The correct answer is: 'U' may indicate that a router along the path did not contain a route to the destination address and that the ping was unsuccessful.
The most common indicators of a ping issued from the Cisco IOS are "!", ".", and "U". The "!" indicates that the ping completed successfully, verifying connectivity at Layer 3. The "." may indicate that a connectivity problem, routing problem, or device security issue exists along the path and that an ICMP destination unreachable message was not provided. The "U" indicates that a router along the path may not have had a route to the destination address, and that it responded with an ICMP unreachable message.
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