BGP Fundamentals and Protocol Operation
BGP Fundamentals and Protocol Operation
This domain tests whether you understand BGP as a path-vector protocol carried over TCP, its message types, the finite state machine, and how the well-known path attributes drive loop prevention and decision-making. SR OS questions here usually ask which attribute caused a route to be rejected, or which FSM state a stuck session is sitting in.
Core mechanics
BGP runs over TCP port 179 and therefore inherits TCP's reliability - it has no retransmission timers of its own. Every message begins with a 19-byte header: 16-byte Marker (all ones), 2-byte Length (19–4096), 1-byte Type. Types are OPEN (1), UPDATE (2), NOTIFICATION (3), KEEPALIVE (4) and ROUTE-REFRESH (5, RFC 2918). An UPDATE carries Withdrawn Routes, Path Attributes and NLRI; a prefix is withdrawn explicitly or implicitly when re-advertised with new attributes.
The FSM runs Idle → Connect → Active → OpenSent → OpenConfirm → Established. "Active" does not mean working - it means the router is retrying the TCP connection, so a session stuck there usually points at reachability, TTL or a filter rather than BGP policy.
Loop prevention differs by session type. eBGP prepends the sender's AS to AS_PATH and a router rejects any route whose AS_PATH already contains its own AS. iBGP does not prepend and does not re-advertise iBGP-learned routes to other iBGP peers, which is why full mesh or route reflectors are required.
Worked example
Router A (AS 65001) receives 203.0.113.0/24 from eBGP peer B (AS 65002) with AS_PATH "65002". A advertises it to iBGP peer C inside AS 65001 with AS_PATH still "65002" and NEXT_HOP still B's address. C cannot use the route until A sets next-hop-self or C has a route to B's next hop - the classic "route present but not best" symptom.
Traps
Confusing Active with Established; forgetting iBGP leaves NEXT_HOP unchanged; assuming KEEPALIVE carries a hold time; and mixing up well-known mandatory attributes (ORIGIN, AS_PATH, NEXT_HOP) with optional nontransitive ones such as MED.