Finding ip from mac address

[Manoj Wadhwa]

Hi Friends,

One of my switches is experiencing errors on fa0/7. I am not sure which device is connected to fa0/7 remotely. CDP neighbors does not show any cisco device connected to fa0/7. I have the mac address of the device that i got from sh mac-address-table. Any way, we can find out the ip address from the given mac address other than having someone check physically what is connected to the port. The arp table too does not have this mac address.

sh mac-address-table

Dynamic Address Count: 6

Secure Address Count: 0

Static Address (User-defined) Count: 0

System Self Address Count: 75

Total MAC addresses: 81

Maximum MAC addresses: 8192

Non-static Address Table:

Destination Address Address Type VLAN Destination Port

------------------- ------------ ---- --------------------

00e0.7b05.4111 Dynamic 1 FastEthernet0/7

[pavansuraj]

Hi Manoj,

Please try to find the ip address in a router or a L3 switch connected nearby to this switch by using show arp command.

HTH, Please rate if it does

Pavan

[Manoj Wadhwa]

Hi Pavan,

As mentioned earlier, the mac address does not show up in the sh arp. Is there any other way we can get the mapping b/w mac and ip.

[pavansuraj · Accepted solution]

Hi Manoj,

Let me be more specific.

After taking down the mac address from the local switch using show mac-address-table command, log on to nearby router or L3 switch and issue the show arp to get the IP address.

I hope you must be able to find the IP address from the router as the device connected to F0/7 of switch has to comminicate over the router at some point of time.

Please note: Tha arp table of switch may not has that entry whereas the router's arp table may contain.

HTH, Please rate if it does

Pavan

[Manoj Wadhwa]

Hi Pavan,

Thx man. That worked. I logged in to nearby router and was able to find the ip and also the device. The device was actually a nortel box and was configured as duplex to auto. Once i changed it to full, issue was resolved. Appreciate ur assistance

- Manoj

[pavansuraj]

Hi Manoj,

Thanks for your rating and am happy that your issue has been resolved.

Have a nice time

Pavan