BGP Fundamentals and Protocol Operation
This domain tests whether you know exactly what BGP puts on the wire and how a Nokia SR OS router turns a received UPDATE into a BGP RIB entry and then into a best path.
Messages and transport
BGP-4 (RFC 4271) runs over TCP port 179 and is incremental: a prefix is advertised once and stays valid until withdrawn or the session drops. The four base messages are OPEN (version, AS number, hold time, router ID, optional capabilities), UPDATE (NLRI plus path attributes, or withdrawals), KEEPALIVE (sent at one third of the negotiated hold time) and NOTIFICATION (error code/subcode, then close). ROUTE-REFRESH (RFC 2918) is negotiated as a capability and lets an SR OS router re-request a given AFI/SAFI without bouncing the peer.
Attribute classes
- Well-known mandatory: ORIGIN, AS_PATH, NEXT_HOP.
- Well-known discretionary: LOCAL_PREF (iBGP scope only), ATOMIC_AGGREGATE.
- Optional transitive: COMMUNITY, EXTENDED COMMUNITY, AGGREGATOR.
- Optional non-transitive: MED (MULTI_EXIT_DISC).
SR OS configuration model
configure router bgp holds group and neighbor containers. A neighbor carries peer-as, the enabled family list, timers and import/export policy. Neighbour-level policy overrides the group-level policy it inherited, so attaching a policy at neighbour level can silently wipe out the group filter - a classic outage cause.
Worked example
A router in AS 65001 peers with 65010 and receives 10.10.10.0/24 with AS_PATH "65010 65001" and NEXT_HOP 192.0.2.1. SR OS inspects the AS_PATH on receipt from the eBGP peer, finds its own AS number, and discards the update before any import policy is evaluated. show router bgp routes 10.10.10.0/24 shows nothing, and adding an accept policy changes nothing. The correction must happen on the sender, or be an intentional local-as/loop-detection design decision.
Traps that cost marks
- Believing an import policy can rescue a looped AS_PATH: loop detection runs first.
- Expecting LOCAL_PREF to cross an eBGP boundary; it never leaves the local AS.
- Treating MED as transitive; it is not passed to the next AS.