FHRP First Hop Redundancy Protocol
FHRP is not a single protocol, but rather a family of high-availability protocols designed to protect the "first hop"—the default gateway. By pooling multiple physical routers together into a single logical group, FHRPs ensure that if the primary router dies, a standby router instantly takes over without the end-user endpoints ever knowing an outage occurred.
The Virtual IP Illusion
If you have two routers (R1 and R2) acting as gateways for a subnet, you cannot configure half your PCs to point to R1 and half to R2. If R1 dies, those PCs are permanently stranded. FHRP solves this by creating a Virtual IP (VIP) and a Virtual MAC Address.
- Administrators group R1 and R2 into an FHRP cluster.
- R1 and R2 hold an election to decide who is "Active" and who is "Standby".
- The cluster generates a shared Virtual IP (e.g.,
192.168.1.1). - Every PC on the network is configured to use
192.168.1.1as their default gateway. - The Active router answers all ARP requests for the VIP. If the Active router stops sending "Hello" packets, the Standby router promotes itself, assumes control of the VIP and Virtual MAC, and begins routing the traffic seamlessly.
The FHRP Family
| Protocol | Vendor & Mechanics |
|---|---|
| HSRP | Hot Standby Router Protocol: Cisco Proprietary. Uses the terms "Active" and "Standby". Supports up to 255 groups. This is the primary FHRP tested on the CCNA. |
| VRRP | Virtual Router Redundancy Protocol: IEEE Open Standard (802.3ad). Uses the terms "Master" and "Backup". Often used when mixing Cisco and non-Cisco equipment. |
| GLBP | Gateway Load Balancing Protocol: Cisco Proprietary. Unlike HSRP/VRRP which leave the backup router sitting idle, GLBP actively load-balances traffic across multiple routers simultaneously using multiple Virtual MACs. |
CCNA Exam Gotchas
Preemption is Disabled by Default
In HSRP, if Router A (Priority 150) fails, Router B (Priority 100) takes over. If Router A reboots and comes back online 10 minutes later, it will NOT take the Active role back. By default, HSRP does not preempt an active session to prevent constant flapping. If you want Router A to force its way back to the Active state upon recovery, you must manually configure standby [group] preempt.