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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