Networking Q - Suse

This is a discussion on Networking Q - Suse ; Greetings, I have an ethernet best described as being in the shape of a "Y". Picture the wireless router/gateway at the bottom of the letter and a few cpus at each remaining fork of the letter on separate switches. One ...

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Thread: Networking Q

  1. Networking Q

    Greetings, I have an ethernet best described as being in the shape of a "Y".
    Picture the wireless router/gateway at the bottom of the letter and a few
    cpus at each remaining fork of the letter on separate switches. One gateway,
    two switches.

    Do I need the entire subnet to be gigabit speed to obtain that speed between
    2 cpus that are both on the same switch? In other words, do I need one
    gigabit switch where I require the speed or must I revamp the whole
    enchilada network?

    Hoping for the cheaper answer and thanks for reading...

  2. Re: Networking Q

    Pete Puma wrote:

    > Greetings, I have an ethernet best described as being in the shape of a
    > "Y". Picture the wireless router/gateway at the bottom of the letter and a
    > few cpus at each remaining fork of the letter on separate switches. One
    > gateway, two switches.
    >
    > Do I need the entire subnet to be gigabit speed to obtain that speed
    > between 2 cpus that are both on the same switch? In other words, do I need
    > one gigabit switch where I require the speed or must I revamp the whole
    > enchilada network?
    >
    > Hoping for the cheaper answer and thanks for reading...


    It is my understanding that the difference between a "switch" and a "hub" is
    that each port on a switch is independent of the other ports. This means
    that if you have a device that can operate at gigabit speed, the
    communication between that device and a switch will use gigabit speed
    regardless of any other devices connected to the switch. Therefore, if you
    have two gigabit devices connected to the same switch, the communication
    between those devices through that switch will also be at gigabit speed,
    again regardless of any other devices connected.

    A "hub", OTOH, operates with all ports simultaneously. This means that
    every device connected to a hub is limited to the speed of the slowest.

    The best part, assuming you actually have switches and not hubs, is this is
    the cheaper answer, just like you wanted!


  3. Re: Networking Q

    Larry Bristol wrote:

    > Pete Puma wrote:
    >
    >> Greetings, I have an ethernet best described as being in the shape of a
    >> "Y". Picture the wireless router/gateway at the bottom of the letter and a
    >> few cpus at each remaining fork of the letter on separate switches. One
    >> gateway, two switches.
    >>
    >> Do I need the entire subnet to be gigabit speed to obtain that speed
    >> between 2 cpus that are both on the same switch? In other words, do I need
    >> one gigabit switch where I require the speed or must I revamp the whole
    >> enchilada network?
    >>
    >> Hoping for the cheaper answer and thanks for reading...

    >
    > It is my understanding that the difference between a "switch" and a "hub"
    > is
    > that each port on a switch is independent of the other ports. This means
    > that if you have a device that can operate at gigabit speed, the
    > communication between that device and a switch will use gigabit speed
    > regardless of any other devices connected to the switch. Therefore, if you
    > have two gigabit devices connected to the same switch, the communication
    > between those devices through that switch will also be at gigabit speed,
    > again regardless of any other devices connected.
    >
    > A "hub", OTOH, operates with all ports simultaneously. This means that
    > every device connected to a hub is limited to the speed of the slowest.
    >
    > The best part, assuming you actually have switches and not hubs, is this is
    > the cheaper answer, just like you wanted!



    Thanks, Larry.
    At worst, I guess I'll add them one at a time until these files start moving
    faster.

  4. Re: Networking Q

    On Wed, 5 Nov 2008, Pete Puma wrote:-

    >Greetings, I have an ethernet best described as being in the shape of a "Y".
    >Picture the wireless router/gateway at the bottom of the letter and a few
    >cpus at each remaining fork of the letter on separate switches. One gateway,
    >two switches.
    >
    >Do I need the entire subnet to be gigabit speed to obtain that speed between
    >2 cpus that are both on the same switch?


    Larry has already answered you, but I'll throw in my 2p worth. No, you
    don't. The ports on a switch are independent of each other and you can
    have mixed 10Mbit, 100Mbit and 1Gbit passing data to each other, each of
    them unaware that the other systems speed may not be the same as their
    own, and data is sent out through only one port.

    This is different to a hub where the speed is (usually? always?)
    10Mbit/s and data coming in on one port is sent out via every other
    port. This has both the advantage and disadvantage that you can snoop on
    traffic passing through the hub to see what's there just by running
    wireshark and putting an ethernet card into promiscuous mode.

