Catalyst 2950 & 802.1Q Trunking

Discussion in 'Cisco' started by mark, Jun 22, 2004.

  1. mark

    mark Guest


    it says:

    "Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a
    Point-to-Point Protocol. However, some internetworking devices might forward DTP
    frames improperly, which could cause misconfigurations.

    To avoid this, you should configure interfaces connected to devices that do not
    support DTP to not forward DTP frames, that is, to turn off DTP. "

    OK then.... if a 2950 switch port is a trunk, but DTP is turned off... is its
    data transmission the same as any other port? In other words, can it communicate
    with a Westell 2200 DSL modem that does not support trunking?

    mark, Jun 22, 2004
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  2. Program ended abnormally on 22/06/2004 14:19, Due to a catastrophic
    What would be the point?

    The DSL modem would be on the native vlan of the trunk and the other vlans would
    not be able to talk to it. You're better off turning trunking off.
    Francois Labreque, Jun 23, 2004
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  3. mark

    mark Guest

    The point of this exercise is to share a DSL line among more than one VLAN while
    not allowing the PC's on different VLANs to see each other.
    Well....... actually.... that's not quite true.
    "By default, a trunk port sends traffic to and receives traffic from all VLANs"

    The ONLY way for a port (the DSL line) to be shared by more than one VLAN on a
    catalyst 2950 is if it's a trunk.

    My question is, what other implications does turning trunking on have on the
    neighboring device attached to it? And if you turn DTP off, will it work with a
    device that does not support trunking at all?
    mark, Jun 23, 2004
  4. Program ended abnormally on 23/06/2004 05:34, Due to a catastrophic
    You can't - at least not with vlans. What you are looking for is "protected ports"

    Configure all your PC ports to be protected and the DSL modem port unprotected
    and away you go.
    Yes it is. You are misinterpreting the statement.
    Yes by default a trunk port will send and receive traffic for all vlans, but the
    switch will expect all traffic coming in that port to be tagged with an 802.1Q
    vlan ID. Packets without a tag will be deemed to be on the "native vlan" and
    only ports on the switch that are on that same vlan will be able to see those

    Packets going out that trunk port in vlans other than the native vlan will also
    have an 802.1Q tag and I have no idea what your DSL modem will do with those...
    but don't rely on it working.
    Answered above.
    Francois Labreque, Jun 23, 2004
  5. mark

    mark Guest

    Thanks for your reply.

    I see what you're saying.... or to quote chapter and verse:
    ...... An IEEE 802.1Q trunk port supports simultaneous tagged and untagged
    traffic. An 802.1Q trunk port is assigned a default Port VLAN ID (PVID), and all
    untagged traffic travels on the port default PVID. All untagged traffic and
    tagged traffic with a NULL VLAN ID are assumed to belong to the port default
    PVID. A packet with a VLAN ID equal to the outgoing port default PVID is sent
    untagged. All other traffic is sent with a VLAN tag.

    Private ports are not possible, because that would prevent the PC's within each
    group from seeing each other.

    So I guess the 2950 can't do what I want it to do after all.... final
    mark, Jun 24, 2004
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