Find IP address for trunk devices?

popquizkid95popquizkid95 Posts: 39Member ■■□□□□□□□□
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.

Comments

  • networker050184networker050184 Posts: 11,962Mod Mod
    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.
    An expert is a man who has made all the mistakes which can be made.
  • popquizkid95popquizkid95 Posts: 39Member ■■□□□□□□□□
    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?
  • joelsfoodjoelsfood Posts: 1,027Member ■■■■■■□□□□
    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.
  • NVLadyNVLady Posts: 51Member ■■□□□□□□□□
    Show IP apr | i (insert IP or MAC) is one of my favorite layer 3 commands. Give it a try. :)
  • popquizkid95popquizkid95 Posts: 39Member ■■□□□□□□□□
    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?
  • networker050184networker050184 Posts: 11,962Mod Mod
    The hosts keep arp table too. But at that point you could just look at it's IP!
    An expert is a man who has made all the mistakes which can be made.
  • popquizkid95popquizkid95 Posts: 39Member ■■□□□□□□□□
    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.
  • HAMPHAMP Posts: 163Member
    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
  • SimridSimrid Posts: 327Member
    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.
    Network Engineer | London, UK | Currently working on: CCIE Routing & Switching

    sriddle.co.uk
    uk.linkedin.com/in/simonriddle
  • HAMPHAMP Posts: 163Member
    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?)
  • popquizkid95popquizkid95 Posts: 39Member ■■□□□□□□□□
    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.
  • SimridSimrid Posts: 327Member
    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.
    Network Engineer | London, UK | Currently working on: CCIE Routing & Switching

    sriddle.co.uk
    uk.linkedin.com/in/simonriddle
  • james43026james43026 Posts: 303Member
    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.
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