5.4Redundancy & high availability

Guaranteeing that a system "does not go down" is not about eliminating failure entirely, but about designing a configuration that minimizes business impact even when a failure occurs. This section covers redundancy techniques for each network layer—gateway, link, route, and device—along with the availability calculations that quantitatively back their effect, aiming to build the judgment needed to decide "which redundant configuration achieves a given availability target."

5.4.1Gateway redundancy (VRRP/HSRP)

• A host recognizes its default gateway as a fixed single IP address and MAC address, so if that single router fails, the host loses reachability to the gateway. VRRP (Virtual Router Redundancy Protocol, a standard) and HSRP (Hot Standby Router Protocol, Cisco proprietary) let multiple physical routers share a virtual IP address and virtual MAC address, presenting hosts with what always appears to be a single virtual router.
• Normally, the master (active) router actually handles traffic to the virtual IP, while the backup (standby) router waits and monitors the master's liveness (via advertisements). If the master stops responding, the backup automatically promotes itself and takes over handling the virtual IP/MAC, enabling failover without any configuration change on the host side.