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MB_F5_beginner_'s avatar
MB_F5_beginner_
Icon for Nimbostratus rankNimbostratus
Jan 04, 2017

UDP Health Monitor SEND/RECEIVE STRING SYNTAX

Hi Expert,

 

I am in the process of configuring a LB application for SNMP agent server pool for a network monitoring tool. I wanted create a custom UDP health monitor. I was reading the following:

 

Its states that you only need the 2nd monitor if you do not implement a SEND/RECEIVE string in the monitors, but I cannot find any documentation on what type of receive string I could put together for this SNMP agent to send back. I do not want to place a generic udp and icmp monitor on the pool. Any help would be greatly appreciated.

 

MB

 

application delivery
devops
LTM
TMSH

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  • Leonardo_Souza's avatar
    Leonardo_Souza
    Icon for Cirrocumulus rankCirrocumulus
    Jan 04, 2017

    Interestingly, both UDP and TCP monitors have send and receive string. However, I never saw these monitors implemented with send and receive string.

     

    I guess this would be useful if you know the application, so what text to send and what it would reply. Same idea as with HTTP/HTTPS monitor.

     

    The idea for the second monitor (ICMP) is simple. If the host is there, and the port is down, it will reply and F5 will mark it down. If not there, it will not respond, and continue up. This is why you need ICMP to make sure the server is marked down when is not alive.

     

    If you use a send and receive string, if the string is not receive, it is marked down after the timeout. In this case, you don't need the ICMP to cover the situation when the server is down.

     

  • Steven_J__Willi's avatar
    Steven_J__Willi
    Icon for Nimbostratus rankNimbostratus
    Oct 30, 2018

    I have a UDP monitor probing my VIP/Pool for UDP 1812, and the nodes and pool show UP, but a netstat -a on one of the nodes does not list 1812 so how is it up exactly?

     

    • Leonardo_Souza's avatar
      Leonardo_Souza
      Icon for Cirrocumulus rankCirrocumulus
      Oct 30, 2018

      Take a tcpdump to confirm the traffic is arriving in the server and the server is responding.