HSRP
Hot Standby Router Protocol — Cisco's proprietary gateway failover protocol. Two routers share virtual IP + MAC. Predecessor to open-standard VRRP. RFC 2281 (1998, informational).
Active router responds to ARP for virtual IP. Standby takes over on failure (3 sec default). Group ID selects master via priority. Track features: priority lowered if uplink interface fails → secondary becomes master. v2 supports IPv6 + IPv4. Cisco-exclusive — if you run multi-vendor network, use VRRP instead. Classic CCNA curriculum theme. Modern data center: ECMP + BGP CLOS makes HSRP/VRRP unnecessary in fabric design, but access layer (schools, offices) still uses it.