few questions on L2 switching - Network

This is a discussion on few questions on L2 switching - Network ; Hello, All! I often come across on these terms in technical manuals related to switches, L3 routers and so on. And I could not get the exact, clear meaning of it so far. Could someone clarify for me? 1) line-rate ...

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  1. few questions on L2 switching

    Hello, All!

    I often come across on these terms in technical manuals related to switches,
    L3 routers and so on. And I could not get the exact, clear meaning of it so
    far. Could someone clarify for me?

    1) line-rate switching (is it same as wire speed switching?)
    2) L2 aging of entries mechanism (is it related to purging of entries in
    address table?)

    Thanks in advance.

    With best regards, Roman Mashak. E-mail: mrv@tusur.ru



  2. Re: few questions on L2 switching

    On 2006-01-05, Roman Mashak wrote:

    > I often come across on these terms in technical manuals related to switches,
    > L3 routers and so on. And I could not get the exact, clear meaning of it so
    > far. Could someone clarify for me?


    First, do beware the Sales Pitch, indeed - nothing like lab tests to confirm
    whether or not your multi-layer switch is going to perform well.

    > 1) line-rate switching (is it same as wire speed switching?)


    Line-rate and wire-speed are generally interchangeable marketing buzz words. I
    don't like "wire speed" because there is no such thing - a copper wire can
    drive a 56Kbps modem or 100Mbps Ethernet - it's up to the signaling used
    on the cable and the maximum bandwidth that can be supported by the
    electrical signaling method on the cable.

    In general, a vendor is trying to indicate with this statement that they can
    keep up with the speed of the network port - so, they can switch 1Gbps if
    they offer a 1Gbps port. There's a concept of "blocking" or "non-blocking"
    switching here that enters as well - the switch backplane, or "fabric" as
    it's sometimes called, needs to support a rate equal to or higher than
    the rate of all the ports combined, otherwise, not all data could pass at
    "line rate". Simple example would be a switch with an 8Gbps backplane, that
    supported 16 1Gbps ports - not all ports could pass 1Gbps at the same time
    in that theoretical configuration.

    Another thing to consider is to ask how the switch performs at various
    frame sizes. Usually, lower-end switches experience performance
    degradation the smaller the frame size. Why? A switch is really a special
    purpose computer that moves data really quickly using fast hardware for
    data storage. The smaller the data pieces that need to move, the more times
    per second the computer needs to do something. For example, a switch
    processing a small Ethernet frame stream @ 128 bytes has to move data
    11 more times than a large Ethernet frame stream @ 1518 bytes. ( Math :
    1518 / 128 = 11 )

    > 2) L2 aging of entries mechanism (is it related to purging of entries in
    > address table?)


    Yes - this is related to purging unused L2 MAC address entries from the
    switch's port-to-MAC-address table. Cisco calls this the "CAM aging timer".
    It can be desirable in large-scale networks for this timer to be user-
    settable to accommodate for some Spanning-Tree (STP) and host fail-over
    considerations.

    Hope this helps some.

    -DMFH

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