CCNP SWITCH: VLAN Trunking Protocol (VTP)

 

  • VTP (VLAN Trunking Protocol) is a layer 2 protocol that maintains VLAN configurations, managing addition, deletions and changes of VLANs within a VTP domain.
  • A VTP domain is one switch or multiple connected switches (via a trunk link) that share the same VTP configuration.
  • Only 1 VTP Domain supported per switch.
  • VTP domain is “null” by default
  • The default VTP mode of a switch is SERVER, but Cisco switches do NOT propagate VTP information out trunk interfaces until a management domain name is specified or learned.
  • VTP has 3 modes: Server, Client and Transparent


     

VTP Modes

 

Mode

Features

Server

Creates, modifies and deleted VLANS

Sends and forwards advertisements to other switches

Synchronises VLAN configuration with latest information received from other switches in management domain

Saves VLAN configuration in NVRAM

Client

Cannot create, change or delete VLANs (via the CLI)

Forwards advertisements to other switches (does not initiate/send advertisement)

Synchronises VLAN configuration with latest information received from other switches in the management domain

DOES NOT save VLAN configuration in NVRAM (unlike switch operating in server mode)

Transparent

Maintains its own VLAN database (can create, modify and delete VLANs BUT on the local switch ONLY)

Will neither advertise or accept VLAN database information from other switches

Forwards VTP advertisements received from other switches

 

VTP Message Types

 

Summary Advertisements: communicates with adjacent switches the VTP domain name and configuration revision number. The VTP server increments the configuration revision number when VLANs are added, deleted or modified and issues the Summary Advertisements

Subset Advertisements: follows the Summary Advertisement and contains a list of VLAN information

 

VTP Updates

 

  • Every time a VTP server modifies its VLAN information the configuration number increments by 1, it then sends VTP advertisement with the revision number.
  • A VTP switch will synchronise an update if the revision number is higher than the current database.
  • Cisco switches transmit VTP summary advertisements using a layer 2 multicast frame every 300 seconds (5 minutes) or anytime a change to the VLAN database occurs.
  • VTP Updates are sent across the management VLAN (using VLAN 1)
  • VTP propagates configuration changes only across trunk links

 

VTP Versions

 

  • 3 versions (1,2 and 3)
  • Versions are NOT interoperable

 

Version 2: Token ring support, Unrecognised TLV, Version independent transparent mode, Consistency checks

Version 3: Support for extended VLANs (1025 – 4094), Support for creation and advertising Private VLANs, Improved server authentication, Protection of incorrect database accidentally coonected to VTP domain, Configurable on a per-port basis

 

VTP Pruning

 

  • Prevents unnecessary VLAN broadcasts or multicast traffic flooding a trunk link
  • VTP Pruning blocks data frames for a VLAN on the trunk link if no ports are associated to the VLAN on the access switch. When an access port is later assigned to a VLAN that was originally pruned, the access switch will communicate with the VTP server instructing it that it now requires the frames for that VLAN. The VTP server will now start forwarding frames for that VLAN.
  • VTP Pruning is disabled by default
  • Can only be enabled on the VTP Server

     

 

Commands

 

Configure the switch as a VTP Server

switch (config)# vtp mode server

Configure domain name

switch (config)# vtp domain LAB

Change VTP version

switch (config)# vtp version 2

Specify VTP Password

switch (config)# vtp password CISCO123

Optional (VTP Pruning)

switch (config)# vtp pruning

switch (config)# interface gigabitethernet 0/1

switch (config)# switchport trunk pruning

Show commands (VTP status)

switch# show vtp status

 


 

 

Summary

 

There are some dangers running VTP. Connecting a switch that is already pre-configured with VLANs and VTP configured into a network could override the existing VLAN database by adding or worse removing existing VLANs, thus causing a major outage. Advise would be to configure switches in transparent mode “vtp mode transparent” and manually add VLANS as needed. If VTP is used ensure proper procedures are followed when connecting additional switches to the network.

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