VTP Trunk question

mikearamamikearama Member Posts: 749
in CCNA & CCENT
Having a hard time with this little tidbit...

By default, all vlans can traverse a trunk, right? Don't we have to "disallow" the vlans that we don't want to trunk?

I ask, because an exam prep I have, by David Hucaby (CCIE), asks this question:

Which vlans are allowed on a new trunk between a new switch and an existing VTP domain?

I went with,
All Vlans are allowed by default.

The correct answer was:
Each Vlan (or Vlan range) that is specified with the VTP Domain command.

I cannot find, in any of my ccna books, any indication that there even is a VTP Domain command, let alone how to use it. Any help?

Much obliged,
Mike
There are only 10 kinds of people... those who understand binary, and those that don't.

CCIE Studies: Written passed: Jan 21/12 Lab Prep: Hours reading: 385. Hours labbing: 110

Taking a time-out to add the CCVP. Capitalizing on a current IPT pilot project.
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Comments

  • darkchronosdarkchronos Member Posts: 4 ■□□□□□□□□□
    i believe the vtp domain command your looking for is in the vlan database
    Switch(vlan)#vtp domain domainname
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  • YossarianYossarian Member Posts: 14 ■□□□□□□□□□
    If you have access to a sim go into global configuration and type vtp ?

    You will see the domain command there.
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  • dtlokeedtlokee Member Posts: 2,378 ■■■■□□□□□□
    It's not the command vtp doman, but what vlans are active in the domain. When the new switch is added it needs to be added to the domain to propigate the vlan configuration from the existing domain. If there are no vlans (other than vlan 1) on the switch it can't forward traffic out any port including trunk ports if the vlan does not exist. If the new switch has vlans that have been created that are not active in the vtp domain the switch in the vtp domain will frop the tagged frames when they arrive.
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  • NetstudentNetstudent Member Posts: 1,693 ■■■□□□□□□□
    I would have answered the same as you. I don't remember reading about allowing VLANS with the vtp domain command. I'v read about pruning and the "switchport trunk allowed vlan remove|add 101-499" command. That Q seems a little out there to me. That seems poorly worded. I found this article though that is interesting.

    "Add a Switch to the VTP Domain
    A recently added switch can cause problems in the network. It can be a switch that was previously used in the lab, and a good VTP domain name was entered. The switch was configured as a VTP client and was connected to the rest of the network. Then, you brought the trunk link up to the rest of the network. In just a few seconds, the whole network can go down.

    If the configuration revision number of the switch that you inserted is higher than the configuration revision number of the VTP domain, it propagates its VLAN database through the VTP domain.

    This occurs whether the switch is a VTP client or a VTP server. A VTP client can erase VLAN information on a VTP server. You can tell this has occurred when many of the ports in your network go into the Inactive state but continue to assign to a nonexistent VLAN.

    Note: Refer to Flash Animation: VTP for a demonstration of this problem.

    Complete these steps in order to avoid this issue when you add a switch to the network:

    Before you connect the new switch to the network, change the VTP mode of the switch to Transparent.

    This resets the Configuration Revision number to zero ('0').

    Connect the switch to the network and configure the appropriate trunk links.

    Configure the VTP attributes:

    Configure the VTP domain name to match the VTP domain name of the network.

    Configure the VTP version and password (if any required).

    Change the VTP mode to Client.

    The Configuration Revision number is still zero ('0'). VLANs start to propagate from the VTP server(s) that exists in the network.

    Verify if all required VLANs are available in the new switch and in the VTP servers of the network.

    If any VLAN is missing, a quick workaround is to add it from one of the VTP servers."

    http://www.cisco.com/en/US/tech/tk389/tk689/technologies_configuration_example09186a00807d5d42.shtml#new_switch
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  • mikearamamikearama Member Posts: 749
    That was some interesting stuff, NS... wouldn't have guessed that a Client could ever overwrite a Server. Neat.

    Chronos... I didn't explain very well. I get the vtp domain command. The answer, though, was "Each Vlan (or Vlan range) that is specified with the VTP Domain command."

    How do you specify a vlan with the vtp domain command?
    Better yet, how would you, on the Server, NOT specify a vlan so that it wasn't propogated to a new client? Is that possibe, to have vlans on the server that are not shared with clients?

    Mike
    There are only 10 kinds of people... those who understand binary, and those that don't.

    CCIE Studies: Written passed: Jan 21/12 Lab Prep: Hours reading: 385. Hours labbing: 110

    Taking a time-out to add the CCVP. Capitalizing on a current IPT pilot project.
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  • dtlokeedtlokee Member Posts: 2,378 ■■■■□□□□□□
    When you create a vlan on the vtp server it is propigated over the "default" vlan so disallowing the vlan would not stop it from being sent to other members in the VTP domain. This is one of the reasons for vtp transparent, if you want a switch to have a different vlan database than other switches.

    You can't create a vlan with the "vtp domain" command, the vtp domain simply sends the vlan database (vlan.dat) to other switches.
    The only easy day was yesterday!
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