PlantUML DiagramHSRP State Machine
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Two HSRP groups configured on the same router, with different roles. Group 1: this router is active (priority 110, preempt). Group 2: this router is standby (priority 100, no preempt). By running two groups with priorities favoring different routers, you get active/active load balancing across both paths.
The state progression is deterministic. Initial is the starting point. Learn and Listen are transitional. Speak means the router is participating in election. Standby is the backup. Active is the winner. Transitions between Active and Standby happen when priorities change or failures occur.
Tracking ties HSRP priority to the state of another interface. If a tracked interface goes down, the HSRP priority decrements by the configured amount. If it drops below the standby's priority, HSRP preempts — the better-connected router takes over.
! Configure HSRP with trackingR1(config)# interface gigabitEthernet0/1R1(config-if)# standby 1 ip 10.10.10.1R1(config-if)# standby 1 priority 110R1(config-if)# standby 1 preemptR1(config-if)# standby 1 track gigabitEthernet0/0 decrement 20R1(config-if)# exit! Verify tracking stateR1# show standby | include Track
The `track gi0/0 decrement 20` command reduces the priority by 20 if Gi0/0 goes down. If R1's priority was 110 and drops to 90, the standby (priority 100) preempts. This ensures that a router with a broken WAN uplink doesn't stay active — the client gateway should follow the working path.
Without tracking, HSRP only reacts to the interface that owns the virtual IP. If that interface is up but the router's WAN uplink is down, the router stays active and silently drops traffic destined for the internet. Tracking fixes this — it makes HSRP aware of the router's actual reachability.
| HSRP version | Virtual MAC prefix | Notes |
|---|---|---|
| HSRPv1 | 0000.0C07.ACxx | xx = group number in hex (00-FF, groups 0-255) |
| HSRPv2 | 0000.0C9F.Fxxx | xxx = group number (000-FFF, groups 0-4095) |
| HSRPv2 group 1 | 0000.0C9F.F001 | Example |
The virtual MAC is derived from the group number. Group 1 under HSRPv1 uses `0000.0C07.AC01`; under HSRPv2 it's `0000.0C9F.F001`. If you capture traffic on a subnet running HSRP, you'll see ARP replies with these MACs — a definitive way to identify HSRP on the wire.
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