Final answer:
If remote sites have overlapping subnets for an IPSEC VPN, traffic routing becomes unpredictable, as the system cannot distinguish between identical subnets for correct data path determination, potentially causing routing conflicts and misdirected data.
Step-by-step explanation:
When remote sites have overlapping subnets in an IPSEC VPN scenario, the most common outcome is that traffic routing becomes unpredictable. This occurs because the network devices cannot differentiate between the two identical subnets to route the traffic correctly. If two different devices across the VPN are assigned IP addresses that are in the same subnet, the system will struggle to determine the correct path for data intended for those addresses. Moreover, automatic routing decisions that rely on subnet information will not function properly, which could lead to network conflicts or data being sent to the wrong destination.
In some cases, depending on the VPN configuration and the devices involved, the VPN tunnel may not establish successfully (option a), but this is not always the immediate result of subnet overlap. Instead, the issues most often manifest as routing problems once the tunnel is up. It's incorrect to assume that VPN tunnels will function normally (option c) or prioritize local subnets (option d) without specific configurations in place to handle such scenarios.