GLBP

GLBP, or Gateway Load Balancing Protocol, is a Cisco-proprietary protocol that provides automatic gateway load balancing and redundancy for hosts on a single LAN segment. It enables multiple routers to participate as gateways, allowing active usage of several gateways rather than relying on a single primary router.

GLBP groups routers into a GLBP group identified by a virtual IP address (VIP). Within the group, one router is elected as the Active Virtual Gateway (AVG). The AVG answers ARP requests for the VIP and distributes forwarding responsibilities by designating one or more Active Virtual Forwarders (AVFs). Each AVF uses its own virtual MAC address, which is associated with the VIP; switches forward traffic destined for the VIP to the MAC of the chosen AVF. Other routers in the group act as Backup Virtual Routers (BVRs).

Traffic distribution is configurable via weights and preemption settings, allowing different AVFs to handle varying portions of traffic and enabling takeover if the current AVG or an AVF fails. If an AVF or the AVG fails, another AVF or BVR takes over to maintain service continuity. GLBP supports quick switchover and provides redundant gateways without requiring a single active path.

GLBP is defined by Cisco as a proprietary protocol and is not an open standard like VRRP. It is designed for IPv4, operates on LANs, and is commonly used in enterprise networks with multiple routers providing gateway services. Because AVFs use distinct virtual MAC addresses, GLBP also enables load-balanced downstream traffic while preserving the single VIP used by hosts.