FHRP Explained: HSRP, VRRP, and GLBP for Reliable Network Access

 FHRP (First Hop Redundancy Protocol)

FHRP (First Hop Redundancy Protocol) is a family of networking protocols designed to ensure gateway redundancy in IP networks. Its primary goal is to prevent a single point of failure at the default gateway, the first router a host contacts when sending traffic outside its local subnet.

Why FHRP Is Needed

In a typical network, hosts rely on a single default gateway. If that gateway fails, all connected devices lose access to external networks. FHRP solves this by allowing multiple routers to share a virtual IP address, so if the active router fails, a backup router can take over automatically and seamlessly.

How FHRP Works

• Routers in an FHRP group share a virtual IP and MAC address.
• One router is elected as the active router (handles traffic).
• Another is the standby router (ready to take over).
• Hosts use the virtual IP as their default gateway.
• If the active router fails, the standby router takes over without requiring host reconfiguration.

Popular FHRP Protocols

1. HSRP (Hot Standby Router Protocol)

• Cisco proprietary
• Uses multicast address 224.0.0.2 and port 1985
• Routers exchange hello messages every 3 seconds
• Election based on priority and IP address
• Preemption (automatic takeover by a higher-priority router) is disabled by default

2. VRRP (Virtual Router Redundancy Protocol)

• Open standard (IP protocol 112)
• Uses multicast address 224.0.0.18
• Preemption is enabled by default
• Versions:
• VRRPv2: IPv4 only
• VRRPv3: IPv4 and IPv6 (not simultaneously)

3. GLBP (Gateway Load Balancing Protocol)

• Cisco proprietary
• Adds load balancing to redundancy
• Multiple routers can actively forward traffic

Failover Process

1. Active router fails.

2. Standby router detects failure via missed hello messages.

3. Standby router assumes the virtual IP/MAC.

4. Hosts continue using the same gateway IP, no disruption.

Benefits of FHRP

• High availability: Ensures continuous network access.
• Automatic failover: No manual intervention needed.
• Scalability: Supports large enterprise networks.
• Transparency: Hosts are unaware of gateway changes.