The term ‘ImpNull’ in the CLI output indicates that Label 0, also known as the “Implicit Null” label, is used for the prefix displayed. This label instructs the penultimate router in an MPLS network to pop the label before forwarding the packet to the ultimate hop, which is the egress router. Therefore, Label 0 will not be part of the MPLS label stack for packets destined for 192.168.10.10 (Option A). This process is known as Penultimate Hop Popping (PHP) and is used to optimize the forwarding process by reducing one level of lookup at the egress router (Option D).
References: For more detailed information, please refer to the MPLS section in the Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) course materials.
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