Simple definition
Cisco Gateway Load Balancing Protocol provides a virtual default gateway and can distribute client traffic across multiple routers.
Purpose and use
GLBP, Gateway Load Balancing Protocol, is a Cisco mechanism combining gateway redundancy with distribution of clients across multiple routers.
How does it work?
An Active Virtual Gateway answers for the virtual IP address using MAC addresses of routers acting as Active Virtual Forwarders.
Example and verification
Compare ARP responses and forwarder counters to verify distribution and continuity after a failure.
Points to watch
Sharing clients does not guarantee equal byte counts: individual clients may generate very different traffic volumes.
In short
GLBP distributes local gateway use according to policy; it does not automatically combine multiple WAN links into one connection's bandwidth.