4.5. First Hop Redundancy Protocols

💡 First Principle: Every PC has one default gateway configured. Just one. If that router fails, the PC can reach the local subnet but nothing else—no internet, no remote servers, no cloud applications. Rebooting the PC won't help. Changing DNS won't help. Until someone either fixes the router or manually reconfigures every PC, users are stuck.

Consider this scenario: Your company has 500 users on a VLAN, all pointing to 192.168.1.1 as their gateway. That router's power supply fails at 9 AM Monday. Without FHRP, you now have 500 users unable to reach email, cloud apps, or the internet. IT scrambles to either fix the router (not fast) or change every PC's gateway (not faster). With FHRP, a standby router takes over the 192.168.1.1 address within seconds. Users experience a brief pause, then everything works again. No manual intervention required.

FHRPs solve this by creating a virtual gateway that floats between physical routers. Think of it like an airline with backup pilots—passengers (PCs) don't care which pilot is flying, they just know the flight number (virtual IP). If the primary pilot becomes incapacitated, the copilot takes over seamlessly. The PCs point to a virtual IP address shared between routers. If the active router fails, another takes over within seconds. From the PC's perspective, the gateway "just works."