1. Longest Prefix Match
e.g. /24 is chosen over /16 because it is more specific.
2. Administrative Distance (AD)
AD is used when there are multiple routes for the same destination and prefix length, for example static (10) wins over BGP (20).
3. Route Priority
If the prefix and AD are the same, then priority is used.
Route A: 172.21.2.0/24 [10/5]
Route B: 172.21.2.0/24 [10/10]The lower priority wins, i.e. 5.
4. BGP best-path selection
For multiple BGP paths to the same prefix, BGP attributes determine which path wins.
Weight
Local Preference
Locally originated
AS-path length
Origin
MED
eBGP vs iBGP
IGP cost to next hop