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 Path Attributes and Algorithm Overview 1 2 3 BGP General Operation and Messaging

The last two topics have described the fundamentals of how BGP devices store and manage information about routes to networks. This included an overview of the four route information management activities performed by BGP speakers: route storage, update, selection and advertisement. Route storage is the function of the Routing Information Base (RIB) in each BGP speaker. Path attributes are the mechanism by which BGP stores details about routes and also describes those details to BGP peers.

As we have seen, the RIB also contains sections for holding input information received from BGP peers, and for holding output information each BGP device wants to send to those peers. The functions of route update, selection and advertisement are concerned with analyzing this input information, deciding what to include in the local database, updating that database, and then choosing what routes to send from it to peer devices. In BGP, a mechanism called the Decision Process is responsible for these tasks. It consists of three overall phases:

1. Phase 1: Each route received from a BGP speaker in a neighboring AS is analyzed and assigned a preference level. The routes are then ranked according to preference and the best one for each network advertised to other BGP speakers within the autonomous system.
   
   
2. Phase 2: The best route for each destination is selected from the incoming data based on preference levels, and used to update the local routing information base (the Loc-RIB).
   
   
3. Phase 3: Routes in the Loc-RIB are selected to be sent to neighboring BGP speakers in other ASes.

BGP Path Attributes and Algorithm Overview 1 2 3 BGP General Operation and Messaging