This lesson begins our journey into First-Hop Redundancy Protocols (FHRP). The term FHRP refers to a category of solutions that includes three options: HSRP, VRRP, and GLBP, with the most popular being the Hot Standby Router Protocol (HSRP). 

We begin by establishing context so that you can understand what the first hop in the network is and why it is essential. Then, we briefly touch on what FHRP is and how it works. In the following lessons, we zoom into each protocol in more detail and demonstrate how we configure and troubleshoot it.

What is a Single Point of Failure (SPOF)?

A single point of failure (SPOF) is a component in the infrastructure that, if it fails, causes an outage for an entire section of the network. These points are undesirable in network design because they compromise the infrastructure's overall reliability, availability, and fault tolerance.

Imagine a LAN network with multiple servers and switches. Everything inside the LAN is redundant. However, take a look at the components with red arrows. If any of these components (1-5) fail, the servers lose connectivity to the outside world.

The network must have redundancy at every component to be highly available and resilient. The following diagram shows a fully redundant network. There are no single points of failure (SPOFs). Any element in the network, such as a power supply, cable, or optical module, might break, or even an entire device might lose power, and the network still works fine.

Physical redundancy is very easy to understand. The network must have at least two of every physical component to be highly available -  two power supplies, two switches, two cables, two WAN routers, two links, etc. 

However, physical redundancy alone is not enough. The network needs protocols that can utilize the redundancy. If you have two WAN links but don't have a dynamic routing protocol, if one link goes down, the traffic might not be forwarded to the other link. Having multiple switches in the LAN is also not enough on its own. You must use the Spanning-Tree Protocol (STP) to calculate a loop-free topology and recalculate the new paths if one switch goes down.

What is the First Hop?

The first hop in an IP routing path is the first device that receives a packet as it leaves the sender's device. Usually, this is the default gateway, such as a router, which is responsible for forwarding the packet to its destination. It acts as the initial step in the packets' journey through the network. The first hop decides the next step based on its routing table.

However, the IPv4 protocol does not utilize redundancy by default. IPv4 focuses on delivering packets from a source to a destination but does not include built-in mechanisms for ensuring redundancy. IP hosts rely on a single configuration with one default gateway IP address that never changes. For example, a Windows host has the following IP configuration:

C:\Users\User>ipconfig
Windows IP Configuration
Ethernet adapter Ethernet 2:
   Connection-specific DNS Suffix  . :
   IPv4 Address. . . . . . . . . . . : 10.1.1.150
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 10.1.1.1

The host's default gateway is 10.1.1.1, which will never change until an administrator reconfigures it. This is pretty straightforward if there is only one router (10.1.1.1) in the local LAN. But what if there are two routers in the LAN? Router 1 has 10.1.1.2, and router 2 has 10.1.1.3, as shown in the diagram below. 

This raises an interesting question: Which IP address should a host use for the default gateway?

• If the server uses 10.1.1.2 (R1) as a default gateway, what happens when R1 goes down? Well, you can guess that the server loses connectivity to the outside world. 
• If the server uses 10.1.1.3 (R2) as a default gateway, it is the same story - it loses connectivity to the outside world if router R2 goes down. 

The following diagram visualizes what happens if the server uses R2 as the default gateway and R2 goes down.

You can see that having redundancy alone is not enough to have a resilient network. Even though there are two local routers in the LAN, the server cannot take advantage of the redundancy. That is where the first hop redundancy protocols come into play.

First-Hop Redundancy Protocols (FHRP)

A default gateway is the local router hosts use to send traffic outside their connected network. If that gateway fails, hosts lose connectivity to the outside world. Hence, the availability of the default gateway is critically important for hosts to stay connected. However, all operating systems, such as Windows, MacOS, and Linux, allow only one default gateway address to be configured on a host. Therefore, it is the network's responsibility to ensure that this default gateway address is highly available. 

First Hop Redundancy Protocol (FHRP) is a protocol that ensures high availability for hosts' default gateway. It allows multiple routers to appear as a single gateway address to hosts, as shown in the diagram below.

FHRP protocols work by creating a group of routers that collectively act as a single virtual gateway for devices in the network. These routers share a virtual IP address (VIP) and a virtual MAC address. One router in the group is chosen as the active router, which handles all traffic sent to the virtual gateway. Another router is designated as the standby router, ready to take over if the active router fails.

The routers in the group communicate with each other using periodic messages to share status updates. If the active router stops sending these messages, the standby router detects the failure and takes over the role of the active router. This failover process is seamless for network devices, so they continue to communicate without reconfiguring their default gateway or noticing any interruption.