BGP uses different metrics to select which path to send data. This is how BGP chooses paths.
| Attribute | Description/Notes | Winning Metric |
|---|---|---|
| Weight | local to the router | Highest |
| LOCAL_PREF | Within the ASN, 100 by default | Highest |
| Locally Originated | Local paths that are sourced by the network or redistribute commands are preferred over local aggregates that are sourced by the aggregate-address command | Local |
| AS_PATH | An AS_SET counts as 1 | Shortest |
| Origin Type | IGP < EGP < INCOMPLETE | Lowest (IGP) |
| MED | multi-exit discriminator | Lowest |
| External | Prefer eBGP over iBGP paths | eBGP |
| IGP metric | lowest IGP metric to the BGP next hop | Lowest |
| Oldest Path | Choose oldest to prevent flapping | Oldest |
| Router ID | router ID is the highest IP address on the router (prefer loopbacks) | Lowest |
For more details on how BGP best paths are chosen, see http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html#anc2
