Virtual Router Redundancy Protocol (VRRP) is an Internet Engineering Task Force (IETF)standard FirstHop Redundancy Protocol (FHRP) that can use object tracking and preemption to provide Layer 3 failover. FHRPs are protocols that are used to provide Layer 3 gateway redundancy, such as failover and load balancing.
Providing Layer 3 redundancy ensures that hosts on a LAN will have a backup path to external networks should a primary path fail or become too congested to forward traffic. Layer 3 devices in an FHRP configuration typically share a virtual IP address that is then configured as the default gateway on each host for which the device is to forward traffic. The FHRP devices might also share a virtual Media Access Control (MAC) address or multiple virtual MAC addresses, depending on the protocol. FHRPs typically use a priority system to elect a primary Layer 3 forwarding device, which is known as an active virtual gateway (AVG), an active router, or a master router, depending on the protocol. The same priority system elects either a single or multiple backup-forwarding devices.
VRRP can be configured to use object tracking to influence the priority of a router in a group and therefore force the election of a different master router when certain conditions are met. When combined with VRRP preemption, which enables a VRRP router to automatically assume the master router role when priority values change, object tracking enables VRRP to adjust the priority of a router based on the line protocol status of a specific interface or the availability of a given route to a destination. For example, if RouterA and RouterB in a VRRP configuration had different paths to the Internet, VRRP could be configured to monitor RouterA's outbound interface and to automatically set RouterA's VRRP priority to a value lower than RouterB's if RouterA's outbound interface were to go down.
RouterB would then become the master router, and Layer 3 traffic would be forwarded through its outbound interface instead of through RouterA's.
Gateway Load Balancing Protocol (GLBP) is a Ciscoproprietary FHRP, not an IETF standard. However, GLBP does support object tracking and preemption. GLBP is different from both VRRP and Hot Standby
Router Protocol (HSRP) in that it is, by default, capable of load balancing traffic between all routers in a GLBP group. VRRP and HSRP are primarily failover protocols. GLBP elects an AVG and up to four primary active virtual forwarders (AVFs). The routers in a GLBP group receive traffic sent to a virtual IP address that is configured for the group. Each GLBP group contains an AVG that is elected based on which router is configured with the highest priority value or the highest IP address value if multiple routers are configured with the highest priority value. The other routers in the GLBP group are configured as primary or secondary AVFs.
The AVG in a GLBP group assigns a virtual MAC address to up to four primary AVFs? all other routers in the group are considered secondary AVFs and are placed in the listen state. When the AVG receives ARP requests that are sent to the virtual IP address for the GLBP group, the AVG responds with different virtual MAC addresses. This provides load balancing, because each of the primary AVFs will participate by forwarding a portion of the traffic sent to the virtual IP address.
HSRP is a Ciscoproprietary FHRP, not an IETF standard. However, like VRRP, HSRP is capable of using object tracking and preemption to modify the priority of the HSRP active router and force the standby router to take over if a specific interface goes down or a path to the destination becomes unavailable.
Link Aggregation Control Protocol (LACP) is not an FHRP. LACP is an Institute of Electrical and Electronics Engineers (IEEE) protocol that is used to enable link aggregation on EtherChannel links.