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352-001 · Question #332
352-001 Question #332: Real Exam Question with Answer & Explanation
The correct answer is B: The RR must advertise the default route target filter toward the PE routers.. BGP Constrained Route Distribution per RFC 4684 requires both PE and RR support, and the RR must advertise a default RT filter to maintain reachability for PEs that have not yet signaled their RT membership.
Question
You are performing a BGP design review for a service provider that offers MPLS-based services to their end customers. The network is comprised of several PE routers that run iBGP with a pair of route reflectors for all BGP address families. Which two options about the use of Constrained Route Distribution for BGP/MPLS VPNs are true? (Choose two.)
Options
- AThis feature must be enabled on all devices in the network at the same time.
- BThe RR must advertise the default route target filter toward the PE routers.
- CThe RRs do not need to advertise any route target filter toward the PE routers.
- DBoth PE and RR routers must support this feature.
Explanation
BGP Constrained Route Distribution per RFC 4684 requires both PE and RR support, and the RR must advertise a default RT filter to maintain reachability for PEs that have not yet signaled their RT membership.
Common mistakes.
- A. RFC 4684 is explicitly designed for incremental deployment, allowing operators to enable it on individual devices without requiring simultaneous activation across the entire network.
- C. The RR is required to advertise the default RT filter toward PEs that have not yet signaled RT membership; omitting this advertisement would prevent those PEs from receiving any VPN routes and break their services.
Concept tested. BGP Constrained Route Distribution RFC 4684 for MPLS VPNs
Reference. https://www.rfc-editor.org/rfc/rfc4684
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