BROCADE IP PRIMER

How Does GSLB Decide?Check 6: FlashbackThe year is 1983… Oh, sorry. Wrong flashback. If we still have multiple sites tochoose from at this point, we'll base the next decision on the Flashback time.This is the response time of the site during the GSLB ServerIron's healthcheck. It will favor the site with the fastest time.This uses a tolerance percentage, too. By default, it's 10%. This means thatthe Flashback response times must be greater than a 10% variance. Otherwise,the GSLB will favor them equally.For Flashback time, the GSLB ServerIron will first compare the speeds of theLayer 7 health check response (if it's configured). If there's not a big enoughvariance there, it will use the Layer 4 health check as a tie breaker.Check 7a: Least Response SelectionLet's say we've passed through all six of these checks, and we still have morethan one site to choose from! Well, the GSLB ServerIron has to make a decision.It needs a “tie-breaker.” So it decides to send it to the site that it haschosen the least. For example, let's say you have three sites. The GSLBServerIron has resolved requests to Site 1: 50% of the time, Site 2: 30% of thetime, and Site 3: 20% of the time. If it comes down to Lease Response, it willresolve to Site 3. Site 3 has been chosen the least often for DNS resolutions atthis point.Check 7b: Round-Robin SelectionLet's say, for whatever reason, you didn't want the last criteria to be LeastResponse Selection. Least Response is enabled by default. It can be overridden,if you enable Round-Robin. If this is enabled, it will take the place (at theend of the criteria) of Least Response.Round-Robin works the same way it has worked with everything else we'vedescribed. It blindly takes the next site in line. If it resolved to Site 1 last time,it'll resolve to Site 2 this time, and so on. You would use Round-Robin if youdidn't want the GSLB ServerIron to favor sites that have recently been enabled,or sites that had gone down, but have now been restored.AffinityAffinity allows you to set a GSLB preference to a particular site, given a certainrange of clients. For example, let's say that you know clients from the123.1.2.0/24 network would always be best served to go to Site 3. You woulddefinite a GSLB affinity for clients from 123.1.2.0/24 to always resolve to Site3, unless it is down. In this sense, affinity acts kind of like Check 1.5. TheServer Health check still supersedes it, but, if it's defined, it will take precedenceover all others. You can define up to 50 affinities.Brocade IP Primer 395