Objectives
| # | Objective |
|---|
| 1 | Create matching VLANs 10 and 20 on both SW1 and SW2 (manually — VTP not yet active) |
| 2 | Configure the inter-switch link as an 802.1Q trunk — allow VLANs 10 and 20 |
| 3 | Verify the trunk with show interfaces trunk and understand DTP negotiation |
| 4 | Configure VTP — set SW1 as server, SW2 as client, and observe automatic VLAN propagation |
| 5 | Assign access ports on both switches and confirm cross-switch intra-VLAN ping works |
| 6 | Open 802.1Q Frame Inspector — step through how a tagged frame crosses the trunk |
| 7 | Knowledge check — trunking, DTP and VTP concepts |
Key Concepts
| Concept | What it means |
|---|
| 802.1Q trunk | A link that carries frames from multiple VLANs, each tagged with a 4-byte VLAN ID header. Access ports carry one VLAN untagged; trunk ports carry many. |
| 802.1Q tag | A 4-byte field inserted into the Ethernet frame between the source MAC and EtherType. Contains the TPID (0x8100) and 12-bit VLAN ID. |
| Native VLAN | The one VLAN whose frames cross a trunk untagged. Both ends must agree or frames get misdelivered. Default is VLAN 1. |
| DTP | Dynamic Trunking Protocol — Cisco proprietary. Allows two switches to automatically negotiate trunk mode. Best practice: disable it and configure trunks manually. |
| VTP | VLAN Trunking Protocol — propagates VLAN database from a VTP Server to VTP Clients across trunk links. Removes the need to manually create VLANs on every switch. |
| VTP domain | A group of switches sharing the same VTP domain name. VTP updates only propagate within the same domain. |
Topology — Two Switches, Trunk Link
| Device | Port | Role | VLAN / Mode |
|---|
| SW1 Gi0/0 | → PC1 | Access | VLAN 10 |
| SW1 Gi0/1 | → PC2 | Access | VLAN 20 |
| SW1 Gi0/3 | → SW2 Gi0/3 | Trunk | VLANs 10,20 |
| SW2 Gi0/0 | → PC3 | Access | VLAN 10 |
| SW2 Gi0/1 | → PC4 | Access | VLAN 20 |
| SW2 Gi0/3 | → SW1 Gi0/3 | Trunk | VLANs 10,20 |
PC Addressing
| Device | IP | VLAN | Switch |
|---|
| PC1 | 192.168.10.10/24 | VLAN 10 | SW1 Gi0/0 |
| PC2 | 192.168.20.10/24 | VLAN 20 | SW1 Gi0/1 |
| PC3 | 192.168.10.11/24 | VLAN 10 | SW2 Gi0/0 |
| PC4 | 192.168.20.11/24 | VLAN 20 | SW2 Gi0/1 |
Key Commands
| Command | Purpose |
|---|
| switchport trunk encapsulation dot1q | Set trunk encapsulation to 802.1Q (needed before mode trunk on some platforms) |
| switchport mode trunk | Force port into trunk mode (disables DTP negotiation) |
| switchport trunk allowed vlan 10,20 | Allow only VLANs 10 and 20 on the trunk |
| switchport nonegotiate | Disable DTP — stops sending DTP frames |
| show interfaces trunk | Show trunk links, allowed VLANs, native VLAN |
| show interfaces Gi0/3 switchport | Verify trunk mode and encapsulation on a specific port |
| vtp mode server / client / transparent | Set VTP operating mode |
| vtp domain <name> | Set VTP domain name (must match on all switches) |
| show vtp status | Show VTP mode, domain, revision number |
Once the trunk is up and you can ping across switches, click 🏷️ Frame Inspector to step through exactly how the 802.1Q tag is added, carried, and stripped.
Tab to complete · ↑↓ for history · Switch between SW1, SW2, and PC tabs using the toolbar.