Using same LB for servers on multiple subnets

Hi Randy, I wondered that a while back and did some testing using nc and telnet while running tcpdump. My tests were run on early 10.x BIG-IP LTM using an HTTP monitor. Note sure which exact version. From my notes:

 

 

The interval takes effect whenever an HTTP health monitor connects to the pool member. This behavior is different if the health monitor passes or fails. For a passing monitor, the succesful GET will close the connection and mark the pool member up. The LTM will then use the interval timer to establish a new connection. For the default interval/timeout 5/16, the health monitor may invoke a new connection and GET around every 5 seconds.

 

 

For a failed monitor, the LTM will establish a connection to the pool member and send up to 4 GET's, per the interval set, before closing the connection. If, after 4 GET's, the timeout has not been met, the LTM will establish a new connection and will continue this behavior until the timeout has been exceeded. At that time the LTM will mark a pool member down.

 

 

This post has additional detail as well:

 

 

http://devcentral.f5.com/Default.aspx?tabid=53&view=topic&postid=19455&ptarget=19509

 

 

interval: how often in seconds to send a request

 

 

timeout: how long to wait for a successful response before marking the member down

 

 

By default, these values are set to interval and timeout of 5 sec and 16 seconds (timeout = 3 x interval + 1). So the monitoring daemon starts a timer equal to the value of the timeout. It will send a request every five seconds--the length of the interval. When it receives a successful response, the countdown is reset to the timeout value. In this scenario, the node basically has three chances to respond before being marked down. Requests are still sent every interval even when the node is marked down. This allows for automatic resumption of use of the node when it responds correctly to a request. If you want to keep the node marked down even after it responds again, you can enable 'manual resume'.

 

 

If you want to give the node more chances to respond before marking it down, you could extend the timeout length. Setting the interval and timeout to 5 and 31 would mean the node would get sent six requests before being marked down.

 

 

Aaron