BGP is an Interdomain routing protocol connecting different networks, and is primarily used to Interconnect different entities and organizations. So when we design our network infra as a dual or multi homed network and providing partial or full internet routing to a downstream customer, we will use BGP.

An Autonomous System (AS) is a single or groups of networks under single administration. AS shares the routing information to each other using an Interior Gateway Protocol (IGP).

BGP tries to narrow its path selection down to one best path; it does not load balance by default. To do so, it examines the path attributes of any loop-free, synchronized (if synchronization is enabled) routes with a reachable next-hop in the following order:

• Choose the route with the highest weight.
• If weight is not set, choose the route with the highest local preference.
• Choose routes that this router originated.
• Choose the path with the shortest Autonomous System path.
• Choose the path with the lowest origin code (i is lowest, e is next, ? is last).
• Choose the route with the lowest MED, if the same Autonomous System advertises the possible routes.
• Choose an EBGP route over an IBGP route.
• Choose the route through the nearest IGP neighbor as determined by the lowest IGP metric.
• Choose the oldest route
• Choose a path through the neighbor with the lowest router ID.
• Choose a path through the neighbor with the lowest IP address.

BGP chooses a route to a network based on the attributes of its path. Four categories of attributes exist as follows:
* Well-known mandatory: Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers. For example, AS path, origin, and next hop.
* Well-known discretionary: Must be recognized by all BGP routers and passed on to other BGP routers but need not be present in an update, for example, local preference.
* Optional transitive: Might or might not be recognized by a BGP router but is passed on to other BGP routers. If not recognized, it is marked as partial, for example, aggregator, community.
* Optional nontransitive: Might or might not be recognized by a BGP router and is not passed on to other routers, for example, Multi-Exit Discriminator (MED), originator ID.

This is feature is used to split an autonomous system into smaller autonomous systems or the reverse which is to combine several autonomous systems into one.