When VPRN Outbound Route Filtering (ORF) is enabled, each PE sends its ORF list to its peer using which of the following BGP message types?

Outbound Route Filtering (ORF) is a BGP capability that allows a router to send a set of filters to its peer. The peer then uses these filters to restrict the routing updates it sends back. According to RFC 5291, which defines this mechanism, the ORF entries (the actual filter list) are communicated from the receiving PE to the sending PE within a BGP ROUTE-REFRESH message. While the capability to perform ORF is negotiated in the OPEN message, the dynamic exchange and updating of the filter set itself is accomplished using ROUTE-REFRESH messages. This reduces the amount of unwanted routing information sent between BGP peers in a VPRN context.

A. Open: The OPEN message is used to negotiate BGP capabilities, including ORF support, but it does not carry the actual filter list.
B. Update: The UPDATE message carries routing information (NLRI and path attributes) from the sender to the receiver, not filtering policies from the receiver to the sender.
D. Notification: The NOTIFICATION message is used exclusively for signaling errors and results in the termination of the BGP session.

1. Nokia 7750 SR OS Border Gateway Protocol Guide, Release 22.10.R1: In the "Outbound Route Filtering" section, it states, "The ORF entries are sent in BGP ROUTE-REFRESH messages." This document confirms the specific mechanism used in Nokia's implementation.

2. IETF RFC 5291, "Outbound Route Filtering Capability for BGP-4", August 2008: Section 3, "Operation", paragraph 1, explicitly states: "The ORF entries are carried in the BGP ROUTE-REFRESH message... A BGP speaker sends a ROUTE-REFRESH message to its peer to communicate its ORF entries." (DOI: https://doi.org/10.17487/RFC5291)

3. Nokia Data Center Fabric with SR Linux, Student Guide, Rev 1.0: Module 6, "Inter-subnet Forwarding with BGP EVPN," discusses advanced BGP features. It clarifies the roles of different BGP messages, reinforcing that ROUTE-REFRESH is used for policy-based re-advertisement requests like ORF, distinct from the initial capability negotiation in OPEN messages.