Do VLANs have to be defined for trunking?

Discussion in 'Cisco' started by, Jan 15, 2005.

  1. Guest

    I have a 3508 that the only ports in use are defined as trunks.

    Do I have to explicitly create vlan entries in the vlan database in
    order for the trunks to
    pass tagged vlan packets?


    joe mcguckin
    , Jan 15, 2005
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  2. BradReeseCom Guest

    BradReeseCom, Jan 15, 2005
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  3. In article <>,
    <> wrote:
    :I have a 3508 that the only ports in use are defined as trunks.

    :Do I have to explicitly create vlan entries in the vlan database in
    :eek:rder for the trunks to
    :pass tagged vlan packets?

    If you have the ports defined as trunks, then you must define
    the VLANs that you want passed, as you don't usually want all
    trunks to be able to pass all VLANs.

    If, though, you have a situation in which all the ports are to be
    considered equivilent for trunking purposes, and you just need the
    switch to perform switching functions on the packets [e.g, consult the
    MAC table to figure out where to pass them on to] then you may be able
    to define the ports as access ports and just have the switch pass on
    the tagged traffic as if it didn't know anything about tags.

    You should check the allowed packet size for the interfaces. Generally
    speaking, sometimes if trunking is not enabled, the maximum packet size
    permitted on input is the usual ethernet maximum packet size, whereas
    when trunking is enabled, slightly larger packets are allowed in order
    to accomedate the VLAN tag (or possibly two layers of VLAN tag.) And on
    some devices, the larger packets will be permitted through if trunking
    is not enabled, but the packet will be counted in the interface
    statistics as being a "baby giant" instead of a regular maximum-sized
    packet as it would if the port were defined as a trunk.

    If you do use this mechanism of defining the trunks as access
    ports and just having the packets passed on as raw ethernet data,
    then you run into a possible problem with per-VLAN spanning trees.
    With the ports defined as access ports, the switch is going to
    believe that -all- instances of the same MAC address should be delivered
    the same way, which might not be correct: a particular MAC in
    one VLAN could have a different path than the same MAC in a different
    VLAN. This supposed that you have the possibility of having
    the same MAC in different VLANs (e.g., you have policy-map VLAN
    classification, or sone of your devices are doing protocol-based
    classification, or you have QoS-related classification into
    different VLANs), and supposed that there may be VLAN-dependant
    topology loops further down.
    "Mathematics? I speak it like a native." -- Spike Milligan
    Walter Roberson, Jan 16, 2005
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