Border Gateway Protocol

aliases:

A specialized dynamic routing protocol for use in routing domains, internetworks, and connections between ISPs.

While other protocols like EIGRP and OSPF are used to route communications within a single routing domain (called an Autonomous System, or AS), BGP is used for routing between ASs. It avoids the impossible amount of storage that would be needed to store routing information between every Internet location. Instead, each ISP has an edge router that shares only the information necessary to reach other edge routers belonging to other ISPs. They are identified through Internet-Assigned-Numbers-Authority-assigned autonomous system numbers (ASNs) via the many regional registries.

BGP works on Transmission-Control-Protocol port 179.

BGP uses multiple metrics to for path selection, including:

hop count
weight
local preference
origin
community

The higher amount of metrics used in BGP makes it slower to converge.