    >In other words, do I need one
    >gigabit switch where I require the speed or must I revamp the whole
    >enchilada network?
    >
    >Hoping for the cheaper answer and thanks for reading...


    Larry's already given you the answer but, as a real world example, I'll
    describe my arrangement. I have a 4-port DSL router. Attached to that is
    one system and another 100Mb switch. Attached to the 100Mb switch are
    six other systems and a connection to a gigabit switch. Attached to the
    gigabit switch there are presently four systems with 100Mb ethernet
    ports and two with gigabit ports. Between the gigabit systems, I can
    routinely transfer (very) large files and achieve transfers exceeding
    60MBytes/s while also having 10+MByte/s transfers taking place between
    the slower connections.


    Regards,
    David Bolt

    --
    Team Acorn: http://www.distributed.net/ OGR-NG @ ~100Mnodes RC5-72 @ ~1Mkeys/s
    SUSE 10.1 32 | | openSUSE 10.3 32b | openSUSE 11.0 32b
    | openSUSE 10.2 64b | openSUSE 10.3 64b | openSUSE 11.0 64b
    RISC OS 3.6 | TOS 4.02 | openSUSE 10.3 PPC | RISC OS 3.11

  5. Re: Networking Q

    On Wed, 05 Nov 2008 06:53:09 -0500, Pete Puma typed this message:

    > Greetings, I have an ethernet best described as being in the shape of a
    > "Y". Picture the wireless router/gateway at the bottom of the letter and
    > a few cpus at each remaining fork of the letter on separate switches.
    > One gateway, two switches.
    >
    > Do I need the entire subnet to be gigabit speed to obtain that speed
    > between 2 cpus that are both on the same switch? In other words, do I
    > need one gigabit switch where I require the speed or must I revamp the
    > whole enchilada network?
    >
    > Hoping for the cheaper answer and thanks for reading...


    Yes but that depends on the Gigabit switch. Most switches are auto-
    sensing but some switch auto-sense the entire network to the lowest
    connected port speed while others connect individual port speeds.

    My guess is that most SOHO router/switches operate all ports at the speed
    of the slowest connected port but if the router/switch has indicators to
    show individual port connection speeds that's your answer.

  6. Re: Networking Q

    David Bolt wrote:

    > Larry has already answered you, but I'll throw in my 2p worth. No, you
    > don't. The ports on a switch are independent of each other and you can
    > have mixed 10Mbit, 100Mbit and 1Gbit passing data to each other, each of
    > them unaware that the other systems speed may not be the same as their
    > own, and data is sent out through only one port.


    I find this very interesting, so let me pass this thought: I was figuring
    that because the gateway (Linksys wireless) was assigning DHCP to each node,
    that every switch (no hubs) had to stop by the gateway and get the IP addy
    from the gateway--and once there, it's in non-gigabit territory.
    How often it would have to ask the gateway for address data as it sends
    packets, I would have no idea, if at all.
    Or is it more of a, once found, IP sender deals as directly as possible with
    IP receiver?

  7. Re: Networking Q

    Hi Pete

    Two different things here. The router/gateway/linksys thing hands out an
    IP address to the PC client. How long the address stays with the client
    is a function of "lease time", available IP's and how many "other"
    devices get connected. Generally speaking in most small/home installs
    the same machine gets the same IP each time. Leases get refreshed but
    are often set for hours or days.

    The next part is knowing what machine is what IP. This has nothing to do
    with DHCP. (well you can setup DHCPD to do permanent maps between MAC
    and IP addresses but that isnt relevant here) When one machine wants to
    connect to a machine in the local network it broadcasts the question
    "Who has IP xxxxxx?" down its wire. If the nearest switch already knows
    the answer it sends back the information "IP address xxxxx is on MAC
    address XX:XX:XX:XX:XX:XX". This information is retained in the PC
    clients ARP table (Address Resolution Protocol) which is also subject to
    a timeout (I dont know what it is but am sure it is more than 30 seconds
    at least..)

    (It is conceivable that the nearest switch could also work by
    broadcasting that it is the "owner" of that IP and L2 route data
    accordingly. I doubt this is the case but the purists out there would
    complain if I didnt mention it!)

    If the nearest switch doesnt know the answer it broadcasts to all of its
    outputs asking the same question.. Switches of course retain their own
    ARP table.

    As you can see these are pretty long time periods unlikely to make much
    of a dent on network throughout.

