BGP AS Path

  • Multiple path attributes to determine the best path for a certain prefix.
  • When no path attributes are set, BGP uses the AS_PATH to determine the best route.
    • Shortest AS path wins
  • AS_SEQ is the component of the AS_PATH attribute.
    • 10.10.10.0/24 20 10
      • AS-SEQ (in order)
  • With summarization:
    • 10.10.0.0/16 20 10 { 1 2 5 4 3 }  
    • Prefix AS-PATH AS-SEQ  { AS-SET } 
      • AS-SET are not in order
  • Prevents Routing loops.
  • If a BGP router received an update with it’s own AS#, it indicates a loop.
  • If a BGP router received an update with it’s own AS#, it drops the route.

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