What are BGP route reflectors?
Why Interviewers Ask This
Advanced questions like this reveal whether a candidate has internalized Networking deeply enough to make architectural decisions. Strong answers demonstrate both breadth and depth of experience.
Answer
In a full-mesh iBGP network, every router must have a BGP session with every other router — resulting in N*(N-1)/2 sessions. For 100 routers that is 4,950 sessions — unscalable. Route Reflectors (RR) solve this by relaxing the BGP split-horizon rule: normally, a router learned a route from an iBGP peer does not advertise it to other iBGP peers. A Route Reflector can re-advertise routes received from clients to other clients and non-clients. Clients only need sessions with the RR, not with each other. The RR reflects routes while preserving the original NEXT_HOP, LOCAL_PREF, and AS_PATH — client routers still make their own forwarding decisions. Cluster ID and Originator ID attributes prevent routing loops. Multiple RRs (redundancy) in a cluster: clients have sessions with multiple RRs. Hierarchical RR design scales to thousands of routers. The alternative is BGP Confederation (dividing the AS into sub-ASes) — less common but valid.
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