The default gateway is the single point of failure nobody notices until it dies. First Hop Redundancy Protocols — HSRP, VRRP, and GLBP — put a virtual IP and virtual MAC in front of two or more routers so that when one fails, the other takes over without a host ever changing its gateway. This is a CCNA 200-301 topic 3.5 deep dive: how a virtual gateway works, the HSRP state machine and its v1/v2 differences, priority and preemption, interface tracking, the open-standard VRRP, Cisco's active-active GLBP, and the IOS configuration and verification commands that make it real.
Glbp
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CCNA: First Hop Redundancy Protocols — HSRP, VRRP, and GLBP