Border Gateway Protocol (BGP) is a path-vector routing protocol used to exchange routing and reachability information between autonomous systems (ASes) on the Internet. BGP is the protocol of choice for Internet Service Providers (ISPs) and is used to route traffic across the Internet. BGP is a critical component of the Internet's infrastructure and is responsible for ensuring that data is routed to its intended destination.
When it comes to routing traffic, BGP relies on the best path selection algorithm to determine the most optimal route for each packet. This algorithm is responsible for selecting the route that has the fewest number of hops and the best path characteristics. The BGP best path selection algorithm takes into account various criteria such as origin, local preference, AS path length, MED, and IGP metric.
The origin attribute is the first criteria that the algorithm will consider. This attribute determines the route source, which can be either internal or external. Internal routes are typically given a higher preference than external routes.
The local preference attribute is a value that is assigned to routes by the router itself. This attribute is used to influence the selection of the best path to a particular destination. Routes with a higher local preference value are preferred over routes with a lower value.
The AS path length attribute is a metric that is used to measure the number of Autonomous Systems (ASes) that a packet must travel through in order to reach its destination. The shorter the AS path length, the more desirable the route will be.
The MED (Multi-Exit Discriminator) attribute is a metric that is used to influence the selection of the best path to a particular destination. MED is used to compare routes that have been learned via different external BGP peers. A route with a lower MED value will be preferred over a route with a higher MED value.
Lastly, the IGP metric is a metric used to determine the best route based on its cost. The lower the cost, the more desirable the route will be.
The BGP best path selection algorithm is responsible for selecting the best route for each packet. In order to do this, it takes into account various criteria such as origin, local preference, AS path length, MED, and IGP metric. By taking these factors into consideration, the algorithm is able to ensure that data is routed to its intended destination in the most efficient manner possible.