What is the BGP path selection criteria?

 

BGP select a best route on the basis of Path attributes. EachPA defines something different about the path.

 

Weight, Local preference and AS_Path are used to influence outbound routes, and MED is used to influence inbound routes.

 

1)  Weight : Cisco proprietary, higest is preferred. it is not shared with any BGP peer. 2) Local preference: Higest is preferred, shared between IBGP neighbors in single autonomus system. It is used to influence best route to go out of AS.

3) Locally injected:   2 methods to inject a route : Network command, Redistribute routes in BGP.

4) AS Path:  Shortest AS_Path is preferred. 

5) Origin Type : Prefer the path with the lowest origin type. IGP is prefferd over EGP over incomplete routes. 6)  MED : a numeric value, advertised by routers in one AS, to impact the BGP decision of routers in theother AS. Smaller metric is better.

 

7)  Neighbor Type :   Prefer eBGP over iBGP paths.  8)  lowest IGP metric to the BGP next hop is preferred.

if still best path is not determined. then

Oldest (longest-known) eBGP routeLowest neighbor BGP RIDLowest neighbor IP address

Border Gateway Protocol (BGP) routers typically receive multiple paths to the same destination.
The BGP best path selection criteria decides which is the best path to install in the IP routing table and to use for traffic forwarding.
Following is the path selection criteria-1.Prefer the path with the highest WEIGHT.2.Prefer the path with the highest LOCAL_PREF.3.Prefer the path that was locally originated via a network or aggregate BGP command or through redistribution from an IGP.4.Prefer the path with the shortest AS_PATH.5.Prefer the path with the lowest origin type.6.Prefer the path with the lowest multi-exit discriminator (MED).7.Prefer eBGP over iBGP paths.8.Prefer the path with the lowest IGP metric to the BGP next hop.9.Determine if multiple paths require installation in the routing table for BGP Multipath.10.When both paths are external, prefer the path that was received first (the oldest one).11.Prefer the route that comes from the BGP router with the lowest router ID.12.If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.13.Prefer the path that comes from the lowest neighbor address.

BGP considers the following criteria for best path selection.
1. 
     Prefer the path with the highest WEIGHT.
2. 
     Prefer the path with the highest LOCAL PREF.
3. 
     Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.
4. 
     Prefer the path with the shortest AS_PATH.
5. 
     Prefer the path with the lowest origin type.
6. 
     Prefer the path with the lowest multi-exit discriminator (MED).
7. 
     Prefer eBGP over iBGP paths.
8. 
     Prefer the path with the lowest IGP metric to the BGP next hop.
9. 
     Prefer the route that comes from the BGP router with the lowest router ID.
10. 
If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.
11. 
Prefer the path that comes from the lowest neighbor address.