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| 9.2 |  | Basic BGP Operation |
| 9.2.1 |  | BGP routing updates |

BGP has been most recently defined in RFC 1772. The function of BGP is to exchange routing information between autonomous systems and guarantee the selection of a loop free path. BGP4 is the first version of BGP that supports CIDR and route aggregation. Common IGPs such as RIP, OSPF, and EIGRP use technical metrics. BGP does not use technical metrics. Instead, BGP makes routing decisions based on network policies, or rules.

This section offers a brief overview of how BGP works and is followed by a more detailed examination of the various types of BGP packets and relationship states.

BGP updates are carried using TCP on port 179. In contrast, RIP updates use UDP port 520, while OSPF does not use a Layer 4 protocol. Because BGP requires TCP, IP connectivity must exist between BGP peers. TCP connections must also be negotiated between them before updates can be exchanged. Therefore, BGP inherits those reliable, connection-oriented properties from TCP.

To guarantee loop free path selection, BGP constructs a graph of autonomous systems based on the information exchanged between BGP neighbors. BGP views the whole internetwork as a graph, or tree, of autonomous systems. The connection between any two systems forms a path. The collection of path information is expressed as a sequence of AS numbers called the AS Path. This sequence forms a route to reach a specific destination.