What is a First Hop Redundancy Protocol (FHRP)

The gateway is the first stop or “hop” your packets take when leaving your local network to reach the internet. If it ever goes down, you instantly lose access to anything outside your local network. FHRP solves that problem by letting a backup router take over the gateway.

How do I use FHRP?

FHRP is a category of protocols, similar to how “soft drink” refers to a whole group of beverages. The three commonly used FHRP protocols are HSRP, VRRP, and GLBP (which also adds load balancing).

How does it work?

Let’s use VRRP as the example.

VRRP uses a virtual IP and MAC address. Clients use this as their gateway. The master router forwards traffic with this virtual address. If the master router goes down, the backup takes over the same virtual IP and MAC address and becomes the master router.

Picture a company with two employees but only one published phone number. The phone number is the virtual IP and MAC. Let’s say the first employee answers all calls. If they become unavailable, the second employee can immediately take over the same phone number. Callers still dial the same number and they never notice the switch.

Let’s take it a step further. If you have hundreds or thousands of callers, you wouldn’t send every call to one employee. You’d spread the calls across the whole team. With more employees, you can handle more calls. GLBP works the same way. It distributes traffic across multiple gateways. As a side effect, you also get redundancy because several routers are active and forwarding traffic, which is an improvement over having a single gateway with only a backup.

What is a virtual IP address (VIP)?

Each router still has its own unique IP and MAC address so they can talk to each other and exchange status messages. The VIP is real address, but it isn’t locked to one router. The VIP will move to whichever gateway is active. If Router A fails, Router B will take over the VIP. From the client’s point of view, they keep using the same gateway address.


Example:
Router A: 192.168.0.2
Router B: 192.168.0.3
VIP: 192.168.0.1