BGP Decision Process

  1. Step 0
    • Next hop: is it reachable?
    • If no route to reach Next_Hop, router cannot use this route.
  2. Step 1
    • Weight
    • Bigger is better; Cisco proprietary
  3. Step 2
    • Bigger is better
  4. Step 3
    • Locally injected routes;
    • Locally injected is better than iBGP/eBGP learned
  5. Step 4
    AS_PATH length; Smaller
  6. Step 5
    Origin; Favor internal over external, external over ?
  7. Step 6
    MED; Smaller
  8. Step 7
    • Neighbor Type
    • Prefer eBGP over iBGP
  9. Step 8
    • IGP metric to Next_Hop;
    • Smaller
  10. Step 9
    Oldest (longest-known) eBGP route
  11. Step 10
    Lowest neighbor BGP RID
  12. Step 11
    Lowest neighbor IP address
  13. Idle
    The BGP process is either administratively down or awaiting the next retry at-tempt.
  14. Connect
    The BGP process is waiting for the TCP connection to be completed. You can-not determine from this state information whether the TCP connection cancomplete.
  15. Active
    The TCP connection has been completed, but no BGP messages have beensent to the peer yet.
  16. Opensent
    The TCP connection exists, and a BGP Open message has been sent to thepeer, but the matching Open message has not yet been received from theother router.
  17. Openconfirm
    An Open message has been both sent to and received from the other router.The next step is to receive a BGP Keepalive message (to confirm all neighbor-related parameters matched) or BGP Notification message (to learn there issome mismatch in neighbor parameters).
  18. Established
    All neighbor parameters match, the neighbor relationship works, and the peerscan now exchange Update messages.
Card Set
BGP Decision Process