devices attached to a Cisco port

leblanc
leblanc used Ask the Experts™
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I have a Cisco Catalyst 3750 and I'd like to find out all the IP addresses (devices) connecting to a port. the sh mac add add <MAC_ADDR> only shows the port of a known MAC address. In my case, I know that the port is a trunk, I'd like to find out how many IP addresses are behind that one port. I believe there is a VMWare host connecting to that port. Thx
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Matt MinorTechnical Systems Analyst

Commented:
You have a couple of options.

You could try show ip arp inspection log

If you have DHCP snooping enabled, you could also use show ip DHCP snooping binding
leblancAccounting

Author

Commented:
sh ip arp ... does not show any good info. I do not have dhcp snooping enabled
Uros Gaberdirector

Commented:
Try pinging your networks broadcast address from the switch and check arp table again, you should have more or less full table.
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Matt MinorTechnical Systems Analyst
Commented:
Well, it seems to me that you are not likely to get the required information from that L3 switch itself. Have you considered using an external tool to gather the required information?

http://www.spiceworks.com/free-pc-network-inventory-software/

Completely free, and will map out your network, routers&switches with connection information included.
Or you could try SPAN port to capture all the traffics from that trunk

Commented:
What you are asking for is not actually the job of a L2/L3 switch. The IP/MAC/Port details are transient and will appear in and time out of the switch's cache as necessary (this is just how Ethernet works and how it dynamically converges in the event of topology changes). All you can do is use different tools to gather the info you need.

Not sure how big your network is, can you not ask the person running the VMWare instance how many servers\desktops are behind that port?
leblancAccounting

Author

Commented:
I sure can ask the person for the number of hosts behind a port. I can also use SPAN and Wiresharl. But it will be great if there is a command that can list all the MAC address behind a port. Now that I think about it, how about the sh CAM table. I will try it tomorrow.
Don JohnstonInstructor
Top Expert 2015
Commented:
The "show cam" command is for CatOS platforms.  You would use the "show mac address dynamic" command to show learned addresses.

But if there is another switch connected to the port with hosts that communicate local to that switch, then you won't necessarily see all the addresses.
Network Architect
Commented:
On the Catalyst 3750, you can use the following command to check active MAC addresses on an interface:

show mac address-table interface x

Replace "x" with the physical interface you want information on.

This will include all MACs sources that have been learned by the switch at layer 2 on the indicated interface.
Jody LemoineNetwork Architect

Commented:
There's no way to get information on a MAC address that hasn't been active on the network, so by generating traffic and following it up with the "show mac" command, you'll have better results.

Using tclsh and doing a ping sweep of the subnets with a zero timeout will quickly generate ARP responses to populate the switch's database and give you useful information.

tclsh
for { set i 1} { i < 255 } { incr i } { ping x.x.x.$i repeat 1 timeout 0 }
exit
TimotiStDatacenter Technician
Top Expert 2012

Commented:
Do you use that switch as a layer 2 or layer 3 device?
If layer 3, it might have ARP info.
If layer 2, you'll need at least DHCP snooping to get any kind of MAC-to-IP database built on it, as I understand.
leblancAccounting

Author

Commented:
Jody,

show mac address-table interface x

That is the command that I was looking for. When I did that command, I see about 30 devices behind my port. Thanks

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