The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)                      9  TCP/IP Border Gateway Protocol (BGP/BGP-4)                           9  BGP Fundamentals and General Operation

BGP Autonomous System Types, Traffic Flows and Routing Policies 1 2 BGP Path Attributes and Algorithm Overview

The job of the Border Gateway Protocol is to facilitate the exchange of route information between BGP devices, so that each router can determine efficient routes to each of the networks on an IP internetwork. This means that descriptions of routes are the key data that BGP devices work with. Every BGP speaker is responsible for managing route descriptions according to specific guidelines established in the BGP standards.

Conceptually, the overall activity of route information management can be considered to encompass four main tasks:

• Route Storage: Each BGP stores information about how to reach networks in a set of special databases. It also uses databases to hold routing information received from other devices.
  
  
• Route Update: When a BGP device receives an Update from one of its peers, it must decide how to use this information. Special techniques are applied to determine when and how to use the information received from peers to properly update the device's knowledge of routes.
  
  
• Route Selection: Each BGP uses the information in its route databases to select good routes to each network on the internetwork.
  
  
• Route Advertisement: Each BGP speaker regularly tells its peers what it knows about various networks and methods to reach them. This is called route advertisement and is accomplished using BGP Update messages. We'll learn more about these messages in the next few topics, and examine them in detail in the detailed section on BGP message formats.

BGP Autonomous System Types, Traffic Flows and Routing Policies 1 2 BGP Path Attributes and Algorithm Overview