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MAC to IP mapping

 
 
adam.rothschild@gmail.com
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      01-11-2006
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.

 
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zulu-1-three
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      01-11-2006

http://www.velocityreviews.com/forums/(E-Mail Removed) 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.

 
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papaia
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      01-13-2006
On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:

>
> (E-Mail Removed) 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?
 
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Arnold Nipper
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      01-13-2006
On 13.01.2006 03:11 papaia wrote

> On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:
>
>>
>> (E-Mail Removed) 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
 
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papaia
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      01-13-2006
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:
>>
>>>
>>> (E-Mail Removed) 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 ...

 
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Walter Roberson
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      01-14-2006
In article <(E-Mail Removed)>,
papaia <(E-Mail Removed)> wrote:
>On Wed, 11 Jan 2006 13:53:29 -0800, zulu-1-three wrote:


>> (E-Mail Removed) 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.)
 
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papaia
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      01-15-2006
On Sat, 14 Jan 2006 21:08:35 +0000, Walter Roberson wrote:

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

>
>>> (E-Mail Removed) 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
 
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Walter Roberson
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      01-15-2006
In article <(E-Mail Removed)>,
papaia <(E-Mail Removed)> wrote:
>On Sat, 14 Jan 2006 21:08:35 +0000, Walter Roberson wrote:


>>>> (E-Mail Removed) 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.
 
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