How does BGP prevent routing loops between internal peers?

BGP employs two distinct loop-prevention mechanisms depending on whether peers are external or internal to the same autonomous system. Between external peers (EBGP), loop prevention relies on the AS path attribute: every time a route crosses an AS boundary, the local AS number is prepended to the path, and a router rejects any incoming route whose AS path already contains its own AS number, since that would indicate the route has looped back around. Between internal peers (IBGP) within the same AS, however, the AS path attribute never changes, because IBGP does not add AS numbers as routes are readvertised internally — meaning AS path alone cannot detect an internal loop. Instead, IBGP enforces a strict split-horizon-style rule: a router that learns a route via IBGP must never readvertise that route to another IBGP peer. This rule guarantees that every IBGP speaker within the AS must be directly peered with every other IBGP speaker (a logical full mesh) in order for all routers to receive all routes, since no IBGP router will relay IBGP-learned routes onward on another router's behalf. This full-mesh requirement is precisely why techniques such as route reflection and confederations were later developed — they preserve the same loop-prevention guarantee while relaxing the physical full-mesh peering burden. VRRP and BFD serve entirely unrelated purposes (gateway redundancy and fast failure detection, respectively) and play no role in BGP loop prevention. Reference topics: Junos Enterprise Routing – BGP, IBGP Split-Horizon and the Full-Mesh Requirement.