PlantUML DiagramFHRP Virtual Gateway
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A campus has two distribution switches, both capable of routing. Users' default gateway is configured as 10.10.10.1 — the primary switch. That switch reboots for a software upgrade. Every user's traffic to the internet and to other VLANs stops. The secondary switch could take over, but nobody's default gateway points to it. Two hours of downtime for a routine upgrade — unless you use FHRP.
First Hop Redundancy Protocols solve exactly this. They let two (or more) routers share a virtual IP address that clients use as their gateway. Whichever router is active answers for that IP. If it fails, another takes over — clients never notice.
The virtual IP is the key. Clients configure a single gateway IP. The routers cooperate to decide which one answers for it. When the active router fails, the standby takes over within seconds. From the client's perspective, nothing changed — the gateway is still reachable.
The virtual MAC is what makes failover invisible. Clients ARP for the virtual IP and get a virtual MAC. On failover, the new active router adopts both the virtual IP and the virtual MAC. ARP caches don't need to update — the traffic just flows to a different physical device behind the same virtual identity.
If your hosts run a routing protocol themselves (rare on client devices, common on servers), FHRP isn't needed — they'll route around failures dynamically. If your network has only one router, FHRP isn't possible — there's nothing to failover to. FHRP matters when you have redundant routers and non-routing hosts.
FHRP solves the default gateway single-point-of-failure problem. Virtual IP + virtual MAC + elected active router = transparent failover. Next lesson compares the three FHRP protocols: HSRP, VRRP, and GLBP.
Finish the lesson once you have worked through the material. This awards ★ 15 XP.