What is the difference between up and down link on LAN switch?

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Hello all -
My new desktop LAN switch have 1 up-port and 5 down-ports. If I connect my
cable-Modem to the up-port and my PC to the down-port it doesn't work. If I
connect both to the down-ports it works? So my question is: What is the
difference between up and down plugs? Only crossing the cable lines? The
same difference as in the patch cable crossed and not crossed version?
Or is a form of priority or special routing implemented on the up-port?

BTW: How does the switch knows where the packets have to go?

regards -
Henry

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Re: What is the difference between up and down link on LAN switch?
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The only difference is which way the pairs are wired.  Also, for
gigabit switches and some newer 100 Mbit switches there is no
difference at all; they work either way.

So yes, the difference is *exactly* the same as using a crossover
cable.

This is the modern equivalent of the old DTE/DCE confusion in the
RS-232 standard; it makes sense when you know what is a DTE (terminal)
and what is a DCE (modem), but gets really confusing when people try
to be clever about it (I have an embedded system wired as a DCE, for
example.)  Routers, which includes cable modems, are functionally
hosts, equalling computers (DTEs), and hubs/switches are the
equivalent of a DCE.  However, since you need to be able to connect
two hubs/switches together you have to switch one of the ports to
"uplink" mode, which makes that port wired as a host.

    -hpa




Re: What is the difference between up and down link on LAN switch?
H. Peter Anvin schrieb in Nachricht ...
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Thank you H. Peter for your lengthly explanation!
How many switches can be cascaded? What is the time delay between ports of a
switch?

I know the problem with DCE-DTE variations on RS-232 (I worked approx. 20
years with embedded systems).

My problem at the moment is, that my two PCs don't work with the Motorola
cable-modem at the SAME time. Only after resetting the modem the first
come-up PC gets the internet connection. Motorola says, up two 32 PCs can be
running with the integrated DHCP server. My provider KabelBW says,
connecting a couple of PCs is allowed and should work.

cu -
Henry


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Re: What is the difference between up and down link on LAN switch?

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As many as you want.  There is a limit to the number of hubs
that can be connected in a single path, but switches create an
isolated collision domain on every port, so there is no limit.

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Not much.  It depends on the switch, but I would expect it to
be not much more than the packet length plus a few hundred
microseconds of overhead.

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Re: What is the difference between up and down link on LAN switch?

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Well, there are some limits that might bite...

Normal spanning tree only supports up to 7 hops.  You can usually
adjust this max slightly, and non-stp participant bridges and hubs
don't count as hops, but it's still a limit of some importance in a
large network.

Other bridge-layer protocols might impose similar limitations.  The
only other one I recall that propogated beyond a single link is GARP,
and I don't remember any hop limitations; no doubt it runs atop the
spanning tree to avoid the issue.


There is also the scalability issue of having many hosts in a single
broadcast domain.  Depending on the protocols in use, some smaller
hosts (print servers, network cameras, etc) can be measurably affected
by the overhead of receiving lots of broadcast "noise".  

The broadcast problem is slightly worse if you make heavy use of IP
multicast; in large installations with lots of multicasting there are
little traffic "leaks" in all the various elements from nics to
routers that add a bit of extra work all around.

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Grant Taylor
Embedded Linux Consultant
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Re: What is the difference between up and down link on LAN switch?
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OK Grant -
I think your're working on really great networks of hundreds of stations or
more. Hopefully I don't have so much children chatting all the day ;-)

So long -
Henry

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Re: What is the difference between up and down link on LAN switch?
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You need NAT, not DHCP.  You should get a 'cable router', which is a little
switch with integrated NAT.  Otherwise I'd expect the behavior you're seeing.

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Ben Jackson
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Re: What is the difference between up and down link on LAN switch?
Ben Jackson schrieb in Nachricht ...
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be
seeing.
Hm. I will see what the provider says. Is it possible to use a switch with
integrated NAT and leave the cabel-modem think there ist only one PC
connected? How is the "switch with NAT" called, so I can buy one?

cu -
Henry

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Re: What is the difference between up and down link on LAN switch?
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Usually described as a cable/broadband router. Linksys do the BEFSR41
(There's also the WRT54G with wireless). Netgear have one as well (RP614).


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Re: What is the difference between up and down link on LAN switch?
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It's just crossing the TX/RX pair.  An 'uplink' port is like a network
card port and can be plugged into the same things.  The other 'down' ports
have their pairs swapped so they can be connected with straight-through
cables to network cards.

For ambiguous devices like a cable modem just try both and see how it's
wired.

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There are such things on managed switches (called trunk ports) but it
has nothing to do with the physical wiring.

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Initially it doesn't.  It broadcasts packets (to all ports) destined for
MACs that it doesn't recognize.  When the target responds the switch
sees what port it was on and remembers it for a while.  If you power cycle
your switch and then watch the activity lights you can see it happen.

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Ben Jackson
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Re: What is the difference between up and down link on LAN switch?

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Yes.


Yes.


No.  Not for your average consumer-grade switch.

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They have a cache that keeps track of which MAC addresses are
sending packets in which ports.  The switch assumes that
packets _to_ MAC address aa:bb:cc:dd:ee:ff should be sent out
the port where packets _from_ MAC address aa:bb:cc:dd:ee:ff
come in.

If the switch needs to send a packet to an MAC address from
which it hasn't seen any packets, then it just sends it out all
the ports.

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Re: What is the difference between up and down link on LAN switch?
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OK!

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Re: What is the difference between up and down link on LAN switch?

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1 MDIX 5 MDI

first one is crossover

Pozdrawiam.
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Re: What is the difference between up and down link on LAN switch?
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Äh? What means your first sentence?

cu -
Henry

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Re: What is the difference between up and down link on LAN switch?

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http://www.firebrick.co.uk/manuals/network.html

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Re: What is the difference between up and down link on LAN switch?
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OK RuSH! Thanks and dobredanje *lol

- Henry

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