Gateway Load Balancing builds on existing first hop redundancy protocols by providing gateway redundancy and an additional benefit of load balancing capabilities.
GLBP is made up of two roles, the active virtual gateway and the active virtual forwarder.
Active Virtual Gateway
All the participating routers in a GLBP group will elect one active virtual gateway. The active virtual gateway will respond to ARP requests within that group for the active virtual forwarder
Active Virtual Forwarder
The active virtual forwarder routes traffic that it receives from hosts within the GLBP group. Each active virtual forwarder has a unique virtual MAC address assigned to it by the active virtual gateway. The active virtual gateway will respond with this MAC address and other active virtual forwarder MAC addresses to ARP requests in the GLBP group.
There can be up to four active virtual forwarders, and one active virtual gateway per GLBP group. A router can be a virtual gateway and a virtual forwarder at the same time.
If an active virtual gateway fails, there will be another virtual gateway on standby in the same group ready to take on the role of the active virtual gateway.
If an active virtual forwarder fails, another router in the GLBP group will take on the virtual MAC address in addition to its own and forward traffic on behalf of the failed device
GLBP balances the traffic between active virtual forwarders in a round robin fashion. There are three difference types of load balancing that gateway load balancing protocol is capable of:
Round robin rotates between different virtual forwarder MAC addresses sequentially to load balance traffic.
Introduces weights assigned to virtual active forwarders to define a ratio of load balancing between devices. Routers with better bandwidth capabilities can be assigned a larger portion of traffic
The hosts MAC address is used in calculating in which active virtual forwarder is used for that host. By utilising an algorithm the host will always receive the same active virtual forwarder