I am troubleshooting a LAN and the problem is to make sure each host can reach each other at layer 3. Each host is configured to the same subnet, but some hosts cannot reach other at layer 3 (e.g. ping says host other is down). Before checking for layer 3 problems, we want to establish that Ethernet (up to layer 2) is working properly and that each of the hosts can discover each other. We have the set of each host MAC address to connect.

Relying on only layers 1-2, what are the suggested methods to make sure layer 2 works?


  • tools which make the host interface run in promiscuous mode
  • some use of FF:FF:FF:FF:FF:FF broadcast
  • There was a related layer 2 question which hinted at some possible topology discover protocols but it was closed.

This might not be fancy enough for what you're wanting, but I like to stick with built-in tools for troubleshooting whenever possible. ARP is a layer 2 protocol based on broadcasts. The command line arp tool included in most operating systems can show you the status of a pending ARP. You can always use a tool like Wireshark if you want to actually see the packets on-screen, too. Since you already have IP addresses in the same subnet assigned to both machines you can start some PINGs from one machine to the other and watch the wire for the ARP requests and replies.

  • And hopefully the business LAN you are troubleshooting has a managed switch so you can verify that mac-addresses of end hosts are known to the switch.
    – cpt_fink
    Apr 13 '13 at 8:20
  • simple is good. Not looking for fancy or to download the latest tool. LLDP looks like another possibility, but I don't see a lot of support for it.
    – T. Webster
    Apr 15 '13 at 2:14

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