1. GLBP
GLBP: Gateway Load Balancing Protocol. It is a Cisco proprietary protocol. We can Load Balance between the Gateways. The Load can be distributed among the Gateways. In GLBP we create a single Virtual IP (VIP) with multiple MAC Addresses. These Virtual MAC Addresses represent different Switches or Routers. A Single AVG or Active Virtual Gateway acts as ‘Point Man’ Point man manages MAC address pool. It knows about all MAC addresses other switch / routers using. Other Switches / Router acts like a Active Virtual Forwarders (AVF). If AVG fails other Backup Routers takes its place.

2. GLBP
How It Works ? Based on the settings, the AVG can distribute the request load by assigning different Switches/ Routers (AVG co-workers) to work as Gateway for specific client. For Example if Computer A & B wants to access Internet, AVG can decided which Switch /Router will act as Gateway for these computers, A & B. AVG decides based on weight or Round Robin fashion. Because Communication starts with ARP (MAC Addresses are involved), AVG replies with the Virtual MAC address to Devices. The Devices think that they are communicating with the same Gateway (As same VIP is setup in their network adapter settings)

3. GLBP
Configuration: Router (config)# int fa 0/0  Router (config-if)# glbp  Note: 20 is group number, VIP is (which all share), then Router (config-if)# glbp 20 priority 150  Priority is to setup AVG “ Higher the better”. If this router goes down the next higher priority takes as AVG. Router (config-if)# glbp 20 timer <as requried in msec or secs> Then Router (config-if)# glbp 20 load-balancing round-robin  Note: If priorities of two switches are equal GLBP used IP address of that interface to chose AVG. The higher real IP wins elections and become AVG. Thanks, Hyp3ri0n