First Hop Redundancy Protocols (FHRP) are essential mechanisms in network design that provide gateway redundancy for hosts on a local network segment. When a default gateway fails, FHRP ensures continuous network connectivity by allowing multiple routers to work together as a single virtual gateway…First Hop Redundancy Protocols (FHRP) are essential mechanisms in network design that provide gateway redundancy for hosts on a local network segment. When a default gateway fails, FHRP ensures continuous network connectivity by allowing multiple routers to work together as a single virtual gateway.
The primary FHRP implementations include HSRP (Hot Standby Router Protocol), VRRP (Virtual Router Redundancy Protocol), and GLBP (Gateway Load Balancing Protocol).
HSRP is Cisco proprietary and operates by designating one router as active and another as standby. Both routers share a virtual IP address and virtual MAC address. The active router handles all traffic destined for the virtual IP, while the standby router monitors the active router through hello messages. If the active router becomes unavailable, the standby router assumes the active role within seconds.
VRRP is an open standard protocol (RFC 5798) functioning similarly to HSRP. It uses a master and backup router configuration. The master router responds to packets sent to the virtual IP address, and backup routers take over when the master fails.
GLBP, another Cisco proprietary protocol, provides both redundancy and load balancing. Unlike HSRP and VRRP, GLBP allows multiple routers to actively forward traffic simultaneously using different virtual MAC addresses mapped to a single virtual IP address.
Key FHRP concepts include priority values that determine which router becomes active or master, preemption settings that allow higher-priority routers to reclaim the active role after recovery, and tracking features that adjust priority based on interface or object states.
FHRP protocols use multicast addresses for communication between participating routers. Timers control hello intervals and hold times, affecting failover speed. Proper FHRP configuration ensures network resilience, minimizes downtime, and provides seamless failover capabilities for end users who remain unaware of any gateway changes occurring in the background.
FHRP Concepts - Complete Study Guide
Why FHRP is Important
First Hop Redundancy Protocols (FHRPs) are critical in enterprise networks because they provide gateway redundancy for end devices. When a default gateway fails, hosts lose connectivity to remote networks. FHRPs solve this problem by allowing multiple routers to work together, presenting a single virtual gateway address to clients. This ensures continuous network availability and is a fundamental concept for network engineers.
What is FHRP?
FHRP is a category of protocols that provide default gateway redundancy. The main protocols include:
VRRP (Virtual Router Redundancy Protocol) - Open standard (RFC 5798) similar to HSRP, uses master/backup terminology
GLBP (Gateway Load Balancing Protocol) - Cisco proprietary, provides both redundancy AND load balancing
How FHRP Works
1. Virtual IP Address: Multiple routers share a virtual IP address that hosts use as their default gateway
2. Virtual MAC Address: A virtual MAC address is associated with the virtual IP, ensuring ARP resolution works properly
3. Election Process: Routers exchange hello messages to determine which router becomes active/master
4. Priority Values: Higher priority values determine the active router (HSRP default: 100, VRRP default: 100)
5. Preemption: When enabled, allows a higher-priority router to take over the active role when it comes back online
6. Failover: If the active router fails, the standby router takes over using the same virtual IP and MAC
Key Protocol Comparisons
HSRP: - Active and Standby routers - Default priority: 100 - Hello timer: 3 seconds, Hold timer: 10 seconds - Virtual MAC: 0000.0c07.acXX (XX = group number in hex) - Multicast address: 224.0.0.2 (v1) or 224.0.0.102 (v2)
VRRP: - Master and Backup routers - Default priority: 100 - Advertisement interval: 1 second - Virtual MAC: 0000.5e00.01XX - Multicast address: 224.0.0.18 - The interface IP can be the virtual IP (making that router the IP owner with priority 255)
GLBP: - Active Virtual Gateway (AVG) and Active Virtual Forwarders (AVFs) - Supports up to 4 forwarders per group - Provides load balancing across multiple gateways
Exam Tips: Answering Questions on FHRP Concepts
1. Know the terminology differences: HSRP uses Active/Standby while VRRP uses Master/Backup. Questions often test this distinction.
2. Remember default values: Default priority is 100 for both HSRP and VRRP. Higher priority wins the election.
3. Understand preemption: HSRP has preemption disabled by default; VRRP has it enabled by default. This is a common exam topic.
4. Virtual MAC format: Be able to identify which protocol is in use based on the virtual MAC address pattern.
5. Proprietary vs Open: HSRP and GLBP are Cisco proprietary; VRRP is an open standard. Multi-vendor scenarios require VRRP.
6. Load balancing: Only GLBP provides load balancing natively. HSRP and VRRP require multiple groups for load distribution.
7. Troubleshooting commands: Know commands like show standby (HSRP), show vrrp, and show glbp
8. Focus on the scenario: Read questions carefully to identify whether redundancy, load balancing, or multi-vendor support is the requirement.
9. Timer relationships: Hold time should typically be 3x the hello timer to avoid false failovers.