Tuesday, May 27, 2014

A10 Load Balancer Default Health Checks

If you work with load balancers, you know that one of the keys to setting up a virtual server (VIP) is the health check that is used to monitor the health of the servers being balanced.  My original experience with load balancers was with F5 LTMs, but in the last few years I’ve added A10 AX to my vocabulary.  

For a long time I assumed that the health check assigned to the server pool (F5 lingo), or service group (A10 parlance), was THE health check that determined the status of the VIP.  However, it turns out that there are two default health checks that A10 uses that I wasn’t aware of (or perhaps I knew at one point and just forgot).

Each server (not virtual server, but actual server) on an A10 AX has a default L3 health check (ICMP), and each port that is defined for the server has a default L4 health check (TCP 3-way handshake).  The overall up/down status of the pool/service group is the logical AND of the L3, L4, and, if defined, L7 health check for each server. If there is one web server in a pool, and the AX cannot ping it, even if it can do an HTTP GET and sees “200 OK”, the pool status will be DOWN and thus the VIP will be DOWN.

To get around this, you can easily disable the default health checks with an example. Consider the following two real web servers. 

slb server WebServerA
  port 80 tcp

slb server WebServerB
   no health-check
   port 80 tcp
      no health-check

In the case of WebServerA, there is a default L3 health check which will periodically ping the server at as well establish and tear-down a TCP connection at If either of these checks fails, then the service group (pool) that this server belongs to will flag the server as down.

For WebServerB, the first “no health-check” command disables the default L3 check and the second iteration of the command disables the L4 test. In this case, the only health check that matters will be the L7 health check assigned to the service group.

I hope this information can prove to be useful to someone else before they pull their hair out as I did before learning about it.

Got questions? Hit me up on Twitter (http://twitter.com/swackhap) or comment below.

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