BROCADE IP PRIMER

Chapter 18: Global Server Load Balancing (GSLB)Check 4: Geographic Location of the ServerIf all things are equal up to this point, the GSLB ServerIron will attempt tochoose the site that suits the client geographically. To do this, the ServerIronmust know where the client is, and where the site is.For the client, it bases this on the IP address of the DNS server that made therequest. The ServerIron can narrow down its geographic location by continent(North America, South America, Asia, or Europe).For the remote site, it determines the geographic location by one of threemethods:• If the site address is a host or an appliance (a physical address, not aVIP), it will determine the region using that address• If the site address is a VIP, the geographic region is based on the managementIP address of the ServerIron hosting the VIP• In the GSLB ServerIron, you can explicitly specify the region, presumingthat the above criteria may not lead to the correct region; this settingsupersedes the two previously-mentioned methodsNow that we know the geographic location of the client and the geographiclocation of the sites, the GSLB ServerIron will favor sites within the same geographicregion.Check 5: Available Session CapacityIf, after the previous four checks, you end up with two or more sites that arewithin the same region, the next thing the GSLB ServerIron checks is the AvailableSession Capacity. This is similar to the Session Capacity Threshold, in thatthe GSLB ServerIron looks at the remote site ServerIrons' session information.Whereas, the Session Capacity Threshold checks to see if a site is too busy,the Available Session Capacity checks to see how busy (or how available) a siteis.This check compares how many unused session entries each site has. To preventfrequent, and unnecessary, changes in site preference, this check uses atolerance percentage. By default, this percentage is 10%. For example, ifServerIron A has 1,000,000 sessions available, and ServerIron B has 800,000sessions available, the GSLB ServerIron will prefer ServerIron A. The differencebetween the two is 200,000 sessions (1,000,000 - 800,000). Ten percent of1,000,000 is 100,000, so this difference is outside of the tolerance percentage.However, if ServerIron A has 1,000,000 sessions available, andServerIron B has 990,000 sessions available, the difference is only 10,000.This is less than the tolerance percentage (1,000,000 x .10 = 100,000). Now,ServerIron A and ServerIron B will be preferred equally. The tolerance percentageis configurable.394 Brocade IP Primer