BGP: Decision Process

The BGP decision process uses PAs to determine the best path to be injected into the IPv4 routing table when there are multiple routes to the same destination in the BGP table.

The following are used by BGP to determine the best path, with three additional tiebreakers listed later:

  1. is the NEXT_HOP reachable? if the router does not have a route to the NEXT_HOP PA for a route it will be rejected in the decision process
  2. Highest administrative weight: Weight is a cisco proprietary PA, the weight can be assigned to each NLRI locally on a router, the value is not communicated to other routers, the higher the value the better the route.
  3. Highest LOCAL_PREF PA: This well known discretionary PA can be set on a router inside an AS, and distributed within the AS only.  As a result this feature can be used by all BGP routers in one AS to choose the same exit point from their AS for a particular NLRI.  The higher the value the better.
  4. Locally injected routes: Picks locally injected routes such as redistribution, summarization, or routes injected via the network command.
  5. Shortest AS_PATH length: the shorter the AS path to get to a destination the better the route.  the length calculation ignores the AS_CONFED_SET and AS_CONFED_SEQ PAs and treats an AS_SET as one ASN regardless of the number of ASNS in the AS_SET.  It counts each ASN in the AS_SEQUENCE as one.
  6. ORIGIN PA: IGP routes are prefferred over EGP routes, which are preferred over incomplete routes
  7. Smalles Multi-Exit Discriminator (MED) PA: This PA allows an ISP with multiple peer connections to a neighboring AS to tell a neighboring AS which of the peer connections is best for reaching a particular NLRI
  8. Neighbor Type: Prefer external BGP (eBGP) routes over internal BGP (iBGP) confederation eBGP is equal to iBGP
  9. IGP metric for reaching the NEXT_HOP: IGP metrics for each NLRI’s NEXT_HOP are compared.  The lower the value, the better the route.

The three tiebreakers are as follows:

10. Keep the oldest eBGP route, If the routes being compared are eBGP and one of the paths is currently the best path retain the existing best path.

11. Choose the smallest neighbor router ID (RID).  Use the route whose next hop RID is the smallest, this is only performed if the bgp bestpath compare-routerid command is configured

12. Smallest neighbor ID, the local router requires at least two neighbor relationships to the same router to get to this step.


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