Border Gateway Protocol (BGP)

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• the internet = many Autonomous Systems (ASes), each with its own routing policy
• inside an AS → OSPF / IS-IS, optimize for shortest path
• between ASes → BGP, optimize for policy (who you'll carry traffic for, who you'd rather pay less to)
• eBGP between ASes, iBGP to sync that view across routers inside one AS

Mechanics

• path-vector protocol — each advertisement carries the full AS_PATH
• loop detection is free: see your own AS in the path → drop
• sessions run over TCP/179
  • open, exchange capabilities
  • dump full table once
  • incremental updates after that (announce / withdraw)
• TCP session = keep-alive; drop the session → withdraw everything learned from that peer

Path Attributes

• AS_PATH — ASes traversed, shorter usually better
• NEXT_HOP — where to forward packets
• LOCAL_PREF — "prefer this exit," set inside your AS, higher wins, not shared with eBGP
• MED — hint to a neighbor AS about which link into you to prefer, lower wins
• COMMUNITIES — opaque tags for policy ("don't export to peers", "prepend twice in Europe")

Best-path tiebreaker (roughly in order)

1. highest LOCAL_PREF
2. shortest AS_PATH
3. lowest origin type
4. lowest MED
5. eBGP over iBGP
6. lowest IGP cost to next hop
7. lowest router ID

policy > distance

Failure modes

• trust — BGP trusts its neighbors
  • AS announces a prefix it doesn't own → neighbors propagate → traffic pulled to the wrong place
  • route leak vs hijack depending on intent
  • defenses: RPKI (signed prefix → origin AS), edge filters. not universal yet
• convergence — a withdrawal can take tens of seconds to minutes to settle globally; every AS on the path re-runs best-path and re-advertises