[lug] Re: switch

[Applegate,Kris]

The easiest way to think of it is as follows:

Anytime you need to connect a NIC to another device, you need to switch
the pairs that do send and receive. Obviously when the NIC is sending
signals down the copper pair the other ends need to be listening rather
than talking down the same pair. Switches and hubs both are set to
listen on the correct pairs by default. That is why you can use a
standard straight cable to connect a NIC to a hub/switch. The uplink
port is set to talk on those pairs, which is why it wont connect to a
NIC. The uplink port is only used for cascading (hooking together) sets
of switches/hubs. However, if you use a cable that switches the send and
receive pairs (crossover cable) the uplink port then is listening on the
correct pairs. It is all a little confusing at first, but once you get
it, it sticks. 


--------------------------------------
Kristopher Applegate




-----Original Message-----
From: Hugh Brown [mailto:hugh at vecna.com]
Sent: Tuesday, June 12, 2001 10:23 AM
To: lug at lug.boulder.co.us
Subject: Re: [lug] Re: switch


So the moral of the story is that some uplink ports require crossover
cables and some don't.  If one doesn't work try the other.

Hugh


"Tkil"
> 
> 
> the best way i've found to remember this sort of thing is to think of
> the situation with serial cables.  
> 
> you had male plugs on the computer ("data terminal equipment" or DTEs
> in RS-232 parlance), and female plugs on the modem or printer ("data
> communication devices" or DCEs).  doing the "usual" connection takes a
> straight-through cable; but, if you want to connect two computers
> together, you need a "null modem" (== twisted-pair ethernet crossover
> cable) and a gender bender (since you need female plugs on both ends
> of the cable)
> 
> with TP ethernet boxes, you have "NIC ports" and "hub/switch ports".
> the usual connection is straight through, and everything works.
> however, when you want to connect one "hub/switch port" to another
> "hub/switch port", you need to use a crossover cable.
> 
> however, most hubs and switches were built with this need in mind.
> some have dedicated "uplink ports" which are really the same "sex" as
> "NIC ports", and are ready to be connected with a *straight* cable to
> a "hub/switch port" on another hub or switch; others have a port that
> is normally a "hub/switch port" but can be toggled to behave as an
> uplink ("nic") port.
> 
> does this help?  did i get it right?
> 
> t.
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