350-501 Services Practice Question
A service provider is implementing Multicast VPN (MVPN) in an MPLS backbone. They use Rosen GRE (draft-rosen) for multicast transport. What is the primary limitation of Rosen GRE compared to mLDP-based MVPN?
⚠ Common exam trap
Cisco often tests the misconception that Rosen GRE requires a full-mesh of tunnels (Option A) because candidates confuse the concept of a shared MDT with the need for individual tunnels, but the real limitation is the flooding of traffic to all PEs regardless of receivers.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
All multicast traffic is sent to every PE in the VPN, even if no receiver exists.
In Rosen GRE (draft-rosen) MVPN, a default MDT (Multicast Distribution Tree) is built using a shared PIM-SM group across all PEs in the VPN. All multicast traffic from any source in the VPN is encapsulated in GRE and sent to this default MDT group, meaning every PE receives the traffic regardless of whether it has any local receivers. This is the primary limitation because it wastes bandwidth and processing on PEs that do not need the traffic, unlike mLDP-based MVPN which builds trees only to PEs with receivers.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It requires full-mesh of GRE tunnels between all PEs.
Why it's wrong here
While Rosen GRE requires a default MDT, it doesn't necessarily require full-mesh of GRE tunnels; it uses a shared tree.
- ✗
It cannot use Source-Specific Multicast (SSM).
Why it's wrong here
Rosen GRE can support SSM with appropriate configuration.
- ✓
All multicast traffic is sent to every PE in the VPN, even if no receiver exists.
Why this is correct
Rosen GRE uses a default MDT that forwards traffic to all PEs, causing waste.
- ✗
It does not support Protocol Independent Multicast (PIM).
Why it's wrong here
Rosen GRE supports PIM.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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