Gateway Load Balancing Protocol (GLBP) can have up to four primary active virtual forwarders (AVFs) provide load balancing across multiple WAN links. GLBP is a Cisco proprietary First Hop Redundancy Protocol (FHRP) that enables up to four routers to act as a single virtual router. The virtual router has its own virtual IP address and up to four virtual Media Access Control (MAC) addresses, one for each of the four primary AVFs in the group. One of the routers in the GLBP group is elected the active virtual gateway (AVG) and performs the administrative tasks for the standby group, such as responding to Address Resolution Protocol (ARP) requests. When a client sends an ARP request for the IPaddress of the default gateway, the AVG responds with one of the virtual MAC addresses in the group.

Because multiple routers in the GLBP group can actively forward traffic, GLBP provides load balancing as well as local redundancy.

Additionally, you can control the percentage of traffic that is sent to a specific gateway byconfiguring weighted load balancing. By default, GLBP uses a round-robin technique to load balance between routers. If you configure weighted load balancing, GLBP can send a higher percentage of traffic to a single GLBP group member based on the weight values assigned to the interfaces of that member.

Hot Standby Router Protocol (HSRP) is a Cisco proprietary protocol that enables two or more routers to act as a single virtual router. However, only one router in the HSRP standby group forwards traffic for the group. Because there is only one AVF in an HSRP standby group, HSRP cannot provide load balancing across multiple WAN links.

Virtual Router Redundancy Protocol (VRRP) is an open-standard protocol that is similar to HSRP. VRRP enables two or more routers to act as a single virtual router, but it does not enable more than a single router to act as an AVF. Therefore, VRRP cannot provide load balancing across multiple WAN links.