interface fastEthernet 0/1 switchport mode access switchport access vlan 10 exit interface fastEthernet 0/2 switchport mode access switchport access vlan 20 F0/3 → VLAN 30. F0/4 → VLAN 10. And so on.
Alex learned the hard lesson: deleting a VLAN from one switch doesn’t delete it from others. But it does break connectivity for any access port still assigned to that missing VLAN on that switch.
switchport trunk allowed vlan add 30 Ping. Success. All three switches now carried all three VLANs. One last test. PC4 (Accounting, S2) → PC6 (Accounting, S3). Works. PC2 (Engineering, S1) → PC5 (Engineering, S2). Works. 3.3.12 packet tracer - vlan configuration.pka
Alex did this for all three switches, matching the color-coded diagram in Packet Tracer. Red for Accounting. Blue for Engineering. Green for Staff.
He walked off. The switches hummed.
But Alex made a classic mistake. On S2, Alex forgot to allow VLAN 30 on the trunk to S3. Suddenly, Staff PCs on S2 couldn’t talk to Staff PCs on S3.
“Right,” Alex groaned. “The switch doesn’t know which PC belongs to which VLAN. It’s like a hotel front desk that doesn’t ask for your room key.” Back on S1: Alex learned the hard lesson: deleting a VLAN
“Allowed VLANs,” Alex muttered. “Add 30.”
interface gigabitEthernet 0/1 switchport mode trunk switchport trunk allowed vlan 10,20,30 Same on S2’s G0/1 and S3’s G0/2. Success
Request timed out. Request timed out.
Professor Lasky walked by, glanced at the screen, and said only: “Three VLANs today. Three hundred in the real world. The logic doesn’t change.”