weird question: dot1q and hub

Discussion in 'Cisco' started by thcollicutt, Jan 25, 2008.

  1. thcollicutt

    thcollicutt Guest

    I know this is a weird question, but someone asked me and I thought
    I'd throw it out. I have no intention of implimenting it to see, but
    it started me thinking.

    A client wanted to know if he could run a dot1q trunk through a hub.
    Basically two cisco switches, trunked together but somewhere between
    them a hub.

    Will the hub break the trunk. I know you'll get a lot of stuff
    happening at the hub, but will the two switches actually trunk if the
    hub is there?
     
    thcollicutt, Jan 25, 2008
    #1
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  2. thcollicutt

    Trendkill Guest

    Re: weird question: dot1q and hub

    On Jan 25, 10:22 am, thcollicutt <> wrote:
    > I know this is a weird question, but someone asked me and I thought
    > I'd throw it out. I have no intention of implimenting it to see, but
    > it started me thinking.
    >
    > A client wanted to know if he could run a dot1q trunk through a hub.
    > Basically two cisco switches, trunked together but somewhere between
    > them a hub.
    >
    > Will the hub break the trunk. I know you'll get a lot of stuff
    > happening at the hub, but will the two switches actually trunk if the
    > hub is there?


    I would think this would work, however this is obviously not a smart
    thing to do. All ports (on the hub) would see that traffic, but as
    long as the endpoints are tagging the frames properly, I don't see why
    this would not work. The hub should not be 'manipulating' the frames
    at all, so I'd be interesting to see the results. If you do try, let
    me know how you fare.
     
    Trendkill, Jan 25, 2008
    #2
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  3. thcollicutt

    Tosh Guest

    > Will the hub break the trunk. I know you'll get a lot of stuff
    > happening at the hub, but will the two switches actually trunk if the
    > hub is there?


    From a hub point of view the only difference from a standard frame and a .1q
    frame is the payload size, wich in .1q frames could be 4 bytes greater, as
    ong as the hub doesn't discard these trames your setup is going to work,
    although not a good practice.
    Bye,
     
    Tosh, Jan 26, 2008
    #3
  4. thcollicutt

    wayne Guest

    thcollicutt wrote:
    > I know this is a weird question, but someone asked me and I thought
    > I'd throw it out. I have no intention of implimenting it to see, but
    > it started me thinking.
    >
    > A client wanted to know if he could run a dot1q trunk through a hub.
    > Basically two cisco switches, trunked together but somewhere between
    > them a hub.
    >
    > Will the hub break the trunk. I know you'll get a lot of stuff
    > happening at the hub, but will the two switches actually trunk if the
    > hub is there?
    >


    Obviously bad practice, but yes the hub (or an unmanaged switch) will
    allow tagged packets to "pass through". Any other hosts connected to
    the hub/unmanaged switch will be belong to the native vlan of the trunk.
     
    wayne, Jan 26, 2008
    #4
  5. thcollicutt

    Thrill5 Guest

    "wayne" <> wrote in message
    news:...
    > thcollicutt wrote:
    >> I know this is a weird question, but someone asked me and I thought
    >> I'd throw it out. I have no intention of implimenting it to see, but
    >> it started me thinking.
    >>
    >> A client wanted to know if he could run a dot1q trunk through a hub.
    >> Basically two cisco switches, trunked together but somewhere between
    >> them a hub.
    >>
    >> Will the hub break the trunk. I know you'll get a lot of stuff
    >> happening at the hub, but will the two switches actually trunk if the
    >> hub is there?
    >>

    >
    > Obviously bad practice, but yes the hub (or an unmanaged switch) will
    > allow tagged packets to "pass through". Any other hosts connected to the
    > hub/unmanaged switch will be belong to the native vlan of the trunk.


    I doubt the hub will allow the dot1q packets because the extra 4 bytes on
    the packet will be seen as an "overrun" and the hub should discard the
    packet. You can try it and it might work, but as the previous poster said
    this would be very bad practice.
     
    Thrill5, Jan 28, 2008
    #5
  6. thcollicutt

    Tosh Guest

    > I doubt the hub will allow the dot1q packets because the extra 4 bytes on
    > the packet will be seen as an "overrun" and the hub should discard the
    > packet. You can try it and it might work, but as the previous poster said
    > this would be very bad practice.

    Said that we all agree on the bad practice, generally todays ethernet
    interfaces are built to manage longer frames than the original 1518 bytes
    limit, never tried with hubs, but I successfuly tried to transparently pass
    trunks through media converters, unmanaged switches and even low end
    wireless bridges.
    Bye,
    Tosh.
     
    Tosh, Jan 28, 2008
    #6
  7. Thrill5 schrieb:
    > "wayne" <> wrote in message
    > news:...
    >> thcollicutt wrote:
    >>>
    >>> A client wanted to know if he could run a dot1q trunk through a hub.
    >>> Basically two cisco switches, trunked together but somewhere between
    >>> them a hub.
    >>>
    >>> Will the hub break the trunk. I know you'll get a lot of stuff
    >>> happening at the hub, but will the two switches actually trunk if the
    >>> hub is there?

    >>
    >> Obviously bad practice, but yes the hub (or an unmanaged switch) will
    >> allow tagged packets to "pass through". Any other hosts connected to the
    >> hub/unmanaged switch will be belong to the native vlan of the trunk.

    >
    > I doubt the hub will allow the dot1q packets because the extra 4 bytes on
    > the packet will be seen as an "overrun" and the hub should discard the
    > packet.


    Hubs aren't sufficiently intelligent for detecting overruns and
    discarding packets. They are just multiport repeaters which blindly
    pass on the signals between their ports. The only thing a hub is
    able to detect is a collision.

    Now if the "hub" was really an unmanaged switch that would be a
    different story ...

    --
    Please excuse my bad English/German/French/Greek/Cantonese/Klingon/...
     
    Tilman Schmidt, Jan 29, 2008
    #7
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