Explain the purpose and basic operation of HSRP, VRRP, and GLBP in providing gateway redundancy.

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HSRP (Hot Standby Router Protocol), VRRP (Virtual Router Redundancy Protocol), and GLBP (Gateway Load Balancing Protocol) are First Hop Redundancy Protocols (FHRPs) that provide gateway redundancy for hosts on a LAN segment. They ensure continuous network connectivity even when a default gateway router fails, which is critical for enterprise network availability.

HSRP, developed by Cisco, creates a virtual router with a virtual IP address and MAC address shared between two or more physical routers. One router becomes the active router, forwarding packets sent to the virtual IP, while another serves as the standby router, ready to take over if the active router fails. The routers exchange hello messages to monitor each other's status. If the standby doesn't receive hellos from the active router within a specified time, it assumes the active role and begins forwarding traffic. HSRP uses priority values to determine which router becomes active, with higher priority winning.

VRRP operates similarly but is an open standard protocol defined in RFC 5798, making it vendor-neutral and interoperable across different equipment manufacturers. Like HSRP, it elects a master router that owns the virtual IP address and handles traffic forwarding, with backup routers standing ready. VRRP routers advertise their status and priority, and the router with the highest priority becomes master. A key difference is that VRRP can use a physical router's actual IP address as the virtual IP, whereas HSRP always requires a separate virtual IP.

GLBP extends the redundancy concept by adding load balancing capabilities. Instead of having only one active forwarder at a time, GLBP allows multiple routers to simultaneously forward traffic for the same virtual IP address. It accomplishes this by using a single virtual IP but multiple virtual MAC addresses. The Active Virtual Gateway (AVG) assigns different virtual MAC addresses to group members and responds to ARP requests with different MAC addresses, distributing the load across multiple Active Virtual Forwarders (AVFs).

The choice between these protocols depends on your environment. HSRP is ubiquitous in Cisco environments and provides solid failover. VRRP offers the advantage of vendor interoperability when using mixed equipment. GLBP provides both redundancy and load distribution, making better use of available bandwidth and hardware resources, though it adds complexity and is Cisco-proprietary.

All three protocols significantly improve network resilience by eliminating the default gateway as a single point of failure, ensuring that host connectivity remains intact during router failures with minimal interruption.