I was wondering about BGP's decision making process when it comes to whether a route is considered valid or not.
I know that a route can be considered valid sometimes when it is not even accessible in the case of NBMA networks or external routes learned from iBGP neighbors. I know that a route is considererd invalid if the receiving router AS is in the AS path. I also know that route filters can make a route invalid, but does anyone have a comprehensive list of what makes a route valid.
What is the step by step decision making process a router goes through when attempting to answer the question of route validity?
Thanks in advance.
Kindly check this Cisco document for a similar list:
BGP relies on the IGP for the actual traffic forwarding, it reports prefixes and their next-hop, route recursion and the actual forwarding depends on the IGP, and thus its a matter of IGP design and convergence, as long as the next-hop is reachable (according to the IGP), then the route is a candidate of BGP best path selection.