BROCADE IP PRIMER

FOUNDRYNETWORKSCo ns ol ePwrCC U1 PU 2 PU 3Co ns ol ePwrCC U1 PU 2 PU 3TxActRxActTxActRxActActi veActi ve1 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 6J -F16GCJ -F16GCJ-F16GCJ-F16GCServerIron 850J -F16GCJ -F16GCJ-F16GCJ -F16GCJ-F16GCServerIron 850J -F16GCJ -F16GCJ-F16GCJ -F16GCJ-F16GCUDP 53Chapter 5: Load Balancing BasicsIf the protocol defined is UDP, the load balancer has to use a different method.UDP is connectionless. Many UDP protocols do not require replies. In this case,the load balancer will send a UDP segment to the server on the destinationport specified. Here, if the load balancer gets no reply (after a short, specifiedperiod of time), the load balancer deems the server active. If the server is notlistening on the destination port specified, it will reply with an ICMP “PortUnreachable” message. If the load balancer sees this, it will mark the serverinactive. It will try again at the next health check interval.1 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 62 M M S WUDP 53UDP health check PASSED1 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 62 M M S WICMP Port UnreadableUDP health check FAILEDCombining Layer 3 and Layer 4 health checks is definitely more reliable thanjust using Layer 3. With Layer 3 and 4, you now know that your server is configuredcorrectly, and it is listening on the TCP or UDP port you're expecting it to.But what if the program is locked? Normally, you'd think if the program werelocked, the server would stop listening on the TCP or UDP port it's supposed to,right? Unfortunately, in real life, it doesn't work that way. For example, let's sayyou are running a web server. You want the customer to be able to use a specialweb application that is accessed through the web service. The programlistening on HTTP (TCP port 80) is one process. When the customer makes arequest of your web application, the program listening on HTTP will hand therequest to your web application. It's possible (and expected) that the programlistening to HTTP will keep responding to TCP port 80 requests, regardless ofwhether or not your web application is functioning (or even present). Well,that's no good. How will the load balancer know if the server's really active andcapable of accepting requests?106 Brocade IP Primer