BROCADE IP PRIMER

Configuring SLBWhen a real server port is bound to the virtual server, regular Layer 3, Layer 4,and (if configured) Layer 7 health checks initiate automatically. Layer 3 healthchecks are the most frequent. By default, Layer 3 health checks (ARP andping) are performed every two seconds. Upon failure, it will retry four timesbefore declaring a failed check.Layer 4 health checks are performed based on the port that was bound. Forexample, when we created the virtual server “www,” we bound rs1 and rs2 toHTTP. The ServerIron will now be performing Layer 4 health checks on thesetwo servers using TCP 80. To summarize what we covered in Chapter 3, theServerIron will send a TCP SYN on port 80 to the real server. The real serverwill send a TCP SYN/ACK back to the ServerIron. The ServerIron will terminatethe session by sending a TCP RST. This is a healthy health check.Layer 4 health checks may also be disabled, but this is also notrecommended:SLB-ServerIron#conf tSLB-ServerIron(config)#no server l4-checkGenerally, Layer 7 health checks are disabled by default. They must be specificallyrequested in the real server's sub-config. Some well-known protocolsautomatically add a Layer 7 health check line. HTTP, for example, automaticallyadds an extra line to the real server config. When a user types “port http”,a second line is also added: port http url “HEAD /.” This defines a Layer 7health check that performs an HTTP “HEAD /.” If the server's response is anHTTP Status code of 200-299 (or 401), the ServerIron considers the realserver to be alive, and it will pass traffic to it. If it gets any other code (or if theoperation fails in any other way), the real server will be considered down, andthe ServerIron will not pass traffic to it. The HTTP command used for thehealth check may be changed to whatever the user chooses. Some commonexamples are:port http url "GET /index.html"port http url "GET /image.gif"port http url "GET /some/specific/application.cgi"But what about SMTP? We can't use a “port smtp url” command for SMTP. ForSMTP, and many of the well-known ports, you need only add one line to thereal server's config: “port smtp keepalive.” This tells the ServerIron that you'dlike to perform a Layer 7 health check for that protocol. For SMTP, theServerIron will look for the opening SMTP banner followed by a 220 statuscode. If it sees these, it will issue an SMTP “Quit” command, and reset the TCPconnection. This is a successful Layer 7 health check for SMTP.There are many more variations for Layer 7 health checks. For protocols thatare not well-known, custom health checks may be created, but this goesbeyond the scope of this book. For more information, consult Brocade's website (http://www.brocade.com/).Brocade IP Primer 337