Essentially the ISP will be trying to lease the same IP to 2 devices at once.
But you only have one connection from your ISP, so I don't see how this is possible.
this post was submitted on 27 Nov 2023
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Are you asking hypothetically? In real world examples.. Mac addresses are Layer 2, unlike IP addresses which are Layer3. Basically Mac is a hardware address hardcoded to the device and no two devices will have duplicate addresses. that's how the system is designed. Officially, no two network devices will ever have the same Mac address. I am ignoring the fact that Mac spoofing is a possibility (software duping network Mac addresses, in which case duplicate Macs possible.. but that's not your situation here).
If two devices had the same MAC on the same network , it could lead to L2 loops and possible network congestion. Some devices may not like the duplication at all and freeze services all together. Behaviour is unpredictable as dependent on device firmware.
It depends on how the last mile access is configured.
Sometimes layer 2 end segments are shared (like doccis) it would break those 2 devices on doccis.
I don't believe the gpon standard has the same problem.
It would cause a problem on wireless based access or on embedded ISPs in buildings, they usually shortcut and just put a /24 and push users are onto the same vlan.
Again it only breaks those 2 devices, unless the network sees mac-moves and blocks a port.
Having dealt with duplicate MACs in the past on a LAN, it's chaos. Both connections can be affected. If they're on the same network, it comes down to an ARP table race as to who gets to talk. If there's enough separation between them that they don't end up in the same ARP table someplace, it may have no impact at all. It will be similar to an ARP table poisoning attack.
Once I had 2 firewalls configured in High Availability for redundancy, active/passive, so if one dies the other becomes active. They both plug into the same ISP and use MAC address cloning to achieve this. Well the link between the 2 firewalls for heartbeat went bad and both became active with the same MAC.
The effect I experienced was 50% packet drop, every other ping failed as they both fought each other. Lesson learned and now I use at least 2 links between both firewalls in case one fails.