IP address of switch port....

Unanswered Question
Sep 8th, 2010

hi guys,

in switches i can able to see the "What mac address is taken by each port"

In unmanagable switch, is there is any way to find "what ip address is taken by each port?

Tracing of wiring is hard in the case of larger networks. is there is any command to trace the IP address of that port.

I have this problem too.
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coto.fusionet Wed, 09/08/2010 - 19:52

Hi Naveen,

Not sure what you're trying to do.

The switches (at least Layer 2 switches) keep track of MAC address binded to ports (they forward frames based on layer 2 information).

You want to know which IP address are taken by each port?

The switches receive a frame and they forward it out using the correct port (from the MAC address table).

i.e.

If you have a router connecting to port 0/5 on the switch, then all traffic that should be routed by that router will be sent from the switch to the router's MAC address (it does not really matter the destination IP address of the packet).

Please clarify what you're trying to do here.

Federico.

Jon Marshall Thu, 09/09/2010 - 02:04

Naveen

It is a 2 step process -

1) find out off the switch which mac-address is on each port

2) go to the L3 device that routes the vlan for clients on that switch and look in the arp table on the L3 device for the mac-address and this will tell which IP address is associated with it.

Note, if you don't have a L3 device and only have one vlan on L2 switch(es) then you cannot find this out without tracing cables.

Jon

naveen_yummy Thu, 09/09/2010 - 19:34

hi jon,

wehn i check the arp table in layer 2 swittch it only shows the mac address of the layer 3 switch, its own switch and one more server IP.

other than that it didnt show the mac address of each port.

Reza Sharifi Thu, 09/09/2010 - 19:52

Naveen,

You don't see the rest of the devices, because the ARP table has aged out.  Ping the devices that are connect to the switch, than you will see the MAC addresses.  Than follow the steps Jon described.

HTH

Reza

Jon Marshall Fri, 09/10/2010 - 01:21

naveen_yummy wrote:

hi jon,

wehn i check the arp table in layer 2 swittch it only shows the mac address of the layer 3 switch, its own switch and one more server IP.

other than that it didnt show the mac address of each port.

Naveen

You don't check the arp table on the L2 switch, you only check the mac-address table on the L2 switch.

You need to check the arp table on the L3 switch.

Jon

naveen_yummy Tue, 09/14/2010 - 01:59

hi jon,

when i check the L3 switch it shows the mac address learned by all switches.

for example,

if i have 3 switches. sw1 and sw2 are L2 switch.

sw3 is the L3 switch.

when i check the L3 arp table its shows the mac address of both switches.

but if i want to trace the Ip addrsss of the particular port in the sw1 means its tougher to me,

Thats only i want to accurately trace

Jon Marshall Tue, 09/14/2010 - 04:33

naveen_yummy wrote:

hi jon,

when i check the L3 switch it shows the mac address learned by all switches.

for example,

if i have 3 switches. sw1 and sw2 are L2 switch.

sw3 is the L3 switch.

when i check the L3 arp table its shows the mac address of both switches.

but if i want to trace the Ip addrsss of the particular port in the sw1 means its tougher to me,

Thats only i want to accurately trace

Naveen

I'm not sure what the problem is here. If you want to find the IP address of a machine connected to a L2 switchport then you need to -

1) look in the mac-address table on the L2 switch to see which mac-address is associated to that port

2) log onto the L3 switch and look at the arp table. The arp table shows mac-address to IP mappings so you are looking for the mac-address that you found in step 1). This will then show you the IP address.

The L3 switch must the switch that is routing the vlan that the machine is part of on the L2 switch.

Jon

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Posted September 8, 2010 at 7:38 PM
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