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

BGP Routing Information Bases (RIBs)

The heart of BGP's system of routing information management and handling is the database where routes are stored. This database is collectively called the Routing Information Base (RIB), but it is in fact not a monolithic entity. It is comprised of three separate sections that are used by a BGP speaker to handle the input and output of routing information. Two of these sections themselves consist of several individual parts, or copies.

The three RIB sections (using the cryptic names given them by the BGP standards, sorry!) are:

• Adj-RIBs-In: A set of input database parts that holds information about routes received from peer BGP speakers.
  
  
• Loc-RIB: The local RIB. This is the core database that stores routes that have been selected by this BGP device and are considered valid to it.
  
  
• Adj-RIBs-Out: A set of output database parts that holds information about routes that this BGP device has selected to be disseminated to its peers.

Thus, the RIB can be considered either a single database or a set of related databases, depending on how you look at it. (The divisions above are conceptual in nature; the entire RIB can be implemented as a single database with an internal structure representing the different components, or as separate databases.)

The RIB is a fairly complex data structure, not just because of this multi-section structure, but because BGP devices store considerably more information about routes than simpler routing protocols. Routes are also called paths in BGP, and the detailed descriptions of them are stored in the form of special BGP path attributes.

The three sections of the RIB are the mechanism by which information flow is managed in a BGP speaker. Data received from Update messages transmitted by peer BGP speakers is held in the Adj-RIBs-In, with each Adj-RIB-In holding input from one peer. This data is then analyzed and appropriate portions of it selected to update the Loc-RIB, which is the main database of routes this BGP speaker is using. On a regular basis, information from the Loc-RIB is placed into the Adj-RIBs-Out to be sent to other peers using Update messages. This information flow is accomplished as part of the system of route update, selection and advertisement known as the BGP Decision Process.

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