This means that if there are multiple paths within the AS to a BGP next hop, the router prefers the path with the lowest IGP metric to get there. Prefer the path within the autonomous system with the lowest IGP metric to the BGP next hop. You can see an example in the prefer eBGP over iBGP lesson. Prefer eBGP (external BGP) over iBGP (internal BGP) paths. For a detailed explanation, take a look at the MED lesson.

The MED is exchanged between autonomous systems. You can learn how it works in the origin code lesson.

IGP is lower than EGP, and EGP is lower than INCOMPLETE. You can learn more about AS path length here. For example, AS path 1 2 3 is preferred over AS path 1 2 3 4 5. Prefer the path with the shortest AS path length. You can see it in action in the BGP attribute locally originated lesson. A BGP router will prefer routes that it installed into BGP itself over a route that another router installed in BGP. You can get a path in the BGP table through the BGP network command, redistribution, or aggregation. In the BGP table, you will see next hop 0.0.0.0. Prefer the path that the local router originated. To learn more, look at the BGP local preference attribute lesson. We prefer the path with the highest local preference. The local preference is used within an autonomous system and exchanged between iBGP routers. You can learn how it works in the BGP weight attribute lesson. The default value is 0 for all routes not originated by the local router. This is a value that is local to the router, and it’s Cisco proprietary. We will cover these in other lessons in detail. Let me give you a quick overview of each attribute. This path was selected based on the following attributes: Out of all those five paths, why did BGP select this path as the best path? Attributes This path will be installed in the routing table. The > symbol means that BGP has selected this path as the best path. This BGP router has five paths for network 1.0.0.0/24. Network Next Hop Metric LocPrf Weight Path Origin codes: i - IGP, e - EGP, ? - incomplete

R RIB-failure, S Stale, m multipath, b backup-path, x best-external Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, Below, I have the output of the BGP table of a looking glass server: > show ip bgpīGP table version is 781755060, local router ID is 203.202.125.6 If you haven’t seen a BGP table before, check out the how to read a BGP table lesson first. Let’s look at a quick example of an actual BGP router. Which path should it select as the best BGP path? The one from R2, its iBGP neighbor? Or one of the eBGP paths from R3, R4, or R5? In the picture below, R1 learns about 5.5.5.0/24 from multiple BGP neighbors: Whether the router will install the BGP best path in the routing table depends on the administrative distance. The prefix also has to be exactly the same. That means that BGP has to decide which path to use when it learns about the same prefix through BGP. We are talking about the BGP best path here.

Magic subnetting chart