In the HSRP topology with preemption, why does R1 (priority 150) become the active router while R2 (priority 100) becomes standby?

Because R1 has a higher HSRP (Hot Standby Router Protocol) priority (150 > 100) and preemption is enabled, R1 wins the election and forwards all traffic. R2 stands by, monitoring R1's health via Hello messages.

Walking through the election in a typical topology:

                    Virtual Router
                IP: 172.16.10.1/24
              MAC: 0000.0c07.ac01
                    
    R1 (Active)              R2 (Standby)
    Priority: 150            Priority: 100
    172.16.10.2/24           172.16.10.3/24
         |                        |
    ─────┴────────────────────────┴─────
              Hosts use 172.16.10.1
              as default gateway

What the hosts see:

• Default gateway: 172.16.10.1 (the virtual IP (Internet Protocol) — not R1's or R2's real IP)
• ARP (Address Resolution Protocol) for 172.16.10.1 resolves to 0000.0c07.ac01 (the virtual MAC (Media Access Control))
• All frames go to this virtual MAC → R1 (as active) processes them

What happens if R1 fails:

1. R2 stops receiving Hellos from R1
2. After the hold timer (10 sec), R2 becomes active
3. R2 now responds to the virtual MAC 0000.0c07.ac01
4. Hosts still send to 0000.0c07.ac01 → R2 now processes the traffic
5. Zero configuration changes on any host

When R1 recovers (with preemption):

• R1 sends Hellos with priority 150
• R1 preempts R2 and reclaims the active role
• R2 returns to standby

Go deeper:

• Hot Standby Router Protocol (Wikipedia) — highest-priority election and virtual-IP/MAC ownership by the active router.