Gateway Load Balancing Protocol

Gateway Load Balancing Protocol (GLBP) takes HSRP even further. Instead of just providing backup for a failed router, it can also handle the load balancing between multiple routers. GLBP provides this functionality using a single virtual IP address and multiple virtual MAC addresses. Workstations are configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets. GLBP members communicate with each other using hello messages sent every three seconds to the multicast address 224.0.0.102.

Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group. It is the job of other group members to back up for the AVG in the event that the AVG fails. The AVG assigns a virtual MAC address to each member of the GLBP group. The AVG is responsible for answering ARP requests for the virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests with different virtual MAC addresses that the group members will respond to. Although you can use many optional commands with GLBP, the primary command to enable GLBP is as follows:

glbp group ip [ip-address [secondary]]