VLAN Trunking Protocol

December 20, 2019 - Reading time: 3 minutes

VTP is a protocol that lets your switches share the same VLAN list, so you don’t have to create VLANs on every switch one by one. Switches that are in the same VTP domain (a name you choose) exchange this VLAN list across a trunk link (a link that carries traffic for many VLANs at once). On a server switch you add or rename VLANs; client switches automatically copy and stay in sync with that list. A transparent switch simply passes the VTP messages along but keeps its own local VLAN list separate. VTP itself doesn’t carry user traffic—it only keeps the VLAN database consistent across the network. In this lab, you’ll set one switch as a server, one as a client, build a trunk, and confirm the client learns the server’s VLANs.

Lab Topology & Assumptions

• Devices: SW0 (server) — SW1 (client).
  
  Link: 
            SW0                 SW1
           Gi0/1  <— trunk —>  Gi0/1.

• Platform/IOS family: Classic Cisco IOS (Catalyst IOS style).

• VTP domain name: lab_network (chosen to match Cisco’s documented outputs).

• VTP version: 2

Configurations WITH Reasons

SW0 — VTP server

hostname SW0
vtp version 2
vtp domain lab_network
vtp mode server
!
vlan 10
 name SALES
vlan 20
 name ENG

• vtp version 2 enables VTPv2; it is backward-compatible with v1 and allows token-ring VLANs (rare today).

• vtp domain lab_network puts SW0 in the same sharing group.

• vtp mode server makes SW0 the source of VLAN database changes.

• VLANs 10 and 20 are created here so clients can learn them automatically.

SW1 — VTP client

hostname SW1
vtp version 2
vtp domain lab_network
vtp mode client

• Same domain and version ensures SW1 joins the same database.

• vtp mode client makes SW1 learn VLANs from SW0 and blocks local creation.

Interface configuration — single block per device

SW0 — trunk toward SW1

interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 description *** Trunk to SW1 Gi0/1 ***

SW1 — trunk toward SW0

interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 description *** Trunk to SW0 Gi0/1 ***

Trunking lets both VLAN traffic and VTP advertisements travel between switches. Without the trunk, SW1 would not learn VLANs from SW0.

Verification 

Switch# show vtp status
VTP Version capable             : 1 to 2
VTP version running             : 2
VTP Domain Name                 : lab_network
VTP Pruning Mode                : Disabled
VTP V2 Mode                     : Enabled
VTP Traps Generation            : Disabled
MD5 digest                      : 0xC2 0x5B 0x20 0xF5 0xC5 0x43 0x4E 0x4F
Configuration last modified by 0.0.0.0 at 3-1-93 00:03:50
Local updater ID is 0.0.0.0 (no valid interface found)

VTP is running version 2, in domain lab_network, and the switch is in Server mode.

Verify counters (server or client)

Switch# show vtp counters
VTP statistics:
Summary advertisements received    : 7
Subset advertisements received     : 5
Request advertisements received    : 0
Summary advertisements transmitted : 15
Subset advertisements transmitted  : 4
Request advertisements transmitted : 2
Number of config revision errors   : 0
Number of config digest errors     : 0
Number of V1 summary errors        : 0

The device is sending/receiving VTP advertisements on trunks.

Common Mistakes (symptom → cause → fix)

• Domain mismatch → client shows empty VLAN list → check vtp domain lab_network.

• Trunk missing → counters stay zero → enable switchport mode trunk.

• Wrong version (1 vs 2) → VLANs not syncing → force vtp version 2 on all.

• Trying to create VLAN on client → error → client mode blocks local VLAN changes; use server.

• Revision number mismatch after reloading → old switch with higher revision overwrote VLANs → reset revision (change domain, then set back).

User Challenge

1. Add VLAN 30 named HR on SW0 and confirm it appears on SW1 with show vlan brief.
   
   

2. Prediction Challenge:

   • If you delete VLAN 20 on SW0 (server), what will happen on SW1 (client)?

   • If SW1 were in Transparent mode instead, what would happen?