BGP route selection design process on Juniper routers

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When a Juniper router has one or more BGP neighbors, the route/path selection process is:

  • Step 1: Can the BGP next-hop be resolved ?
  • Step 2: Prefer the path with the highest local preference value
  • Step 3: Prefer the path with the shortest AS-path length
  • Step 4: Prefer the path with the lowest origin value
  • Step 5: Prefer the path with the lowest MED value
  • Step 6: Prefer the path learned using EBGP over paths learned using IBGP
  • Step 7: Prefer path with the lowest IGP metrics:
    • a. Examine route table inet.0 and inet.3 for the BGP next-hop, and then install the physical next-hop(s) found in inet.3
    • b. For preference ties, install the physial next-hop(s) found in inet.3
    • c. For preference ties within the same route table, install the physical next-hop(s) where the greater number of equal-cost paths exists
  • Step 8: Prefer paths with the shortest cluster length
  • Step 9: Prefer routes from the peer with the lowest router ID (RID), unless multipath is enabled
    • a. For external routes from different ASs, do not alter the active route based on the lowest RID to prevent MED oscillation

Step 10: Prefer routes from the peer with the lowest peer ID (BGP peering address), unless multipath is enabled

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