1. This forum section is a read-only archive which contains old newsgroup posts. If you wish to post a query, please do so in one of our main forum sections (here). This way you will get a faster, better response from the members on Motherboard Point.

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

Discussion in 'Embedded' started by Henry Kiefer, Oct 11, 2004.

  1. Henry Kiefer

    Henry Kiefer Guest

    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

    --

    <Schau auch mal auf meine Homepage www.ehydra.dyndns.info>
    <u.a. Versand von Wasserflohzuchtansatz, Wasserpflanzen/-schnecken,
    brasilianischer Sauerklee, Natron zum Backen/Baden, u.a.>
    <Alternativ über http://people.freenet.de/algenkocher>
     
    Henry Kiefer, Oct 11, 2004
    #1
    1. Advertisements

  2. Followup to: <>
    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
     
    H. Peter Anvin, Oct 11, 2004
    #2
    1. Advertisements

  3. Henry Kiefer

    Henry Kiefer Guest

    H. Peter Anvin schrieb in Nachricht ...
    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


    --

    <Schau auch mal auf meine Homepage www.ehydra.dyndns.info>
    <u.a. Versand von Wasserflohzuchtansatz, Wasserpflanzen/-schnecken,
    brasilianischer Sauerklee, Natron zum Backen/Baden, u.a.>
    <Alternativ über http://people.freenet.de/algenkocher>
     
    Henry Kiefer, Oct 11, 2004
    #3
  4. Henry Kiefer

    Ben Jackson Guest

    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.
    There are such things on managed switches (called trunk ports) but it
    has nothing to do with the physical wiring.
    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.
     
    Ben Jackson, Oct 11, 2004
    #4
  5. No. Not for your average consumer-grade switch.
    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.
     
    Grant Edwards, Oct 11, 2004
    #5
  6. 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.
    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.
     
    Grant Edwards, Oct 11, 2004
    #6
  7. Henry Kiefer

    Ben Jackson Guest

    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.
     
    Ben Jackson, Oct 12, 2004
    #7
  8. Henry Kiefer

    RusH Guest

    1 MDIX 5 MDI

    first one is crossover

    Pozdrawiam.
     
    RusH, Oct 12, 2004
    #8
  9. Henry Kiefer

    Grant Taylor Guest

    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.
     
    Grant Taylor, Oct 12, 2004
    #9
  10. Henry Kiefer

    Henry Kiefer Guest

    Hello all -
    Äh? What means your first sentence?

    cu -
    Henry

    --

    <Schau auch mal auf meine Homepage www.ehydra.dyndns.info>
    <u.a. Versand von Wasserflohzuchtansatz, Wasserpflanzen/-schnecken,
    brasilianischer Sauerklee, Natron zum Backen/Baden, u.a.>
    <Alternativ über http://people.freenet.de/algenkocher>
     
    Henry Kiefer, Oct 12, 2004
    #10
  11. Henry Kiefer

    Henry Kiefer Guest

    They have a cache that keeps track of which MAC addresses are
    OK!

    --

    <Schau auch mal auf meine Homepage www.ehydra.dyndns.info>
    <u.a. Versand von Wasserflohzuchtansatz, Wasserpflanzen/-schnecken,
    brasilianischer Sauerklee, Natron zum Backen/Baden, u.a.>
    <Alternativ über http://people.freenet.de/algenkocher>
     
    Henry Kiefer, Oct 12, 2004
    #11
  12. Henry Kiefer

    Henry Kiefer Guest

    Ben Jackson schrieb in Nachricht ...
     
    Henry Kiefer, Oct 12, 2004
    #12
  13. Henry Kiefer

    Henry Kiefer Guest

    The broadcast problem is slightly worse if you make heavy use of IP
    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

    --

    <Schau auch mal auf meine Homepage www.ehydra.dyndns.info>
    <u.a. Versand von Wasserflohzuchtansatz, Wasserpflanzen/-schnecken,
    brasilianischer Sauerklee, Natron zum Backen/Baden, u.a.>
    <Alternativ über http://people.freenet.de/algenkocher>
     
    Henry Kiefer, Oct 12, 2004
    #13
  14. Henry Kiefer

    Paul Black Guest

    Usually described as a cable/broadband router. Linksys do the BEFSR41
    (There's also the WRT54G with wireless). Netgear have one as well (RP614).
     
    Paul Black, Oct 12, 2004
    #14
  15. Henry Kiefer

    RusH Guest

    RusH, Oct 13, 2004
    #15
  16. Henry Kiefer

    Henry Kiefer Guest

    Henry Kiefer, Oct 14, 2004
    #16
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.