    The way I describe layer 2 switch action is that for normal IP traffic
    (as against ARP and broadcast stuff) the switch "remembers" which IP/MAC
    is attached to which physical port and shunts data from an input to the
    known output without it appearing on a port where the destination isn't!
    Theoretically you can get full transfer rate from (say) port 1 to port 3
    and port 2 to port 4 without them affecting each other. Of course a
    (say) 16 port 100MB/sec switch is rarely made to allow full rate
    combinations on all ports (ie adding up to 800Mbit/sec HDX or
    1600Mbit/sec FDX) concurrently, but the thought is there! A hub (layer 1
    switch) can only move data as fast as one port goes as all input data
    appears on all outputs.

    Hope this doesn't confuse!

    Cheers Bob

    Pete Puma wrote:
    > I find this very interesting, so let me pass this thought: I was figuring
    > that because the gateway (Linksys wireless) was assigning DHCP to each node,
    > that every switch (no hubs) had to stop by the gateway and get the IP addy
    > from the gateway--and once there, it's in non-gigabit territory.
    > How often it would have to ask the gateway for address data as it sends
    > packets, I would have no idea, if at all.
    > Or is it more of a, once found, IP sender deals as directly as possible with
    > IP receiver?


  8. Re: Networking Q

    On Wed, 5 Nov 2008, Pete Puma wrote:-

    >David Bolt wrote:
    >
    >> Larry has already answered you, but I'll throw in my 2p worth. No, you
    >> don't. The ports on a switch are independent of each other and you can
    >> have mixed 10Mbit, 100Mbit and 1Gbit passing data to each other, each of
    >> them unaware that the other systems speed may not be the same as their
    >> own, and data is sent out through only one port.

    >
    >I find this very interesting, so let me pass this thought: I was figuring
    >that because the gateway (Linksys wireless) was assigning DHCP to each node,
    >that every switch (no hubs) had to stop by the gateway and get the IP addy
    >from the gateway--and once there, it's in non-gigabit territory.


    I don't think so, at least not from looking at what happens on my
    network, but then I'm not a network expert.

    As an example, when my wife's' machine asks for an IP address, initially
    it will ask for the last IP address it was assigned. To do this it
    broadcasts a request for the IP address using a specific format. This
    goes out through the ethernet port and arrives at the ADSL router. As
    this isn't running a DHCP server, it then rebroadcasts that request out
    through its active ports.

    The next switch in the chain does exactly the same, and rebroadcasts the
    request to all the ports. As most of these ports are attached to
    computers, none of which are running a DHCP server, the packets are
    ignored.

    The last port on that switch connects to the gigabit switch, which also
    rebroadcasts the request. Again, most of the ports are connected to
    machines that don't run a DHCP server. The one that is, doesn't have a
    gigabit ethernet port so it sends back the data at 100Mb speeds.

    >How often it would have to ask the gateway for address data as it sends
    >packets, I would have no idea, if at all.


    Now that's quite an interesting question. Using wireshark on one of the
    machines on my network, I set it up to watch only for ethernet broadcast
    packets (destination MAC address of ff:ff:ff:ff:ff:ff) or for DHCP
    requests/reponses, and there seems to be quite a few being sent, some of
    which are send very regularly.

    A large portion of these broadcast packets are ARP packets, where one
    machine asks who has another IP address. When I disconnect and then
    reconnect a machine, I see the initial DHCP request and, after the
    address is given/accepted, a broadcast ARP packet telling the rest of
    the network that a particular IP address can be found at a specific MAC
    address.

    The majority of the packets, broadcast every 31 seconds, is from one of
    the machines saying that its cups-pdf queue is available and idle. It's
    almost like it wants something to do :-)

    >Or is it more of a, once found, IP sender deals as directly as possible with
    >IP receiver?


    That's certainly the way things appear to happen. The switches maintain
    a list of what IP and MAC addresses are accessible through which port.
    When the switch receives a packets destined for specific IP address, or
    MAC address, it looks up which port to send the packet through before
    sending it on its way.


    Regards,
    David Bolt

    --
    Team Acorn: http://www.distributed.net/ OGR-NG @ ~100Mnodes RC5-72 @ ~1Mkeys/s
    SUSE 10.1 32 | | openSUSE 10.3 32b | openSUSE 11.0 32b
    | openSUSE 10.2 64b | openSUSE 10.3 64b | openSUSE 11.0 64b
    RISC OS 3.6 | TOS 4.02 | openSUSE 10.3 PPC | RISC OS 3.11

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