Things to know about VTP

Filed in CCIE, Switching by on November 21, 2007 2 Comments

Some notes about VTP (VLAN Trunking Protocol):

  • Cisco switches running Cisco IOS store VTP and VLAN information in a separate database stored in Flash, in file called vlan.dat.
  • Cisco switches running CatOS store VTP and VLAN information in the main switch configuration file, stored in NVRAM.
  • VTP information is only transmitted over trunk ports.
  • A VTP client does not need a VTP domain name to be configured to learn VLANs. If the domain name is left blank it will configure itself with the domain name learned from the first advertisement from the VTP server.
  • A VTP advertisement contains the VLANs configured in the domain that are allowed on the trunk, the VTP domain name, and the VTP configuration revision number.
  • If the revision number received in a VTP advertisement is greater than the current stored revision number, the switch will accept the new configuration and overwrite its existing vlan.dat file with the newly received VLAN configuration.
  • If a VTP client or server is added to the network with the same VTP domain name configured and with a higher revision number, all other switches in the VTP domain will overwrite their VTP configurations with that of the newly added switch.
  • It is possible for a VTP client to overwrite the VLAN configuration of the network by having a higher configuration revision number when added to the network. A switch will listen to VTP advertisements from both VTP clients and VTP servers.
  • Manually adding and deleting VLANs in configuration mode can only be done on the VTP Server.
  • Only normal range VLANs (1-1005) are available in VTP and stored in the vlan.dat file.
  • VLANs 1 and 1002-1005 are reserved VLANs and cannot be advertised, deleted, or pruned by VTP.
  • The vlan.dat file is stored in Flash, not NVRAM.
  • To configure extended range VLANs (1006-4094) the switch must be in VTP Transparent mode (this mode forwards along VTP advertisements, but does not paticipate in VTP), or VTP off (switch does not forward VTP or participate).
  • When normal range VLANs are configured in VTP Transparent mode, the normal range VLANs are stored in both vlan.dat (in Flash) and in the running configuration. If vlan.dat and the running configuration differ during boot up, the information in vlan.dat is prefered.
  • Because normal range VLANs are stored in vlan.dat in Flash, erasing a switch’s running configuration and reloading will result in the normal range VLANs still being present. To permanently erase the normal range VLAN configuration the vlan.dat file must be deleted from Flash.
    Exec command ‘delete flash:vlan.dat
  • When extended range VLANs are configured they are only stored in running configuration.
  • All citations to vlan.dat in these notes refer to Cisco IOS. Therefore, Cisco CatOS does not use a vlan.dat file and stores all VLAN information in its configuration file.

 

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About the Author ()

Brad Hedlund (CCIE Emeritus #5530) is an Engineering Architect in the CTO office of VMware’s Networking and Security Business Unit (NSBU). Brad’s background in data center networking begins in the mid-1990s with a variety of experience in roles such as IT customer, value added reseller, and vendor, including Cisco and Dell. Brad also writes at the VMware corporate networking virtualization blog at blogs.vmware.com/networkvirtualization

Comments (2)

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  1. John says:

    Brad,

    Thanks for this excellent post about VTP. Easy reading and it cleared up a few questions I had about it.

    John

  2. Hi Brad, appreciate you to share out the proves for the portion quoted in the following statement. I believe you must have something to prove that since you italic it intentionally. I did some testings and seems it is not the case. Thanks. :-)

    A VTP advertisement contains the VLANs configured in the domain that are “allowed on the trunk”…

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