Cisco
400-007 · Question #78
400-007 Question #78: Real Exam Question with Answer & Explanation
The correct answer is B: Enable a GRE tunnel between nodes C2 and C4. To carry multicast traffic across an SP network that does not support multicast, a GRE tunnel between the correct provider-facing nodes creates an overlay that encapsulates multicast packets as unicast for transport over the L3 VPN.
Question
Refer to the exhibit. This enterprise customer wants to stream one-way video from their head office to eight branch offices using multicast. Their current service provider provides a Layer 3 VPN solution and manages the CE routers, but they do not currently support multicast. Which solution quickly allows this multicast traffic to go through while allowing for future scalability?
Exhibit
Options
- AEnable a GRE tunnel between nodes CE1 and CE2
- BEnable a GRE tunnel between nodes C2 and C4
- CEnable a GRE tunnel between nodes C1 and C4
- DImplement hub and spoke MPLS VPN over DMVPN (also known as 2547o DMVPN) between
- EThe service provider must provide a Draft Rosen solution to enable a GRE tunnel between nodes
Explanation
To carry multicast traffic across an SP network that does not support multicast, a GRE tunnel between the correct provider-facing nodes creates an overlay that encapsulates multicast packets as unicast for transport over the L3 VPN.
Common mistakes.
- A. The SP manages the CE routers, so the customer cannot configure a GRE tunnel between CE1 and CE2 without SP cooperation, which contradicts the requirement for a quick, customer-driven solution.
- C. A GRE tunnel between C1 and C4 does not align with the hub-and-spoke distribution path needed to deliver one-way video from the head office to all eight branches efficiently.
- D. Hub-and-spoke MPLS VPN over DMVPN (2547oDMVPN) is a scalable architecture but requires significant SP-side and customer-side configuration effort, making it a slower solution to deploy compared to a simple GRE overlay.
- E. The Draft Rosen solution requires the SP to make changes on their network, which introduces dependency on the SP's timeline and does not meet the requirement of quickly enabling multicast on the customer's terms.
Concept tested. GRE overlay for multicast over non-multicast SP L3 VPN
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