MAC to IP mapping

Discussion in 'Cisco' started by adam.rothschild@gmail.com, Jan 11, 2006.

  1. Guest

    We have catalyst 3500xl switches running
    c3500xl-c3h2s-mz.120-5.WC13.bin. If I issue a show mac I get which mac
    address of a client PC is attached to which fast ethernet port.
    However, I would like to find out the IP as well. I currently use
    LanSpy to match the mac from the switch command to the ip attached to
    the mac in LanSpy. Is there a more detailed option in the switch to see
    which port has which IP? Any help is appreciated.
    , Jan 11, 2006
    #1
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  2. zulu-1-three Guest

    wrote:
    > We have catalyst 3500xl switches running
    > c3500xl-c3h2s-mz.120-5.WC13.bin. If I issue a show mac I get which mac
    > address of a client PC is attached to which fast ethernet port.
    > However, I would like to find out the IP as well. I currently use
    > LanSpy to match the mac from the switch command to the ip attached to
    > the mac in LanSpy. Is there a more detailed option in the switch to see
    > which port has which IP? Any help is appreciated.


    No. Switches are L2 devices so their world revolves around MAC
    addresses. To resolve a MAC address to an IP address you need to
    reference a routers' ARP table. ARP tables map L2 addresses (MAC) to
    L3 addresses (IP). We do this in PERL with NETSNMP.
    zulu-1-three, Jan 11, 2006
    #2
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  3. papaia Guest

    On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:

    >
    > wrote:
    >> We have catalyst 3500xl switches running
    >> c3500xl-c3h2s-mz.120-5.WC13.bin. If I issue a show mac I get which mac
    >> address of a client PC is attached to which fast ethernet port.
    >> However, I would like to find out the IP as well. I currently use
    >> LanSpy to match the mac from the switch command to the ip attached to
    >> the mac in LanSpy. Is there a more detailed option in the switch to see
    >> which port has which IP? Any help is appreciated.

    >
    > No. Switches are L2 devices so their world revolves around MAC
    > addresses. To resolve a MAC address to an IP address you need to
    > reference a routers' ARP table. ARP tables map L2 addresses (MAC) to
    > L3 addresses (IP). We do this in PERL with NETSNMP.


    No kidding! Are you telling me that if I ping a device from the switch,
    the switch won't have an entry in its OWN ARP table, mapping IP to MAC,
    even if it is not a router?
    papaia, Jan 13, 2006
    #3
  4. On 13.01.2006 03:11 papaia wrote

    > On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:
    >
    >>
    >> wrote:
    >>> We have catalyst 3500xl switches running
    >>> c3500xl-c3h2s-mz.120-5.WC13.bin. If I issue a show mac I get which mac
    >>> address of a client PC is attached to which fast ethernet port.
    >>> However, I would like to find out the IP as well. I currently use
    >>> LanSpy to match the mac from the switch command to the ip attached to
    >>> the mac in LanSpy. Is there a more detailed option in the switch to see
    >>> which port has which IP? Any help is appreciated.

    >>
    >> No. Switches are L2 devices so their world revolves around MAC
    >> addresses. To resolve a MAC address to an IP address you need to
    >> reference a routers' ARP table. ARP tables map L2 addresses (MAC) to
    >> L3 addresses (IP). We do this in PERL with NETSNMP.

    >
    > No kidding! Are you telling me that if I ping a device from the switch,
    > the switch won't have an entry in its OWN ARP table, mapping IP to MAC,
    > even if it is not a router?


    Actually, a switch does not need an IP address to do its work. Hence no
    need to have an ARP table. Of course if the switch has an IP addess for
    *management* purposes, it will also maintain its own ARP table. But
    typically you only see the per Vlan MAC table.



    Arnold
    --
    Arnold Nipper, AN45
    Arnold Nipper, Jan 13, 2006
    #4
  5. papaia Guest

    On Fri, 13 Jan 2006 04:25:02 +0100, Arnold Nipper wrote:

    > On 13.01.2006 03:11 papaia wrote
    >
    >> On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:
    >>
    >>>
    >>> wrote:
    >>>> We have catalyst 3500xl switches running
    >>>> c3500xl-c3h2s-mz.120-5.WC13.bin. If I issue a show mac I get which mac
    >>>> address of a client PC is attached to which fast ethernet port.
    >>>> However, I would like to find out the IP as well. I currently use
    >>>> LanSpy to match the mac from the switch command to the ip attached to
    >>>> the mac in LanSpy. Is there a more detailed option in the switch to see
    >>>> which port has which IP? Any help is appreciated.
    >>>
    >>> No. Switches are L2 devices so their world revolves around MAC
    >>> addresses. To resolve a MAC address to an IP address you need to
    >>> reference a routers' ARP table. ARP tables map L2 addresses (MAC) to
    >>> L3 addresses (IP). We do this in PERL with NETSNMP.

