I'm testing a new FTTH CPE in our lab. A Cisco ME3400 switch. So far I'm having issues in provisioning them. I have activated DHCP debugging to see the DHCP packets.

I wonder how can I test to know if there is layer 2 connectivity between the CPE and ME3400?

Some output :

ME3400#sh int fa0/1 FastEthernet0/1 is up, line protocol is up (connected) Hardware is Fast Ethernet, address is 0023.05ab.c883 (bia 0023.05ab.c883)

ME3400#sh mac address-table

Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
1 XXXX.XX29.c942 DYNAMIC Gi0/1
1 XXXX.XXe4.0bdd DYNAMIC Gi0/1
1 XXXX.XX70.113e DYNAMIC Gi0/1
10 XXXX.XXa1.4ca0 DYNAMIC Fa0/1

5 Answers 5


If you see MAC address is 'mac address-table' you have L2 connectivity. So nothing wrong in your test methodology.


You can check the mac address table, but there won't be an entry unless there's been traffic. A quick method I often use is to ping the device (assuming it has an IP). Even if it doesn't respond to ping it should respond to ARP, and there will be an entry in the ARP table for that IP.


you may want to try a Layer2Traceroute:

Router# traceroute mac [interface type interface_number] source_mac_address [interface type interface_number] destination_mac_address [vlan vlan_id] [detail]

Once you have confirmed L2 connectivity, can you explain the problem with the provisioning in more detail please?

  • AFAIK it relies on CDP, and I doubt his FTTH CPE is Cisco or CDP enabled.
    – ytti
    Commented May 28, 2013 at 8:36
  • Thanks for the tips. Here some details on traceroute layer 2 link Because I'm testing remotely the CPE's, I've more issues on test. I ask to reboot the modems and them start to "Discover" DHCP. Now I've some issue on provisioning. I've configured the DHCP server on Switch, obsviously only for straighforward test of CPE. And I get this issue : > DHCPD: client rejected, sent Have any idea ? All this, is for evaluate the CPE's
    – cgasp
    Commented May 28, 2013 at 10:16

Q. how can I test to know if there is layer 2 connectivity

A. Configure LLDP and/or show interface

#show mac address-table

This will show stored MAC addresses in the switch mac address table, these could be statically assigned by the administrator or user. Or they may be dynamically learned by passing traffic through interfaces.

  • hey hey now....Helping someone see they were mistaken...one thing were all on the path of learning to be better engineers. down voting. Thats kind of **bold ** unnecessary =) just saying......
    – Ty Smith
    Commented Sep 13, 2016 at 3:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.