3.1.3. First Hop Redundancy
FHRP (First Hop Redundancy Protocol): Provides gateway redundancy so clients don't need reconfiguration if their gateway fails.
Multiple routers share a Virtual IP (VIP). Clients point to the VIP as their gateway. If the active router fails, the standby takes over the VIP—seamlessly, without client reconfiguration.
| Protocol | Standard | How routers share the gateway |
|---|---|---|
| HSRP | Cisco proprietary | One active router forwards; one standby waits |
| VRRP | Open standard (RFC 5798) | One master forwards; backups wait |
| GLBP | Cisco proprietary | Several routers forward at once (load balancing): ARP replies for the VIP hand out different virtual MACs |
Which router is active? The one with the highest priority (default 100 in both HSRP and VRRP); if priorities tie, the highest IP address wins.
Preemption: Decides whether a higher-priority router that comes back online takes the active role back. HSRP has preemption disabled by default, so a recovered router stays in standby until preemption is enabled on it; VRRP preempts by default (RFC 5798).
Subinterfaces: One physical interface divided into multiple logical interfaces, each with its own IP and VLAN assignment. Used in "router-on-a-stick" configurations where a single router interface handles inter-VLAN routing through 802.1Q trunk subinterfaces.
