Network Load Balancing

Network load balancing (commonly referred to as dual-WAN routing or multihoming) is the ability to balance traffic across two WAN links without using complex routing protocols like BGP.

This capability balances network sessions like Web, email, etc. over multiple connections in order to spread out the amount of bandwidth used by each LAN user, thus increasing the total amount of bandwidth available. For example, a user has a single WAN connection to the Internet operating at 1.5Mbit/s. They wish to add a second broadband (cable, DSL, wireless, etc.) connection operating at 2.5Mbit/s. This would provide them with a total of 4Mbit/s of bandwidth when balancing sessions.

Session balancing does just that, it balances sessions across each WAN link. When Web browsers connect to the Internet, they commonly open multiple sessions, one for the text, another for an image, another for some other image, etc. Each of these sessions can be balanced across the available connections. An FTP application only uses a single session so it is not balanced; however if a secondary FTP connection is made, then it may be balanced so that on the whole, traffic is evenly distributed across the various connections and thus provides an overall increase in throughput.

Additionally, network load balancing is commonly used to provide network redundancy so that in the event of a WAN link outage, access to network resources is still available via the secondary link(s). Redundancy is a key requirement for business continuity plans and generally used in conjunction with critical applications like VPNs and VoIP.

Finally, most network load balancing systems also incorporate the ability to balance both outbound and inbound traffic. Inbound load balancing is generally performed via dynamic DNS which can either be built into the system, or provided by an external service or system. Having the dynamic DNS service within the system is generally thought to be better from a cost savings and overall control point of view.


Microsoft NLB

Microsoft has also developed a technology called Network Load Balancing (NLB) that allows for efficient utilization of multiple network cards.

Server load balancing

When multiple servers are joined to create a cluster. Clusters can use network load balancing whereby simultaneous cluster request are distributed between cluster servers.

Round robin DNS records is one form of cluster load balancing. It works by creating multiple host records (usually A and/or AAAA) for one machine. As clients make requests, DNS rotates through its list of records.

In addition to the before mentioned, to configure a terminal server cluster, one needs a load-balancing technology such as Network Load Balancing (NLB) or DNS round robin. A load balancing solution will distribute client connections to each of the terminal servers.

Terminal Server Session Directory is a feature that allows users to easily and automatically reconnect to a disconnected session in a load balanced Terminal Server farm. The session directory keeps a list of sessions indexed by username and server name. This enables a user, after disconnecting a session, to reconnect to the correct Terminal Server where the disconnected session resides in order to resume working in that session. This reconnection will work even if the user connects from a different client computer.

See also


  • Vector routing is a form of network load balancing

External links

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