17.1.0.3, 17.1.1, 17.1.1.1, 17.1.1.2, 17.1.1.3, 17.1.1.4, 17.1.2, 17.1.2.1, 17.1.2.2, 17.1.3, 17.1.3.1, 17.1.3.2, 17.1.3.4, 17.1.3.5, 17.5.0, 17.5.1, 17.5.1.2, 17.5.1.3, 17.5.1.4, 17.5.1.5, 17.5.1.6, 17.5.1.8, 17.5.1.9, 21.0.0, 21.0.0.1, 21.0.0.2, 21.0.0.3, 21.1.0, 21.1.0.1, 21.1.0.2
Opened: Sep 25, 2026 Severity: 3-Major
A virtual server configured with service-down-immediate-action reset does not immediately reset (RST) new client connections when its default pool has no members. Instead, the virtual server completes the TCP handshake (SYN, SYN/ACK, ACK) and only resets the connection later, during load balancing, with the internal reason "No pool member available" instead of "VIP down". This matches the behavior of service-down-immediate-action none, meaning the configured reset action is effectively ignored
Clients can briefly establish a TCP connection to a virtual server that is expected to immediately refuse it, exposing an unexpected open connection state instead of an immediate reset. This condition persists until the associated pool's configuration is otherwise changed (for example, a pool member is added or removed), which forces the virtual server's availability status to be recalculated
- A virtual server is configured with the "service-down-immediate-action" setting set to "reset" - The default pool associated with this virtual server has no pool members - If the BIG-IP system is restarted (using the command "bigstart restart") or rebooted while this virtual server and pool configuration exist, the following behavior is observed: Only the virtual servers that existed prior to the restart and were reloaded from the saved configuration will exhibit this behavior. Virtual servers that are created or modified after the restart or reboot are not affected
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