Strictly for new comers-By Ravindu Chathuranga Jayalath

Global Server Load Balancing (GSLB) February 19, 2010

There are two well known methods of GSLB called,

1. DNS based Global Server Load Balancing
2. BGP Host Route Injection

The most common and one most common method would be the 1^st one which is DNS based Global Server Load Balancing. GSLB directs DNS requests to the best-performing GSLB site in a distributed Internet environment and enables distribution of traffic across multiple sites, manages disaster recovery, and ensures that applications are consistently accessible. When a client sends a DNS request, the system determines the best-performing site and returns its IP to the client.  Following features are some of features available in this load balancing technique.

• Directs client requests to the geographically closest GSLB site (geographic and network proximity-based traffic redirection)
• Directs client requests to surviving data centers when an outage occurs
• Directs client requests to alternate data centers, when a pre-defined traffic load limit is reached
• Directs client requests to be distributed among multiple data centers

According to above features, load balancing requirements, which users can be directed based on the users network address and failover between sites can be achieved.

Sample Example is shown below. In this case simple failover and load balancing is demonstrated. To achieve more features like route traffic according to source network address, device would be configured with features such as proximity-based traffic redirection and so on.

Fig 01: Example of two sites and users connecting via Internet

Site A in has a virtual IP address (VIP) of 1.1.1.1, and Site B has a VIP of 2.2.2.2. A GSLB device is acting as the authoritative name server for www.mydomain.net. Upon a DNS query for www.mydomain.net, the job of the GSLB is to determine whether to return the IP address 1.1.1.1 or 2.2.2.2.

1.     The stub resolver (a software program running on the client computer) makes a request to the assigned local DNS server.

2.    The client’s DNS server performs an iterative resolution on behalf of the client, querying the root name servers and eventually ending up at the authoritative name server for www.mydomain.net. In this case the GSLB device is that authoritative name server.

3.      The GSLB device performs some sort of communications with devices at each site, gathering information such as site health, number of connections, and response time.

4.      Using the information gathered the GSLB device makes a determination as to the preferred site, and returns the answer to the client’s DNS server. The answer is either IP address 1.1.1.1 or IP address 2.2.2.2.

5.      The DNS answer is returned to the client’s stub resolver.

6. After DNS resolution is complete, the client makes a TCP connection to the preferred site.