Hello Friends, I am back with a new query related to F5 Health Monitor. A custom Parent Monitor (http) is available (Using word available as we did'nt configure this , configured by our implementor) on F5 for one of our application pool members. SEND string is configured with some values and content and RECEIVE String is mentioned as "true". Pool members are down since couple of days and during investigation while we swapping the Health Monitor from above mentioned customized monitor to "icmp". pools members and LTM VIP came UP. please help to investigate the reason for this. We are assuming this as Customized Health monitor issue OR might be problem with Pool Members end something related to their String Services. How and where to verify this. Rgds

The SEND string that has been configured as a monitor is actually related to monitoring the services (LAYER 4) on the pool members, whereas the ping monitor checks the availability (LAYER 3 ) of the nodes forming that pool. The services may be down even if the nodes are UP. Imagine a situation in which you have built the server and assigned it an IP address which is pinging from F5 (In this case the ping monitor will show the pool to be UP). But no services or your application has been configured and hence you will not get services out of that server( the pool would be down as per the customised monitor).

HTTP is layer 7 (Application layer) not layer 4 (Transport layer) https://en.wikipedia.org/wiki/OSI_modelLayer_7:_Application_Layer Everything else you said is correct.

Health Monitor Issue

Amresh008
Nimbostratus
Feb 12, 2019
The SEND string that has been configured as a monitor is actually related to monitoring the services (LAYER 4) on the pool members, whereas the ping monitor checks the availability (LAYER 3 ) of the nodes forming that pool. The services may be down even if the nodes are UP. Imagine a situation in which you have built the server and assigned it an IP address which is pinging from F5 (In this case the ping monitor will show the pool to be UP). But no services or your application has been configured and hence you will not get services out of that server( the pool would be down as per the customised monitor).
- Andy_McGrath
  Cumulonimbus
  Feb 12, 2019
  HTTP is layer 7 (Application layer) not layer 4 (Transport layer) https://en.wikipedia.org/wiki/OSI_modelLayer_7:_Application_Layer
  
  Everything else you said is correct.
- Amresh008
  Nimbostratus
  Feb 12, 2019
  @ Andy, thanks for pointing out the typo.
- EIT_Network
  Altostratus
  Feb 13, 2019
  Thanks Andy and Amy !
  
  What is the importance of RECEIVE String , Which is mentioned as TRUE in my case.
Amy003_314955
Cirrus
Feb 12, 2019
The SEND string that has been configured as a monitor is actually related to monitoring the services (LAYER 4) on the pool members, whereas the ping monitor checks the availability (LAYER 3 ) of the nodes forming that pool. The services may be down even if the nodes are UP. Imagine a situation in which you have built the server and assigned it an IP address which is pinging from F5 (In this case the ping monitor will show the pool to be UP). But no services or your application has been configured and hence you will not get services out of that server( the pool would be down as per the customised monitor).
- Andy_McGrath
  Cumulonimbus
  Feb 12, 2019
  HTTP is layer 7 (Application layer) not layer 4 (Transport layer) https://en.wikipedia.org/wiki/OSI_modelLayer_7:_Application_Layer
  
  Everything else you said is correct.
- Amy003_314955
  Cirrus
  Feb 12, 2019
  @ Andy, thanks for pointing out the typo.
- EIT_Network
  Altostratus
  Feb 13, 2019
  Thanks Andy and Amy !
  
  What is the importance of RECEIVE String , Which is mentioned as TRUE in my case.
Amresh008
Nimbostratus
Feb 12, 2019
The SEND string that has been configured as a monitor is actually related to monitoring the services (LAYER 4) on the pool members, whereas the ping monitor checks the availability (LAYER 3 ) of the nodes forming that pool. The services may be down even if the nodes are UP. Imagine a situation in which you have built the server and assigned it an IP address which is pinging from F5 (In this case the ping monitor will show the pool to be UP). But no services or your application has been configured and hence you will not get services out of that server( the pool would be down as per the customised monitor).
- Andy_McGrath
  Cumulonimbus
  Feb 12, 2019
  HTTP is layer 7 (Application layer) not layer 4 (Transport layer) https://en.wikipedia.org/wiki/OSI_modelLayer_7:_Application_Layer
  
  Everything else you said is correct.
- Amresh008
  Nimbostratus
  Feb 12, 2019
  @ Andy, thanks for pointing out the typo.
- EIT_Network
  Altostratus
  Feb 13, 2019
  Thanks Andy and Amy !
  
  What is the importance of RECEIVE String , Which is mentioned as TRUE in my case.
Andy_McGrath
Cumulonimbus
Feb 13, 2019
Your HTTP monitor receive string is a regular expression search agains the HTTP response from the monitor request.

For a very basic check set to something like ‘200 OK’ assuming that you are getting a standard OK response.

The following two knowledge articles are great to help you construct a good HTTP or HTTPS monitor:

K5917: Using regular expressions in a health monitor receive string

K2167: Constructing HTTP requests for use with the HTTP or HTTPS application health monitor