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352-001 · Question #192
352-001 Question #192: Real Exam Question with Answer & Explanation
The correct answer is B: Multicast traffic is forwarded over GRE tunnels.. The Rosen Draft MPLS L3 VPN multicast design encapsulates customer multicast traffic inside GRE tunnels (MDT tunnels) and forwards it across the provider core using PIM-built multicast trees.
Question
What is a design aspect regarding multicast transport for MPLS Layer 3 VPNs using the Rosen Draft implementation?
Options
- ALDP is the multicast control plane protocol.
- BMulticast traffic is forwarded over GRE tunnels.
- CMulticast traffic is forwarded over LDP or RSVP signaled LSPs.
- DUsing the MDT SAFI in BGP ensures that PIM can be disabled in the core.
Explanation
The Rosen Draft MPLS L3 VPN multicast design encapsulates customer multicast traffic inside GRE tunnels (MDT tunnels) and forwards it across the provider core using PIM-built multicast trees.
Common mistakes.
- A. LDP is the unicast label distribution protocol for MPLS forwarding and does not serve as the multicast control plane; PIM is used in the provider core to build MDT trees in the Rosen Draft.
- C. Rosen Draft uses GRE-based MDT tunnels for multicast transport, not native MPLS LSPs signaled by LDP or RSVP - those LSPs carry only unicast VPN traffic.
- D. MDT SAFI in BGP is used for MDT address discovery and advertisement between PE routers, but PIM remains required in the core to build MDT trees and cannot be disabled in the Rosen Draft model.
Concept tested. Rosen Draft MPLS L3 VPN multicast forwarding over GRE MDT tunnels
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