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The command "show ip cef [prefix-ip prefix mask] [longer-prefixes] [detail]" is specifically used to display detailed information about the Forwarding Information Base (FIB) entries in Cisco devices that utilize Cisco Express Forwarding (CEF). CEF is a critical component for efficient packet forwarding in high-performance routers, enabling them to make quick forwarding decisions based on the already computed FIB.

When this command is executed, it can provide insights into how traffic destined for a specific IP prefix is handled. It details the paths that packets will take through the network, including information about the next-hop router, interface information, and potentially any associated metrics. Particularly, the 'detail' option will present comprehensive data about the FIB entries, such as the associated interfaces, route sources, and more.

The other options do not align with the functionality of this command. While VLAN configurations, Layer 3 routing information, and active NAT sessions are important networking concepts, they fall outside the purview of what the "show ip cef" command specifically addresses. Thus, the core focus of the command on FIB entries is what validates the answer provided.