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  Topic 3
•   VTP basics
•   Components
•   Frames and advertisements
•   Domains and revision numbers
•   VTP operations
•   VTP pruning
•   VTP issues
                 VTP basics
• VLAN Trunking Protocol
• Sends VLAN information to other switches in
  the network
• Makes VLAN configuration changes on the
  switch with the VTP server role
• VTP advertisements distribute and sync VLAN
  changes to other switches
• Layer 2 protocol, Cisco® proprietary protocol
               VTP benefits
• Less work! Do it once on the VTP server and let
  it propagate
• Easy VLAN management on switches
  – Adding, deleting and renaming VLANs
• Configurations are consistent and have less
• Dynamic reporting of new VLANs
• Dynamic trunk configuration when new VLANs
  are added
                     VTP components
• VTP domain (the scope)
   – The switches that accept advertisements and are updated by VTP
• VTP advertisements
   – Messages that distribute and synchronise VLAN configurations over
     trunk links
• VTP switch modes
   – VTP Server:
       • Make VLAN changes on the VTP server, VLAN list for domain stored in
   – VTP client:
       • Receives VLAN information from VTP server advertisements, VLAN list
         stored in RAM (lost when powered off)
   – VTP transparent:
       • Relays VTP advertisements to VTP clients and VTP servers
       • Does not accept VLAN information
       • VLANs can be configured manually and are local to the switch
                  VTP frame
• VTP header and message inserted into data
  field of an Ethernet frame
• Frame is tagged as 802.1q frame
  – EtherType and tag control field inserted behind
    source MAC address field
              VTP advertisements
• Summary advertisements
  – Sent every five mins by client and server switches to
    advertise their current revision number
  – Sent after a configuration change
• Subset advertisement
  – Sent if a change in the VLAN configuration is made
  – Sent in response to an advertisement request – may be
    multiple messages
• Advertisement request
  – Sent to a VTP server to request new information
  – Sent when:
     •   When VTP domain name changes
     •   When an advertisement with a higher revision number is received
     •   Subset advertisement is missed
     •   Switch is reset
                VTP frame details
• Summary advertisement contains
     •   VTP domain name
     •   Updater identity and timestamp
     •   MD5 digest (for integrity)
     •   Configuration revision number
• Subset advertisement contains
     •   VTP domain name
     •   Configuration revision number
     •   VLAN ID
     •   VLAN name
     •   VLAN type
     •   VLAN status
     •   Additional information
                    VTP domains
• Large networks are divided into smaller VTP management
• A VTP domain is one or more connected switches with the
  same VTP domain name (case sensitive)
• A switch can be a member of only one VTP domain at a time
• VTP advertisements are only accepted from switches in the
  same VTP domain (dropped if different domain)
• Each switch (server and client) in the domain sends
  advertisements out each trunk port to a reserved multicast
  address (01-00-0C-CC-CC-CC) every five minutes
   – Advertisements are received by neighbouring switches,
     which update their VTP and VLAN configurations as
   – Currency of information is controlled by the configuration
     revision number
   Configuration revision number
• Determines which advertisement has the most
  recent information
• 32 bit number in the VTP header of summary
• Default is zero, each time a VLAN is added,
  removed or renamed the number is incremented
• Configuration revision number reset to zero if:
  – A VTP domain name change
  – Changing a switch mode to transparent
  – Switch is reset
Default VTP configuration settings
•   VTP version = 1
•   VTP domain name = null
•   VTP mode = Server
•   Config revision = 0
•   VLANs = 1 (referring to VLAN 1)
•   Version 2 supports token ring messages
•   VTP password is used for the integrity check
    – If used the same password must be configured on all
      switches in the domain
                      VTP operations
• Configure VTP domain name on VTP server (must be done before adding
• Configure new VLANs
    – Configuration revision number will increment with each change
• Bring up trunk links, VTP only works over dot1q, ISL and LANE LAN emulation
  trunk links
    – VTP Server sends summary advertisement to connected switches
    – Switches update the VTP domain name from NULL (the default)
    – Summary advertisement arrives at neighbour switch with a higher revision
        • Neighbour switches sends advertisement request
    – VTP server responds with a 2nd summary advertisement followed by subset
        • Switches update VLAN configurations and configuration revision number
• VTP transparent mode switches forward advertisements along trunk links
  but make no updates to their VLAN information
• VTP servers and clients continue sending summary advertisements (5 mins)
                 VTP pruning
• Normally VTP messages are sent to every switch in
  the domain down all trunk links
• VTP pruning prevents unnecessary flooding of VTP
• If VLANs have no switchports on downstream
  switches, VTP messages are not sent
• VLANs can be marked as pruning-eligible and VTP
  reconfigures the trunk links to prune VTP
  messages on the egress port
• Pruning is disabled by default
  – enable pruning on only one VTP server switch in the
• Incompatible VTP versions
   – VTP versions 1 and 2 are incompatible
• VTP password issues
   – No password set or password misconfigured
   – Password must be set manually on each switch
• Incorrect VTP domain name
   – Set the VTP domain name on one VTP server switch to
     avoid incorrect configuration
   – Case sensitive
• All switches set to VTP client mode
   – Configure a VTP server and a backup VTP server switch
                              VTP issues
• Inserting a switch (VTP server or a client) with higher VTP
  configuration revision number
– Switches will update VLANs to those of the new switch, loss of
– Add back the required VLANs and delete the unwanted VLANs on a VTP
– Change VTP mode to transparent or change the domain name to reset the
  revision number
• Inserting a switch with a different domain name between switches
– Trunk links may drop, VTP domain mismatch and no VTP updates will
– Trunk link must be set to ON on both sides of link
        • Trunks set to desirable or any other settings will not form a trunk if the domains
          are different
– All switches in the domain must be adjacent and connected by trunks
        • Messages will not be relayed over different VTP domains (dropped)
– Change the VTP domain name to solve
•   VTP basics
•   Components
•   Frames and advertisements
•   Domains and revision numbers
•   VTP operations
•   VTP pruning
•   VTP issues

  Topic 3

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