weird question: dot1q and hub

thcollicutt

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Jan 25, 2008, 10:22:27 AM1/25/08

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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?

Trendkill

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Jan 25, 2008, 10:51:31 AM1/25/08

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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.

Tosh

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Jan 26, 2008, 3:35:25 AM1/26/08

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> 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,

wayne

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Jan 26, 2008, 12:24:29 PM1/26/08

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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.

Thrill5

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Jan 28, 2008, 12:22:04 AM1/28/08

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"wayne" <wa...@nospam.com> wrote in message
news:479B6CC...@nospam.com...

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.

Tosh

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Jan 28, 2008, 1:18:10 AM1/28/08

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> 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.

Tilman Schmidt

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Jan 29, 2008, 10:05:30 AM1/29/08

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Thrill5 schrieb:

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/...