BROCADE IP PRIMER

Chapter 18: Global Server Load Balancing (GSLB)request “www.something.com” might see “3.4.5.6, 1.2.3.4, 2.3.4.5.” Eachtime the client will pick the first one on the list. It's the DNS server that's usinground-robin to decide which IP address is seen first.Well, that seems reasonable, doesn't it? Problem solved. I'll just put each ofmy three sites' IP addresses in the A record, and let DNS serve them up. Whatif a site goes down? DNS won't make a change. It will keep handing offrequests. Theoretically, if one of your three sites became unavailable, youwould lose approximately 33% of your incoming traffic. About one-third of yourclients would get a connection failure when trying to reach your site. Now, whatif one of your sites was temporarily experiencing problems and was performingmuch slower than usual? Again, about 33% of your incoming clients would seethis congestion, and experience it as well.What if you had an agent who handled DNS much more intelligently? What ifyou had something that checked the health of the site, checked the responsivenessof the site, and maybe even checked the geographic location of theclient in relation to the site being requested? And what if this something usedthat information to decide to which IP address to resolve the name to? EnterGSLB.That's what it does. It adds an intelligent layer to DNS. It makes decisions onthe fly as to which might be the best IP address to resolve to, given the circumstances.If a site is down, it will no longer resolve the name to that site's IPaddress (until it comes back up). If a site is not responding well, it will favorresolving to other sites, until that site is responding better. If a client in Londonis making a request, it will resolve the name to the IP address of the site inLondon, rather than the site in South Korea. It's as if we've got an operatorworking 24x7 watching every DNS request and doing his or her best to changethe reply to suit the needs of the client. That's GSLB.How Does GSLB Decide?GSLB uses several metrics to decide which site would be best for the client.They will be presented to you in their default order. This can be changed. Indefault order, they are as follows:Check 1: Server HealthIf the remote site is dead, GSLB doesn't want to serve that IP address. To avoidthis scenario, it performs a health check. It will perform a Layer 4 health checkand, if configured, a Layer 7 health check. If the site fails the health check, itwill not be considered for the IP resolution.So, to what IP is this health check being performed? It's to whatever IP addressyou configured. It could be a virtual server on the remote site's ServerIron. Itcould be the actual IP address of a remote host. It could be a VIP contrived bysome third-party appliance. It's the site that the client would see, if youresolved it to that site's IP address.392 Brocade IP Primer