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352-001 · Question #332

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…

The correct answer is B. The RR must advertise the default route target filter toward the PE routers. D. Both PE and RR routers must support this feature. 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.

Layer 3 Control Plane

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.

How the community answered

(29 responses)
  • A
    10% (3)
  • B
    66% (19)
  • C
    24% (7)

Why each option

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.

AThis feature must be enabled on all devices in the network at the same time.

RFC 4684 is explicitly designed for incremental deployment, allowing operators to enable it on individual devices without requiring simultaneous activation across the entire network.

BThe RR must advertise the default route target filter toward the PE routers.Correct

The RR must advertise the default route target filter - a wildcard RT membership - toward PE routers to ensure that PEs which have not sent an explicit RT membership NLRI still receive all VPN routes. This is a critical requirement for incremental deployment and backward compatibility. Without the default filter advertisement, PEs that have not yet adopted the feature would be isolated from VPN routes on the RR.

CThe RRs do not need to advertise any route target filter toward the PE routers.

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.

DBoth PE and RR routers must support this feature.Correct

Both PE and RR routers must support RFC 4684 because the feature requires PEs to originate RT membership NLRIs advertising which route targets they import, and RRs to filter VPN-IPv4/v6 prefix advertisements based on those memberships. If either side lacks support, constrained distribution cannot function correctly and VPN reachability is at risk.

Concept tested: BGP Constrained Route Distribution RFC 4684 for MPLS VPNs

Source: https://www.rfc-editor.org/rfc/rfc4684

Topics

#BGP constrained route distribution#route reflector#route target filter#MPLS VPN

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