Which two BGP attributes prevent loops in a route reflector environment? (Choose two.)

Route reflectors (RRs) are used to scale iBGP by relaxing the full-mesh requirement. This modification of the iBGP split-horizon rule necessitates specific loop-prevention mechanisms. The ORIGINATORID attribute is created by an RR and contains the router ID of the client that originally advertised the prefix. If a router receives a prefix with its own router ID in the ORIGINATORID field, it discards the update, preventing it from re-learning its own routes. In hierarchical RR designs, the CLUSTERID is used. An RR adds its CLUSTERID to the CLUSTERLIST attribute. If an RR receives an update that already contains its own CLUSTERID in the CLUSTERLIST, it discards the update, preventing loops between different RR clusters.

B. local preference: This attribute is used within an autonomous system to influence the preferred exit point for outbound traffic; it is not a loop prevention mechanism.
C. origin: This attribute indicates how a route was learned by BGP (e.g., from an IGP or redistributed) and is a factor in path selection, not loop avoidance.
E. ASPATH: This attribute is the primary loop prevention mechanism for eBGP (inter-AS), not for the intra-AS loops that route reflectors are designed to prevent.

1. RFC 4456: BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)

Section 8, "Avoiding Routing Information Loops": "To avoid routing information loops, a BGP speaker MUST discard a received route if the ORIGINATORID attribute is present and is equal to its own BGP Identifier... A BGP speaker that recognizes the CLUSTERLIST attribute MUST discard a route if the CLUSTERID of the local BGP speaker is present in the CLUSTERLIST of the received route."

DOI: https://doi.org/10.17487/RFC4456

2. Cisco Systems, IP Routing: BGP Configuration Guide, Cisco IOS XE Gibraltar 16.12.x

Chapter: BGP, Section: BGP Route Reflector: "The originator ID is a 4-byte, non-transitive BGP attribute that is created by a route reflector. This attribute carries the router ID of the originator of the route in the local autonomous system. If a route is received with the originator ID set to the router ID of the local router, the route is discarded. This is the mechanism that is used to avoid routing information loops."

Chapter: BGP, Section: BGP Route Reflector: "The route reflector uses the BGP cluster ID to avoid routing information loops when you have redundant route reflectors."