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Hoe to find ip address of neighbouring switch

Posted on 2009-05-11
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Last Modified: 2012-06-27
I have a Cisco 1841 series router that it connected to a 3com switch through the FastEthernet port. I am trying to find out the ip address of the switch attached to the router.

I have tried a SH CDP NEI but when I do this it informs me that the router is connected through the interfaces f0/0 and f0/1.

Is there any other command that I can loo at without trying to telnet to all the devices in the arp table?
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Question by:MartynLawson
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Expert Comment

by:jfrady
ID: 24353708
No.  CDP is a proprietary Cisco protocol.  The ways you can find the IP:

Log into the console
Look in the ARP cache on the router and associate with the MAC address on the label of the switch
Use a freeware app to do a ping scan of your network range
Try the default IP of the switch

Looking through aRP caches etc. won't do you much good if the switch is on a different subnet than your network for some reason.  Easiest bet is to go in to the console port.
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Teacish earned 250 total points
ID: 24353840
Also if your computer is connected to the switch then you can type arp -a from the command prompt and compare the mac addresses in the list with the sticker on the bottom of the switch as ifrady said.
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Expert Comment

by:Teacish
ID: 24354067
Although thinking about it if its managed then each port is going to have a mac address.. It would have to be done by a network scanner really.

Try using Angry IP Scanner and scan your network range and see if you can find it on there.
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by:jfrady
jfrady earned 250 total points
ID: 24354163
Teacish - If it is a Layer 2 switch there will only be one MAC address that is associated with the management interface.  The only time a switch will have multiple MAC addresses is if it has multiple Layer 3 networks.  

Using ARP -a will only show the address if it is on the same subnet and your machine has tried to contact it.  Sometimes you can do a ping to x.x.x.255 and then do ARP -a to see devices on your network.  This is not a valid way however as most devices would take that as a smurf attack and not respond.  And in this case we do not know if the switch management interface is even on the same subnet or configured from default.

The easiest way in your case Martyn is to plug in to the console or use one of the other methods mentioned.
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by:MartynLawson
ID: 31580107
Thanks Guys :)
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