# Glossary of Terms

Terms used in descriptions throughout this document are defined below:

adjacency
  A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers becomes adjacent.
autonomous system
  A set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other ASs. Since this classic definition was developed, it has become common for a single AS to use several interior gateway protocols and sometimes several sets of metrics within an AS. The use of the term Autonomous System stresses that even when multiple igps and metrics are used, the administration of an AS appears to other ASs to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it. The AS is represented by a number between 1 and 65534, assigned by the Internet Assigned Numbers Authority.
BGP
Border Gateway Protocol
  One of a class of exterior gateway protocols, described in more detail in the BGP section of the Protocol Overview.
cost
  An OSPF metric. See metric.