While HSRP and VRRP provide gateway resiliency, the standby members
of the redundancy group along with their upstream bandwidth is not used while
the device is in standby mode. Only the active router for the HSRP and VRRP
group forwards traffic for the virtual MAC. Resources associated with the
standby router are not fully utilized. Some load balancing can be accomplished
with these protocols through the creation of multiple groups and through the
assignment of multiple default gateways, but this configuration creates
administrative burden.
Cisco designed Gateway Load Balancing Protocol
(GLBP) to allow automatic selection and simultaneous use of multiple, available
gateways, as well as automatic failover between those gateways. Multiple
routers share the load of frames that, from a client perspective, are sent to a
single default gateway address. With GLBP, resources can be fully utilized
without the administrative burden of configuring multiple groups and managing
multiple default gateway configurations as is required with HSRP and VRRP.