    >>
    >> No kidding! Are you telling me that if I ping a device from the switch,
    >> the switch won't have an entry in its OWN ARP table, mapping IP to MAC,
    >> even if it is not a router?

    >
    > Actually, a switch does not need an IP address to do its work. Hence no
    > need to have an ARP table. Of course if the switch has an IP addess for
    > *management* purposes, it will also maintain its own ARP table. But
    > typically you only see the per Vlan MAC table.
    >
    > Arnold


    I did NOT say that a switch needs an IP address to do its work, I said "if
    I can ping ... from ..." which implies association of an IP address with
    the switch, as ICMP runs ON TOP of IP (the "ping .... from" part). This
    just for clarification.

    As far as [being willing to be] using a 3500 switch w/out remote
    management (aside from a terminal server for console connnection, just to
    cover our basis), thus w/out an IP - I will leave this up to those who
    want to do that ...
    papaia, Jan 13, 2006
    #5
  6. In article <>,
    papaia <> wrote:
    >On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:


    >> wrote:
    >>> We have catalyst 3500xl switches running
    >>> c3500xl-c3h2s-mz.120-5.WC13.bin. If I issue a show mac I get which mac
    >>> address of a client PC is attached to which fast ethernet port.
    >>> However, I would like to find out the IP as well. I currently use
    >>> LanSpy to match the mac from the switch command to the ip attached to
    >>> the mac in LanSpy. Is there a more detailed option in the switch to see
    >>> which port has which IP? Any help is appreciated.


    >> No. Switches are L2 devices so their world revolves around MAC
    >> addresses. To resolve a MAC address to an IP address you need to
    >> reference a routers' ARP table. ARP tables map L2 addresses (MAC) to
    >> L3 addresses (IP). We do this in PERL with NETSNMP.


    >No kidding! Are you telling me that if I ping a device from the switch,
    >the switch won't have an entry in its OWN ARP table, mapping IP to MAC,
    >even if it is not a router?


    Does the switch -have- an IP?
    Deos the switch -have- a ping facility?
    Does the switch have a way to show its "OWN ARP table"?
    Does the switch offer SNMP?
    Does the switch tell the truth about the port number of
    devices that are talking to the switch itself?

    Unmanaged switches are plentiful these days, and they don't have IPs.

    Configurable consumer-level switches usually don't offer SNMP or
    a way to examine the ARP table.

    Managed switches with SNMP -often- offer a standard MIB that would
    allow their ARP table to be examined; sometimes it's a custom
    MIB though, and sometimes it just isn't there.

    When switches do have appropriate MIBs, it is not particularily
    uncommon for them to indicate that the port being used to talk to the
    remote device is one of: the first port; the last port; or a
    "pseudo-port" that exists just for management purposes (or to
    make implementations easier.)
    Walter Roberson, Jan 14, 2006
    #6
  7. papaia Guest

    On Sat, 14 Jan 2006 21:08:35 +0000, Walter Roberson wrote:

    > In article <>,
    > papaia <> wrote:
    >>On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:

    >
    >>> wrote:
    >>>> We have catalyst 3500xl switches running

    !!!!!!!!!!!!!!!!
    >>>> c3500xl-c3h2s-mz.120-5.WC13.bin.

    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    <snip>
    > Does the switch -have- an IP?
    > Deos the switch -have- a ping facility?
    > Does the switch have a way to show its "OWN ARP table"?
    > Does the switch offer SNMP?
    > Does the switch tell the truth about the port number of
    > devices that are talking to the switch itself?
    >
    > Unmanaged switches are plentiful these days, and they don't have IPs.

    <snip>

    This thread is becoming really funny ;)
    papaia, Jan 15, 2006
    #7
  8. In article <>,
    papaia <> wrote:
    >On Sat, 14 Jan 2006 21:08:35 +0000, Walter Roberson wrote:


    >>>> wrote:
    >>>>> We have catalyst 3500xl switches running

    > !!!!!!!!!!!!!!!!
    >>>>> c3500xl-c3h2s-mz.120-5.WC13.bin.

    > !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


    >> Does the switch -have- an IP?
    >> Deos the switch -have- a ping facility?


    >This thread is becoming really funny ;)


    Not really. You do not appear to be the original poster with the 3500xl,
    so when -you- posted talking about the ARP behaviour on pinging
    from "the" switch, your post became a general comment about switches
    rather than a reference to the 3500xl with some particular IOS version.
    Walter Roberson, Jan 15, 2006
    #8
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