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New Member

Can you tell the IP address of a device attached to a 6500 Catalyst Switch

I have devices attached to a Catalyst switch, and I wanted to find out which device is attached where by checking the IP address of a specific device attached to a port. The ARP tables don't show any devices. Anyone know a quick/dirty way to do this?

4 REPLIES
Anonymous
N/A

Re: Can you tell the IP address of a device attached to a 6500 C

Show arp displays you the mappings of IP addresses or "IP aliases" to MAC addresses. So, you first need to set ip alias to ip addresses:

set ip alias mercury 192.168.174.234.

To discover Cisco neighbors you may enable CDP protocol.

New Member

Re: Can you tell the IP address of a device attached to a 6500 C

The arp table on your switch does not show this as the switch does not talk to devices directly and thus does not need this information. Do this:

- From the switch ping devices you want to find where attached to.

- type "sh arp" on the switch. This time it will contain the information you need.

Similar to this:

switch6500> (enable) sh arp

ARP Aging time = 1200 sec

+ - Permanent Arp Entries

* - Static Arp Entries

10.0.0.1 at xx-xx-xx-xx-xx-xx port 1/1 on vlan 15

10.0.0.30 at yy-yy-yy-yy-yy-yy port 1/1 on vlan 15

switch6500> (enable) ping 10.0.0.20

!!!!!

----10.0.0.20 PING Statistics----

5 packets transmitted, 5 packets received, 0% packet loss

round-trip (ms) min/avg/max = 1/1/1

switch6500> (enable) sh arp

ARP Aging time = 1200 sec

+ - Permanent Arp Entries

* - Static Arp Entries

10.0.0.1 at xx-xx-xx-xx-xx-xx port 1/1 on vlan 15

10.0.0.30 at yy-yy-yy-yy-yy-yy port 1/1 on vlan 15

10.0.0.20 at zz-zz-zz-zz-zz-zz port 3/22 on vlan 15

switch6500> (enable)

New Member

Re: Can you tell the IP address of a device attached to a 6500 C

Observations, You can identify what mac address is on what port using the 'sh cam' commands. for example if you needed to know what the mac address of the device in port 2/21 is, you can use 'sh cam mthd 2/21' where mthd is any of dynamic, static, permanent or system.

Using that entry which will be in the form nn-nn-nn-nn-nn-nn, you will need to go to the router that is the default gateway out of the switch and use the command 'sh ip arp nnnn.nnnn.nnnn' to get the IP address of that device.

This is not always going to be useful. It presumes two things. One is that the device in question has put traffic on the network within the cam timeout period. The other is that that traffic has been seen by the router within the arp timeout value. In the environment I work in, we use the default values of 30 min and 255 min respectively. You can adjust the cam timeout values if your policies allow you to.

This is also not useful if the device in question does not speak IP. If it is an older Novel server, only speaking IPX, and your router does not route IPX (granted a strange situation) the device will not be identified at all in the router.

-Rusty

New Member

Re: Can you tell the IP address of a device attached to a 6500 C

I agreed with the post by Rusty the most. My method of finding out which devices is attached to which port is by simply:

1. If the device is ono the same subnet as your PC then simply ping the device and do a "arp -a" command on your PC to obtain the MAC address of the PC. Copy and paste this MAC address to your "show mac nn-nn-nn-nn-nn-nn" on your switch will reveal which port is associated with the MAC address.

2. If the device is on a different subnet then you'll need to log onto the closest router and ping the device. Then do a "show arp | begin . The first entry will be the ARP table on the router that contain the IP and the MAc address. Again copy and paste this MAC address to your switch to obtain the port.

Good luck

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