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Forum Discussion

Ernesto_Mayol_2's avatar
Ernesto_Mayol_2
Icon for Nimbostratus rankNimbostratus
Jan 12, 2018

LB_FAILED. Why exactly?

It is very frustrating not being able to determine why the LB_Failed event was raised. Based on other posts "event info" could be used to determine the reason but that is not returning anything for m...
application delivery
iRules
Andy_McGrath's avatar
Andy_McGrath
Icon for Cumulonimbus rankCumulonimbus
Jan 12, 2018

LB_FAILED even happens when F5 has selected has either:

 

  1. Failed to select a pool member
  2. Pool member selected is not reachable
  3. Pool member fails to respond to connection

First is triggers if not pool is on the VS or no pool members are available, your LB::status in the LB_FAILED event of an iRule should tell you the selected pool member and the status. Can happen if health monitors interval and timeout values are to small. This should also be in the logs so easy to spot normally.

 

Second is triggered if no route is found, can happen if dynamic routing is being used and a route it lost for a short time but not long enough for the health monitor to mark the pool member down. You using dynamic routing on the F5?

 

Third is when the F5 is not getting a response to its connect request. Can happen if the server is really busy and the Maximum Syn Retransmissions setting in the TCP profile is to low. You using a customized TCP profile?

 

A traffic dump (tcpdump) would be the best way to identify if it happens often enough.