What is HSRP preemption, and what happens without it when a higher-priority router comes online?

Without preemption, a router that becomes active stays active even if a higher-priority router later joins. With preemption enabled (standby preempt), the higher-priority router will force a new HSRP (Hot Standby Router Protocol) election and take over.

* How preemption decides reclaiming active. *

Without preemption (default behavior):

1. R1 (priority 150) is down for maintenance
2. R2 (priority 100) becomes active — it's the only router
3. R1 comes back online with priority 150
4. R2 remains active — R1 becomes standby despite having higher priority
5. R1 will only become active if R2 fails

With preemption enabled on R1:

1. Same scenario — R2 is currently active
2. R1 comes online with priority 150 and has standby preempt configured
3. R1 forces a new election and takes over as active
4. R2 transitions to standby

Preemption rules:

• Only works based on priority — a higher priority triggers preemption
• A router with equal priority but higher IP (Internet Protocol) will NOT preempt (priority must be strictly higher)
• Both routers should have preemption enabled for predictable behavior
• Preemption is disabled by default — you must explicitly enable it

When to use preemption:

• You have a preferred primary router (e.g., the one with the faster WAN uplink)
• You want the primary to automatically resume the active role after recovery
• Without preemption, you'd need to manually fail back by shutting down the current active

Tip: A common scenario: "R1 has priority 150 but R2 is active. Why?" — Answer: Preemption is not enabled, and R2 became active first.

Go deeper:

• Virtual Router Redundancy Protocol (Wikipedia) — preemption behaviour (a higher-priority router pre-empting the master), the open-standard analogue.