What are First Hop Redundancy Protocols (FHRP)?
Learn what FHRPs are, how HSRP, VRRP, and GLBP provide gateway redundancy, and how failover works — with networking interview Q&A.
Expected Interview Answer
A First Hop Redundancy Protocol (FHRP) lets two or more routers share a single virtual IP and virtual MAC address as a client’s default gateway, so if the active router fails a standby router takes over transparently without hosts needing to change their configured gateway.
Hosts on a LAN are configured with one static default gateway IP address, which is normally a single point of failure — if that router goes down, every host loses its path off the subnet until an administrator intervenes. HSRP (Cisco proprietary), VRRP (open standard), and GLBP (Cisco, with load balancing) solve this by having a group of routers present one shared virtual IP/MAC as the gateway, while internally electing an active (or master) router and one or more standby routers. The active router forwards traffic and answers ARP for the virtual IP; the standbys exchange periodic hello messages to detect a failure and take over within seconds, assuming the virtual identity so hosts never notice a change. Priorities and preemption settings control which physical router becomes active and whether it reclaims the role after recovering.
- Removes the default gateway as a single point of failure
- Failover is transparent — hosts keep the same gateway IP/MAC
- Standby routers detect failure via hello timers, typically within seconds
- HSRP/VRRP/GLBP differ in vendor support and load-balancing ability
AI Mentor Explanation
An FHRP is like a team naming a vice-captain who is fully briefed and ready to make every on-field decision the moment the captain is injured, so the umpires and opposition see one continuous captaincy even though the actual person changed. The vice-captain does not wait for a selection meeting; they step in using the same authority instantly. Fans in the stands never notice a handover happened because the team still answers to one recognizable leader. This is exactly how a standby router assumes the shared virtual gateway identity the instant the active router disappears.
Step-by-Step Explanation
Step 1
Group formation
Two or more routers on the same LAN are configured into one FHRP group sharing a virtual IP and virtual MAC address.
Step 2
Election
Routers exchange hello messages and elect an active/master router (usually by highest priority) to forward traffic for the virtual IP.
Step 3
Steady state
The active router answers ARP for the virtual IP and forwards packets; standbys listen for periodic hellos and stay idle.
Step 4
Failover
If hellos from the active router stop arriving within the dead timer, a standby promotes itself to active and assumes the virtual identity.
What Interviewer Expects
- Explains the default-gateway single-point-of-failure problem being solved
- Names at least HSRP and VRRP and knows one is Cisco proprietary, one is open standard
- Describes virtual IP/MAC and the active/standby election model
- Understands failover is transparent to end hosts
Common Mistakes
- Confusing FHRP with load-balancing/ECMP at the routing-protocol level
- Thinking every group member forwards traffic simultaneously (true only for GLBP-style load sharing)
- Not knowing hosts point to a virtual IP, not any single router's real IP
- Forgetting preemption controls whether the original active router reclaims its role
Best Answer (HR Friendly)
“Think of a first hop redundancy protocol as a backup default gateway for your network. Normally every device points to one router to leave the LAN, and if that router dies, everyone loses connectivity. With HSRP or VRRP, a second router is standing by, sharing the same virtual gateway address, so it can take over instantly and invisibly if the primary router fails — nobody has to reconfigure anything.”
Code Example
# Show HSRP group status on an interface
show standby brief
# Interface Grp Pri P State Active Standby Virtual IP
# Gi0/1 10 110 P Active local 192.168.1.2 192.168.1.1
# Show VRRP group status (open-standard equivalent)
show vrrp brief
# Interface Grp Pri Time Own Pre State Master addr Group addr
# Gi0/1 10 150 3570 Y Master 192.168.1.1 192.168.1.254Follow-up Questions
- What is the difference between HSRP, VRRP, and GLBP?
- How does preemption affect which router stays active after a failover?
- How does GLBP achieve load balancing across multiple routers?
- What happens to existing TCP sessions during an FHRP failover?
MCQ Practice
1. What problem does a First Hop Redundancy Protocol primarily solve?
FHRPs (HSRP, VRRP, GLBP) provide gateway redundancy so hosts do not lose connectivity if their single configured gateway router fails.
2. Which of the following is an open standard rather than Cisco proprietary?
VRRP is an IETF open standard (RFC 5798); HSRP and GLBP are Cisco proprietary.
3. What do hosts use as their configured default gateway in an FHRP setup?
Hosts always point to the group's virtual IP/MAC, which any elected active router answers for, making failover transparent.
Flash Cards
What is a First Hop Redundancy Protocol? — A protocol (HSRP, VRRP, GLBP) letting routers share a virtual gateway IP/MAC so failover is transparent to hosts.
Is HSRP or VRRP the open standard? — VRRP is the open (IETF) standard; HSRP and GLBP are Cisco proprietary.
What do hosts configure as their gateway? — The group's shared virtual IP address, not any individual router's real IP.
What detects a failed active router? — Missed periodic hello messages within the configured dead/hold timer.