CDP, ping from port

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Sep 14th, 2005
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Is there a way to log into a 2950 or 3550 and ping certain ports in order to find the IP of the device connected to it? Also how would I find the MAC of the device connected to a specific port?

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Josef Oduwo Thu, 09/15/2005 - 01:16
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You can use CDP to find out information about directly connected Cisco devices.


Here is a link on configuring CDP for the 2950 series switches (which generally hold for most access switches)http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a00800d84c4.html


For a particular port:


Switch#show cdp interface [type mod/number] so that


Switch#show cdp interface fastethernet 0/3 displays information only about Fastethernet port 3.


Josef.

wrwiii122 Thu, 09/15/2005 - 18:17
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But this does not let me see the IP for say a PC connected to the port.

Josef Oduwo Thu, 09/15/2005 - 20:49
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No it wouldn't.


To answer your original questions, because switches work at Layer 2 you would only be able to see Layer 2 traffic (MACs) and would not be able to pick up IP addresses of connected devices (Layer 3) by simply logging into a switch. To find the MAC of a device connected to a specific port use the user command:


Switch>show mac address-table interface interface-id [vlan vlan-id] [ | {begin | exclude | include} expression]


More on this command is available here: http://www.cisco.com/en/US/products/hw/switches/ps5528/products_command_reference_chapter09186a00804765f3.html#wp1943113.


You could also use SNMP: http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a00801c9199.shtml#tpic2.


HTH,


Josef.

burhamkri Fri, 09/16/2005 - 02:44
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To find the IP address associated with a given MAC address, connect to the router for the subnet that the device is on and run "show arp"

You can speed up the process by only displaying the MAC/IP combo you are looking for with:

"show arp | include [MAC ADDRESS]"

Just make sure you get the mac-address format right.

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