Provided these are all Cisco switches, this is how I would go about figuring out a topology in a new environment and determining the port a PC is on. The first step is not necessarily required but would be helpful for future troubleshooting moving forward especially if finding the switch port a PC is connected to becomes a regular occurrence.
1) Learn the Network Topology - Using Cisco Discovery Protocol provides you with the following information:
- Hostname
- IP address
- Interface
Using this information, you can map out a network pretty much in no time. I typically use the commands as such:
show cdp neighbor
This will give you a view of any Cisco access points, phones, routers, and most importantly switches. Identify the interfaces that the switches are attached to and use them in the next command to reveal the IP address of the connected device:
show cdp neighbors g1/0/48 detail
Now that you have the IP address of the connected device, you can SSH/Telnet to it and start the process over again and again until you know about the whole topology. There are some paid tools out there that will do all this for you but this doesn't take all that long to do on your own.
2) Find the Switch Interface the PC is Connected to - SSH/Telnet into the router/layer3 switch that is the PC's default gateway. From there, ping the IP address of the PC. Lets say the IP address of the PC is 10.0.0.13:
3750X-Switch# ping 10.0.0.13
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.13, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/5/17 ms
3750X-Switch# show arp | inc 10.0.0.13
Internet 10.0.0.13 1 0022.4d9a.8f60 ARPA Vlan1
You now have the MAC address of the PC.
Caution: do not try to ping the PC from any other device.
Note: You may need to ping the PC again every so often since the ARP table will timeout after 5 minutes.
Now use the following command to find the interface that the switch used to ping the PC:
3750X-Switch#show mac address-table address 0022.4d9a.8f60
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0022.4d9a.8f60 DYNAMIC Gi1/0/48Compare the interface with the "show cdp neighbors" result. If there is a CDP neighbor on that interface, then SSH/Telnet into that neighboring switch and search for the MAC address again using the command above. Continue this progress until you get to an interface that does not have a CDP neighbor attached to the interface. You have now found the switch & interface that the PC is connected to.
