Border Gateway Protocol (BGP) is a routing protocol used in the Internet to exchange routing information between different Autonomous Systems (AS). BGP is based on the path-vector protocol which allows a router to make routing decisions based on the complete path of an IP packet, rather than simply the next-hop router.
BGP path selection is based on a set of rules, which are commonly referred to as the BGP decision-making process. The BGP decision-making process uses a combination of factors to select the best path to a specific destination network. The following are the factors that influence the BGP path selection process:
1. Highest Local Preference: The Local Preference (LP) is used to indicate the preferred path for incoming traffic within an Autonomous System (AS). A BGP router selects the path with the highest Local Preference value as the preferred route. For example, if an AS has two connections with different Local Preference values, BGP selects the path with the higher Local Preference value.
2. Shortest AS Path: BGP also prefers the path with the shortest Autonomous System path. This means that BGP routers will prefer a path that traverses the fewest number of ASes. For example, if an IP packet has two paths, one with three ASes and the other with five ASes, BGP will select the path with three ASes as the preferred route.
3. Origin Type: BGP also takes into account the type of the route’s origin. For example, BGP prefers a route that has an origin in the local AS (e.g., originated by the local router) over a route that has an origin in a neighboring AS.
4. Multi-Exit Discriminator (MED): Sometimes referred to as the "metric," the MED is a value that is used by the local AS to indicate the preferred path for outgoing traffic. BGP selects the path with the lowest MED value as the preferred route. However, MED is only used when comparing routes from the same neighboring AS.
5. eBGP over iBGP: BGP prefers a route that is learned from an External BGP (eBGP) peer over a route learned from an Internal BGP (iBGP) peer. This is because eBGP routes provide better reachability than iBGP routes.
6. Lowest IP address: If all other factors are equal, BGP selects the path with the lowest router ID or IP address as the preferred route.
To illustrate the BGP decision-making process, consider a scenario where there are two paths for a specific destination network. Path 1 has a Local Preference of 200, an AS path length of three, and an origin type of eBGP. Path 2 has a Local Preference of 150, an AS path length of four, and an origin type of iBGP. In this scenario, BGP would select Path 1 as the preferred route because it has a higher Local Preference value, a shorter AS path, and was learned from an eBGP peer.
In conclusion, BGP path selection is a complex process that takes into consideration several factors that influence the routing decision. Understanding these factors can help network engineers optimize their network’s routing policies and ensure that traffic is being routed efficiently.