You can imagine, in fact let’s give a source here, here’s somebody in their house trying to browse Google. Similarly with a MAC address, this interface here has a MAC address on it. I can only step as far as to the next stone. That’s the actual word for it, but I imagine it like stones as you’re walking across a stream. This packet is flying through the network. If I have a router connected to a router connected to a router and so on…Īnd if there’s a packet flying through this network. If the PC on the left in this picture is in a different network, or let’s say VLAN, from the PC on the right, it’s not as consequential, although the real problem here is they’re on the same switch. How far reaching is the MAC address? That is, when does it cease to have network meaning? That’s the real extent of a MAC address is, it only really matters within the same network. Let’s say these two people were users, and they were working for a company, and needed to browse for the Internet, as soon as both machines were live and claiming to have the same MAC address, they would have an issue. 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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