ltm rule event LB FAILED

iRule(1)					  BIG-IP TMSH Manual					     iRule(1)

LB_FAILED
       Triggered when the system fails to select a pool or a pool member. or when a selected resource is unreachable.

DESCRIPTION
       LB_FAILED is triggered when LTM is ready to send the request to a pool member and one hasnXt been chosen (the
       system failed to select a pool or a pool member), is unreachable (when no route to the target exists), has
       reached a queue limit, or is non-responsive (fails to respond to a connection request).

       If the target fails to respond to a connection request, the "Maximum Syn Retransmissions" option in the TCP
       profile will affect the amount of time before LB_FAILED is triggered.

       When a client doesn't receive a response to the SYN, there is a defined algorithm for the specified number of
       re-tries. First retransmission if no response is typically 3 seconds, and typical back-off timer algorithm is
       to double the wait time after each failed attempt. Most TCP stacks set Max Syn Retransmits to 5, producing the
       following progression and timing out after 93 seconds:

       Trying 172.24.2.200...  11:26:24.455153 172.24.2.41.55507 > 172.24.2.200.http: S ...  11:26:27.452465
       172.24.2.41.55507 > 172.24.2.200.http: S ...  11:26:33.452465 172.24.2.41.55507 > 172.24.2.200.http: S ...
       11:26:45.452465 172.24.2.41.55507 > 172.24.2.200.http: S ...  11:27:09.452481 172.24.2.41.55507 >
       172.24.2.200.http: S ...  11:27:57.452466 172.24.2.41.55507 > 172.24.2.200.http: S ...  telnet: connect to
       address 172.24.2.200: Connection timed out

       In past versions, LTM's default TCP profile sets "Maximum Syn Retransmissions" to 4, so with the default
       setting, LB_FAILED would be triggered if server didn't respond in 45 seconds:

       1st SYN:  0 2nd SYN: +3 seconds 3rd SYN: +6 seconds 4th SYN: +12 seconds 5th SYN: +24 seconds
       ====================== LB_FAILED: 45 seconds

       Reducing "Maximum Syn Retransmissions" to 2 will result in the trigger of the LB_FAILED event in 9 seconds,
       which makes recovery logic in the LB_FAILED event much more useful as a backup to monitoring.

Examples
	when DNS_REQUEST {
	    if {[active_members poolName] < 2} {
		pool fallbackPool
	    }
	}
	when LB_FAILED {
	    pool errorPool
	}

HINTS
       Note: The LB_FAILED event is not triggered for Performance L4 virtual servers. There is an enhancement request
       to add this in BZ341776. Contact F5 Support for additional information.

SEE ALSO
CHANGE LOG
       @BIGIP-9.0.0 --First introduced the event.  @BIGIP-9.2.2 --First introduced the event.

BIG-IP						      2020-06-23					     iRule(1)