VTP Topic Notes

VLAN Trunking Protocol Overview

• Cisco Proprietary
• Method to syncronise VLANs between switches without manual configuration
• Default mode is Server
• Switch may only be in one VTP domain
  • Domain name needs to match on all switches in domain
    • Includes Transparent-Mode switches
• Updates only propagated to VTP domain members
• VLAN information stored in flash in VLAN.DAT file
• Information carried in ISL and Dot1Q trunk links

• Following must be configured the same on all switches:
  1. Domain Name
     • 1 – 32 characters
     • Case sensitive
     • Command
       • (config)#vtp domain <domain-name>
  2. VTP Password (optional)
     • 16byte secret value used in MD5 hash
     • Command:
       • (config)#vtp password <password>

VTP Message Types

1. Summary Advertisements
   • Sent out every 5 minutes
   • Sent when there is a topology change
   • Inform adjacent switches of domain name and configuration revision number
   • Contains:
     • VTP Version
     • Timestamp
     • VTP Domain Name
     • MD5 Hash
     • Configuration Revision number
     • Number of subset advertisements incoming
2. Subset Advertisements
   • Sent out after a configuration change
   • Contains change information of VLAN(s)
3. Advertisement Requests from Clients
   • VTP clients requesting specific VLAN information
   • Required if
     • Switch has been reset
     • VTP domain name has changed
     • Received a Summary Advertisement with a configuration number higher than its own
4. VTP join messages

VTP Modes

VTP Mode: Server

• Default Mode
• Only server can make VLAN changes in domain
  • Create, modify, and delete VLANs
• Specify VTP version number
• Specify pruning
• Advertise VLAN information
• Command:
  • (config)#vtp mode server

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VTP Mode: Client

• Can't create, change or delete VLANs
• Passes on advertisements
• Command:
  • (config)#vtp mode client

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VTP Mode: Transparent

• Doesn't advertise or synchronize its own VLAN configuration
  • Local configuration is manually configured
• Doesnt accept VLAN information from VTP advertisements
• Passes on VTP advertisements in VTPv2
• Configuration Revision Number is set to 0
• Command:
  • (config)#vtp mode transparent

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VTP Mode: Off

• Doesn't participate in VTP
• Can be disabled globally or per-interface
• Same as transparent mode except advertisements aren't forwarded
• Only supported in VTPv3
• Command:
  • Globally:
    • (config)#vtp mode off
  • Per-Interface:
    • (config-if)#no vtp

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VTPv1

• Supports VLANs 1 – 1005 only
• Doesn’t support Private VLANs
• Default version
• Command:
  • (config)#vtp version 1

VTPv2

• Supports VLANs 1-4096
• VTPv1 and v2 are not interoperable
• Supports unknown TLV records (Type-Length-Value)
• VLAN database consistency checks are skipped if change was caused by a VTP message.
• Command:
  • (config)#vtp v2-mode
  • (config)#vtp version 2

VTPv3

• Command:
  • (config)#vtp version 3
• Fixes configuration overwrite problem
  • Higher configuration revision number received
  • Checks revision number and source of information
    • Primary ID is MAC address of Primary Server
    • Only a primary server can make the updates
• Supports the following types of VLANs
  • Extended VLANs (1006 – 4094)
  • Private VLANs
  • MST configuration
• VTP password can now be hidden in configuration
  • Command:
    • (config)#vtp password <password> [hidden | secret]
• VTPv3 adds the following fields to the advertisements:
  • Primary Server ID
  • Instance number
  • Window Size
  • Start Index

VTPv3 Roles:

1. Primary Server
   • Can only have 1 Primary server at a time
   • Only server allowed to make changes to VTP config
   • Switch needs to be promoted to VTP Primary server for either VLANs or MST configuration:
     • Command:
       • #vtp primary-server vlan
       • #vtp primary-server mst
   • Need to confirm password to become Primary Server
     • reduces chance of errors
   • VLANs and MST configuration separated out
   • Primary Server option not written to the running-config
     • If reloaded it will loose primary status
2. Secondary server
   • Default configuration
   • Not allowed to make VTP configuration changes

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VTP Configuration Revision Number

• Sequence number for the VTP database
• Highest number is preferred
• Domain is synchronised when number matches on all switches in domain
• Could cause issues if a switch advertises wrong information and has a higher sequence number
  • Reason why VTPv1/v2 isn’t used in most networks
  • Issues fixed with VTPv3

VTP Pruning

• Reduces unnecessary
  • Broadcasts
  • Unknown Unicasts
  • Unknown Multicasts
• VTP pruning reduces VLANs sent across a trunk
• Only sends VLANs in use or active
• Periodic updates every 5 minutes
• Only supported in Client or Server mode
• Can only be enabled on switches in VTP Server mode
• Command:
  • (config)#vtp pruning
• Only VLANs 2-1001 are eligible to be pruned
• VLANs not in eligible list can’t be pruned
  • Can be configured manually
    • Command:
      • (config-if)#switchport trunk pruning vlan <vlan>

Troubleshooting Commands

• #show vtp status – Verifies VTP name, mode, revision number and other information
• #show vtp counters – Indicates if VTP updates are being sent and recevied by switch
• #show vlan – On client configuration it indicates if VTP updates are being received
• #show interface pruning – Shows which VLANs are being pruned