In fact, if I added a router to this picture. If by some chance both of these machines claim to have duplicate MAC addresses, the switch will not know what to do with that traffic. It will have learned the MAC addresses of these two endpoints. The Layer 2 switch passively learns MAC addresses by listening. But we have an intervening network device here. It turns out if one of these computers try’s to perform an ARP process to determine the MAC address of the far side, that machine will answer. I have a topology here if I can bring it live ‑‑ let’s say that I have two IP endpoints and you’ll notice that they are both plugged in to a switch. I’m going to have a problem in the fact that I feel like I’m talking to myself. However, what if the MAC address that that other endpoint has is exactly the same as mine? I say, “Who’s the IP address that has this MAC address?” They reply back to me, and all is well. That means in order for me to ping that device, I have to determine that device’s MAC address. Let’s say I’m a PC and I have another PC next to me that is in the same network that I’m in and I want to communicate with it. What issues arise if there are duplicate MAC addresses?
Instructor Mark Jacob was presenting a Q&A Session in our ICND1 CCNA class and answered a question regarding MAC Addresses and when they get associated to devices. This post is from our Cisco CCNA Training Course.
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