When an Alcatel-Lucent 7750 SR receives a BGP route with an AS Path loop, what action is taken by default?

According to the standard BGP route processing model on the Nokia 7750 SR, all routes received from a BGP peer are first stored in the Adjacency-RIB-In (RIB-In). A fundamental BGP validity check is then performed, which includes the AS Path loop detection. If the router's own Autonomous System Number (ASN) is found within the ASPATH attribute of a received route, the route is considered invalid. It remains in the RIB-In but is flagged as invalid due to "AS Path Loop" and is not considered a candidate for the BGP decision process or installation into the Local-RIB (LOC-RIB).

A. The route is not completely ignored; it is stored in the RIB-In and marked as invalid, which allows for operational visibility and troubleshooting.
B. The LOC-RIB contains only the valid, best paths selected by the BGP decision process. An invalid route due to a loop is never eligible for the LOC-RIB.
D. The RIB-Out is used for storing routes that are to be advertised to a BGP peer, not for routes that are received from a peer.

1. Nokia 7750 SR OS BGP and MPLS Guide, Release 21.10.R1: In the "BGP Concepts" chapter, the section "BGP RIBs" describes the route processing flow. It states, "The Adjacency-RIB-In stores the routing information that has been learned from a BGP peer... before the application of any inbound routing policies." The AS Path check is a protocol-level validation that occurs at this stage. Routes failing this check are marked invalid in the RIB-In.

2. RFC 4271 - A Border Gateway Protocol 4 (BGP-4): Section 9.2, "Phase 2: Route Selection," implicitly supports this. Before a route can be selected, it must be validated. Section 9.1.2.1, "ASPATH," states that if a router finds its own AS in the path, the update is ignored. The Nokia implementation interprets "ignored" as flagging it invalid in the RIB-In and not considering it for the decision process.

3. Nokia Data Center Fabric with SR Linux Cookbook, Section 3.2.1 "BGP Route Processing": While for SR Linux, the BGP processing model is consistent with SR OS. It details that routes are received into the Adj-RIB-In, where initial validation, including AS path loop checks, occurs. Invalid routes are filtered at this stage and not passed to the next stage of policy evaluation or best-path selection. This confirms the invalid route is held in the RIB-In.