Cisco’s classic FHRP: one virtual IP + virtual MAC 0000.0c07.acXX, one Active + one Standby per group. UDP 1985 to 224.0.0.2 (v1, groups 0–255) / 224.0.0.102 (v2, groups 0–4095). Hello 3 / hold 10.

interface vlan 10
 standby 10 ip 10.10.10.254
 standby 10 priority 110
 standby 10 preempt
 standby 10 track 1 decrement 20      ! lose the uplink, lose the crown
track 1 interface Gi1/0/1 line-protocol

• Without preempt the failed owner never takes back control after recovery.
• Tracking is what keeps clients off a gateway that has lost its own route.
• Load sharing: run two groups with opposite priorities (gateway 254 → R1, 253 → R2).
• Verify: show standby brief. Traps: mismatched group ID, mismatched timers, wrong version, VLAN mismatch.