How does Layer 3 VPN traffic traverse an ISP network?

Question

Options

• ADevices on the network use MPLS labels to share VPN routes between P routers in the
• BDevices on the network use LSAs to share routes between P routers in the network.
• CDevices on the network use MPLS labels to move VPN traffic through the network.
• DDevices on the network use GRE tunnels to move traffic between VRFs.

• B
  4% (1)
• C
  93% (25)
• D
  4% (1)

Explanation

In MPLS Layer 3 VPNs, P (core) routers forward customer traffic using MPLS label switching - they read the label on each packet and swap it along a pre-established Label Switched Path (LSP) to the egress PE router, never needing to inspect IP headers or know anything about the customer's VPN routes. This makes C correct: MPLS labels are the mechanism that moves traffic through the ISP core.

Why the distractors are wrong:

• A is close but wrong on two counts: VPN routes are shared between PE routers (not P routers), and that sharing is done via MP-BGP, not MPLS labels. MPLS labels forward traffic; they don't advertise routes.
• B is wrong because LSAs (Link State Advertisements) are an OSPF concept used for the ISP's internal routing, not for distributing VPN customer routes. Again, MP-BGP handles VPN route distribution between PEs.
• D is wrong because standard MPLS L3VPNs use label stacking (an inner VPN label + an outer transport label), not GRE tunnels. GRE is associated with older or alternative VPN designs, not the MPLS/BGP VPN model.

Memory tip: Think of the ISP core as a conveyor belt system - PE routers stamp a label on each package (packet), and P routers just read and swap those labels to keep it moving, blind to what's inside. "Labels move traffic; BGP shares routes."

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