Two Alcatel-Lucent 7750 SRs are established BGP peers with the following add-paths configuration: R1: configure router bgp add-paths ipv4 send 4 receive R2: configure router bgp add-paths ipv4 send 2 If router R1 advertises three routes for a given prefix X, how many routes is router R2 expected to have for prefix X?

The BGP ADD-PATH capability (RFC 7911) allows a BGP speaker to advertise multiple paths for the same prefix. The number of paths advertised is determined by the sender's configuration and the number of paths it has available. In this scenario, router R1 is configured to send 4, meaning it can advertise a maximum of four paths for any given prefix. Since R1 has three paths available for prefix X, and this is less than its configured limit of four, it will advertise all three paths. Router R2's send 2 configuration only affects the number of paths it would advertise to its peers; it does not limit the number of paths it can receive. The ability to receive multiple paths is negotiated during the BGP session establishment. Because R1 advertises the ADD-PATH send capability, R2 will enable its receive capability for the session and will accept the three paths sent by R1.

A. None: Incorrect. ADD-PATH is configured on both peers, and R1 has valid paths to advertise, so routes will be exchanged.
B. 2: Incorrect. This value corresponds to R2's send limit, which dictates how many paths R2 sends, not how many it receives. The sender (R1) determines the number of advertised paths.
D. 4: Incorrect. This is the maximum number of paths R1 is configured to send, but it only has three paths available for prefix X to advertise.

1. Nokia 7750 SR OS BGP Configuration Guide, Release 21.10.R1: In the "BGP Command Reference" section, the description for the add-paths command states: send "specifies the maximum number of paths that are advertised for a prefix." This confirms that the sender's limit and available paths determine the advertisement count. The guide also clarifies that negotiating the send capability with a peer enables the receive capability on the local router.

2. RFC 7911: Advertisement of Multiple Paths in BGP: Section 3, "Operation," states, "The number of paths that a BGP speaker can advertise to a peer for a prefix is a local matter." This establishes that the sending router (R1) controls how many paths it sends, constrained by its local policy and configuration (send 4). R2, as the receiver, processes what is sent.