Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) — Question 480

How does MPLS Layer 3 VPN function?

Answer options

• A. When an EIGRP internal route is redistributed into BGP by one PE and then back into EIGRP by another PE, the originating router ID for the route is changed to the router ID of the first PE.
• B. When a destination PE device receives a labeled packet, it pops the label and uses it to forward the packet to the correct CE device.
• C. When a PE device forwards a packet received from a CE device across the provider network, it labels the packet with the label learned from the source PE device.
• D. When a VPN route is learned from a CE device and injected into IGP, a VPN route distinguisher attribute is associated with it.

Correct answer: B

Explanation

The correct answer is B because, in MPLS Layer 3 VPN, the destination PE device removes the label from the packet to forward it to the corresponding CE device. Option A is incorrect as it describes EIGRP route redistribution, which is unrelated to MPLS Layer 3 VPN operation. Option C is wrong because while a PE does label packets, the correct action in this context involves popping the label, not labeling it. Option D, although related to routing, does not describe the label manipulation process that is central to MPLS Layer 3 VPN functionality.