21. June 2011 18:00
What is GLBP?
GLBP is a protocol for load balancing routers but make them appear to be a single router to other devices on a network. GLBP stands for gateway load balancing protocol. An example of a situation where this could be used is for balancing between two broadband connections for a small to medium office. As far I as am aware it will only work with cisco routers and nobody else support the GLBP protocol. A full description can be found on the cisco web site for the command reference.
How does it work?
It works the same way as other redundant gateway protocols work. Like HSRP or VRRP. However in GLBP instead of one or other router being the active gateway both routers share an ip address to make them both active in the setup. So as an added benefit of load balancing you can also have the added benefit of redundancy for free. It is possible for the device to share a single ip address because it will only ever advertise one mac / arp address to each host that requests the gateway.
Where is the benefit?
What it will not be able to do is to double an internet connection bandwidth to a single machine when nat is involved. What it will do is spread the load from multiple machines into two or more internet connections. So it is best suited to growing offices where there are too many people for a single internet connection but not yet enough to consider getting a costly leased line.