Tuesday, December 10, 2013

Tech Tip: How To Resolve A MAC Addressing / IP Conflict

By Rick Escobar, Product Support Assistant Manager

Anyone who works with equipment that has a MAC address and IP address have run into the main topic of this tech tip. Heck even a banana these days has networking as a feature. 

Problem: Cannot login to a unit or even ping it with a windows operating system.

Explanation:
Let's use two of TOA's NX-100 units, and set them both to maintenance mode. (NOTE: many TOA products have an IP Address of 192.168.1.1 right out of the box)
Connect one of the units to your switcher.
Open a DOS prompt and enter 'ping 192.168.1.1'

Result: The unit will pass the ping test without an issue.

NOTE: From a DOS prompt type 'arp -a' this will display the MAC address currently associated with 192.168.1.1

Now, move the Ethernet cable to the second NX-100 unit and again ping 192.168.1.1

Expected Result: Good chance your ping test will time out. If you type 'arp -a' again, you will see that it is still using the previous NX-100 MAC address.

TIP: You can use your QWERTY up / down arrows to use a previous typed command.

What's going on?:
Your PC is still associating the previous MAC address with 192.168.1.1. This is stored in the Windows Address Resolution Protocol (ARP) Cache. Windows Vista, XP and 7 should release the ARP Cache all on its own after 2 minutes. However, if you are moving quickly, you are very likely to run into this issue, and may even end up in a loop where it may appear as if your unit is defective. Also, I have encountered an operating system that was not releasing the ARP Cache, and forcing a refresh was the only option.

Solution:
From the DOS command prompt enter 'netsh interface ip delete arpcache'. This will force the ARP cache to refresh and allow you to connect to the unit you were unable to connect to before.