Gateway Load Balancing Protocol (GLBP) can be used to replace HSRP in environments where you would like to load balance between multiple nodes, instead of having an active/standby pair.
In order to enhance on the capabilities of Hot Standby Router Protocol (HSRP), Cisco developed GLBP. GLBP provides automatic, first-hop gateway load balancing, which allows for more efficient resource usage and reduced administrative costs. It is an extension of HSRP and specifies a protocol that dynamically assigns responsibility for a virtual IP address and distributes multiple virtual MAC addresses to members of a GLBP group.
In campus networks, Layer 3 VLAN interfaces act as the gateway for the hosts. These Layer 3 VLAN interfaces from different switches are load balanced using GLBP. Layer 3 interfaces from multiple switches form one GLBP group. Each group contains one unique virtual IP address.
Supervisor 720 can have a maximum of 1024 GLBP groups (group numbers 0 to 1023). Supervisor 2 supports only one GLBP group. A GLBP group can have a maximum of 4 members. It means that GLBP can load balance up to 4 gateways
