Unresolved MAC address in ARP table

[Anonymous]

I have a PC that is unable to communicate. Previous to today, the PC successfully pulled an address from a windows base DHCP server. Let's say the address is ending in 15.130. Today the PC will not connect and when I view the arp table I see the address in question with an Unresolved MAC address. I have shut the port down and cleared the arp cache and entry for that IP but it still remains. I even rebooted all devices supporting this issue and the issue remains. I have port security enabled on the port in question and was able to pull back an layer 2/mac address for the system in question. But for some reason I cant ping the address and the MAC address is unresolved in the arp table.

Any help in this matter would be great!

Chris

[jayh]

What Adtran equipment is in the mix here?

Does the PC display an IP as if it has successfully negotiated DHCP?

Is there link up on the port?

Does it work if you turn off port security?

Does it work if you assign the same IP, subnet mask, and gateway statically on the PC?

Start with:

• Port security settings having a different stuck MAC
• DHCP server debugging
• Something blocking broadcast traffic

[Anonymous]

NetVanta 1335. PC was unable to successfully negotiate an address. Link is up. Does not obtain an address or resolve the mac address when I remove port security. The address is stuck in the arp table and is unable to resolve mac address. We have restarted the the DHCP server, 1335, and PC. Even when we shut the PC in question down, the address remains in the arp table. No firewall at location.

Chris

[jayh]

cburgamy wrote:

NetVanta 1335. PC was unable to successfully negotiate an address. Link is up. Does not obtain an address or resolve the mac address when I remove port security. The address is stuck in the arp table and is unable to resolve mac address. We have restarted the the DHCP server, 1335, and PC. Even when we shut the PC in question down, the address remains in the arp table. No firewall at location.

ARP entries are cached for several minutes.

You say "the address" is stuck in the ARP table?  What address?  The MAC of the PC?  The IP you're assigning via DHCP?  Can you post specific examples of what you think is stuck in error and why? 

Does the MAC stuck in the ARP table for the IP match the MAC of the computer?  It's possible that another device is causing an IP conflict.  Might someone have configured a static ARP entry for different hardware at some point? 

Look at the ARP entry for the IP in question.  If the MAC isn't that of the host you're working with, do a "show mac-address-table | inc [MAC of offending device]"  This will tell you to which physical port the machine with the offending duplicate IP is connected. 

Useful commands:

clear arp-cache  -  Clears layer 3 ARP entries routing to or through the switch itself (doesn't affect layer 2 traffic traversing the switch on a single VLAN).

clear mac address-table dynamic - Clears layer 2 bindings of MAC addresses to ports. 

Both of these are ok to run on a production switch, you'll lose at most one packet per flow.

[Anonymous]

- Do you need any further assistance regarding this issue?

Thanks,

Noor

[Anonymous]

Sorry for the late reply very busy.

The IP address of the workstation is what is stuck in the ARP table, even after the PC is turned off, with a message in the sho mac address-table stating that the MAC address is unresolved. There is no MAC address just the IP address. No IP conflict. IP is provided by DHCP server.

I tried both commands listed to free up the ip addresses, but the moment we turned the PCs back on, the IP address returns with an unresolved MAC address.

Chris

[jayh · Accepted solution]

cburgamy wrote:

The IP address of the workstation is what is stuck in the ARP table, even after the PC is turned off, with a message in the sho mac address-table stating that the MAC address is unresolved. There is no MAC address just the IP address. No IP conflict. IP is provided by DHCP server.

I tried both commands listed to free up the ip addresses, but the moment we turned the PCs back on, the IP address returns with an unresolved MAC address.

This is normal behavior.  If you ping an IP on that subnet that doesn't exist at all and then do sho mac address-table you will see it as unresolved.

An IP address is a layer 3 address.  When traffic comes to the router destined for that IP, an ARP packet is broadcast at layer 2, "Who has IP www.xxx.yy.zzz, tell [ip of sender]" If there is no response, the IP address shows up as unresolved.  If there is a response, the IP to MAC mapping will show in the address table for the system ARP timeout.

It sounds like your problem is related to DHCP and not ARP.  Does the PC get an IP address from DHCP? 

[Anonymous]

:

I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post and unmark it and select another in its place with the applicable buttons.  If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.

Thanks,

Levi