BGP-Anycast and DNS Round-Robin

The simplest GSLB that you can deploy is by creating exact replicas (or surrogates) of a site, including the services and IP addressing scheme. Then, to distribute load across the sites, simply advertise the IP prefix into BGP across the Internet from each site. You learned previously that this technique is commonly referred to as BGP-anycast. BGP routers that receive multiple prefixes install those routes in their routing tables that they deem are the most preferable using various BGP metrics, in particular the autonomous system (AS) path attribute. When requests arrive for content for the GSLB, BGP routes the packets to the nearest site at Layer 3, without any need for higher layers of intelligence.

The drawback to BGP-anycast is potentially uneven distribution of client requests across sitesthe request distribution is dependant on the geographic location of your clients. If a large percentage of traffic comes from a particular geographic region, then those packets use the same route and thus the same site. Additionally, BGP-anycast lacks site health check capabilities.

Whereas you must use the same IP addressing scheme across your sites with BGP-anycast, other GSLB methods require a separate routable IP address space per site. For example, another simple GSLB involves installing multiple DNS A records in the domain's TLD DNS server (or the enterprise's intermediary DNS server, if available), with one entry for each location. Any readily available commercial DNS server software, such as Berkeley Internet Name Domain (BIND), can provide simple DNS round-robin load balancing between the entries. DNS round-robin evenly distributes client requests between the sites. An advantage of this design is that you do not need to manage a DNS server at each site. If your intermediary DNS server is delegated authority for the load-balanced domain replicated across your sites, you can locate it at the central location.

Note

Recall that the TLD can delegate authority to an organization's intermediary DNS server, so that you can easily manage subdomains within your organization. If you do not use an intermediary DNS server, you will need the TLD entries to be updated with a single entry per replicated site.