Find IP address for trunk devices?

popquizkid95

Myself and a co-worker are in sort of a study group for the CCNA. I'm still trying to wrap my head around trunking. We have setup a lab situation where we are trying to identify devices connected to a switch where all ports are setup as trunks.

So basically my question, if the ports are trunked, is there no way to tell what the IP address is for the devices on each port? I can find the mac-addresses by "show mac address-table", but there are no IP addresses in the ARP table.

networker050184

Whether the port is trunked or not really has nothing to do with it. To see arp you need to go to the layer 3 device.

popquizkid95

networker050184 wrote: »

Whether the port is trunked or not really has nothing to do with it. To see arp you need to go to the layer 3 device.

Ah! I'm stuck in a layer 2 mindset.. ARP is all done in layer 3.

So next question, what if the network is just a bunch of trunked layer 2 switches, and there is no layer 3 device?

joelsfood

You'll likely still have a Layer 3 address on a management VLAN or physical management network. You also have CDP/LLDP to identify directly connected devices.

NVLady

Show IP apr | i (insert IP or MAC) is one of my favorite layer 3 commands. Give it a try.

popquizkid95

The switches are linked together via trunk links, and they are on the same VLAN 5, with the ip addresses of 192.168.5.x etc. I know the devices have IP addresses of 192.168.1.xxx. Since they are trunk ports, it doesn't matter what IP address they have. But for the purpose of the lab, we're trying to identify the ip addresses by port.

So since I'm stuck in layer 2, and the devices aren't in CDP, are we SOL?

networker050184

The hosts keep arp table too. But at that point you could just look at it's IP!

popquizkid95

Ok, well it looks like there's no easy answer here. Hopefully this won't be on the exam.. lol

Thanks everyone for chiming in.

HAMP

popquizkid95 wrote: »

Ok, well it looks like there's no easy answer here. Hopefully this won't be on the exam.. lol

Thanks everyone for chiming in.

It seems like it was answered in the 2nd post.

Switches are layer2 devices so they keep layers2 information

Routers are layer3 devices so they keep layer3 information.

If you want layer3 information, you will need a layer3 devices and not layer2 devices.

But,

That doesn't go both ways, a layer3 device does have layer2 information.. lol

Simrid

Good real life tip to find particular hosts on switch ports are:

#show mac address-table - Shows MAC addresses which are connected to the switch and neighboring switches.

#show arp | inc mac_address - Shows IP of the MAC in question.

HAMP

Simrid wrote: »

Good real life tip to find particular hosts on switch ports are:

#show mac address-table - Shows MAC addresses which are connected to the switch and neighboring switches.

#show arp | inc mac_address - Shows IP of the MAC in question.

Isn't the first line done on a switch, while the second line is done with a router? (not both on a switch?)

popquizkid95

Yes, HAMP, that's exactly the issue here. All of the switches are layer 2. I can see the mac-addresses of the devices no problem with sho mac address-table. But since there is no ARP info (layer 3 info) for the devices, then the mac addresses won't resolve to an IP address.

So running a sho arp only shows the IP address of the VLAN and default gateway.

Simrid

Apologies if I wasn't clear. Correct, layer 3 devices will have the ARP table for the subnet and that is where you will need to do the show arp.

Remember, you can do layer 3 switching which will hold the arp entries also.

james43026

Try this: on a switch with devices connected to it.
conf t
ip device tracking
interface range g1/0/1 - 24
ip device tracking max 10
end
---- Wait for approximately 10 seconds ----
sh ip device tracking max 10
-- or --
sh ip device tracking interface


You can read more here , I think this feature may be new to IOS 15 though.