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I have been working on this project and I have come to a point where I need to block the user in the network(At most in the CISCO switch)

I know I can send port shut via telnet session with a premade command but my problem is getting the users port.

I have only his IP Address for identification, is it possible to dump or get all the addresses in that switch? via telnet or?

2 Answers 2

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Go to the router for that subnet and find the MAC for that IP -- if they're generating traffic, the router will have the MAC. Then go to each switch with access ports for that subnet (could be a single switch, could be dozens) and look for the MAC in the mac-address-table.

-- Example --

I have forgotten the port for the management interface of my iSCSI SAN switch, but I know the IP. From a machine in the same LAN, I ping it and then look in the ARP table:

[root:pts/5{4}]machine:~/[01:53 PM]:arp -a | grep IP
? (IP) at d0:7e:28:d1:42:01 [ether] on eth0

I first login to sw-r2-3:

sw-R2-3#show mac address-table address d07e.28d1.4201
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
  55    d07e.28d1.4201    DYNAMIC     Po1

I know Po1 (port-channel 1) is the uplink for that switch, so I go to sw-r2-2:

sw-R2-2#show mac address-table address d07e.28d1.4201           
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
  55    d07e.28d1.4201    DYNAMIC     Gi1/0/43

I know G1/0/43 is an access port -- show run int g1/0/43 -- so that's where it's connected.

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  • Ah so its more of I need to access the router to get what I wanted. thank you will try this approach :) Jun 28, 2014 at 0:01
  • Note: "router" could also be a switch. In my example, all the switches are layer-2 devices (they don't do any routing) as such, they have no layer-3 (IP) information. (other than management traffic)
    – Ricky
    Jun 28, 2014 at 0:14
  • Is it possible to contact the router from a an access point in the switch? via telnet or something similar? Also I assume this is possible with ipv6? =) Jun 28, 2014 at 2:13
  • layer-2 (MAC) doesn't care what your layer-3 (IP, IPv6, IPX, etc.) is. It only changes the first step: finding the MAC. For linux, that's ip -6 neigh.
    – Ricky
    Jun 28, 2014 at 5:56
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You can try ARP'ing for the MAC address and find out which machine it belongs to. Checking the ARP table on the switch can also help.

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