One gateway is a single point of failure
A PC is set with one default gateway. Give the LAN two routers for safety, and the PCs still only use one of them. If that router (or its uplink) fails, the PCs are cut off, even though the other router is fine.
The answer: a virtual gateway
With a first-hop redundancy protocol (FHRP), two or more routers share a virtual IP address and a virtual MAC address. The PCs use the virtual IP as their gateway. One router is in charge and answers for it; the others watch. If it fails, another takes over the same virtual IP and MAC, and the PCs never notice.
| HSRP | VRRP | GLBP | |
|---|---|---|---|
| Standard | Cisco | Open (RFC 5798) | Cisco |
| Router in charge | Active (next in line: Standby) | Master (others: Backup) | AVG; all routers forward |
| Default priority | 100 | 100 | 100 |
| Pre-emption | Off by default | On by default | Off by default |
| Virtual MAC | 0000.0c07.acXX (v1), 0000.0c9f.fXXX (v2) | 0000.5e00.01XX | 0007.b400.XXYY |
| Load balancing | Only with several groups | Only with several groups | Built in |
XX is the group number in hexadecimal. The exam expects you to know these differences and to configure HSRP and VRRP.
Remember: PCs point to the virtual IP, never to a router's own address. Highest priority wins (then highest IP), and pre-emption decides if a better router takes back over when it returns.