BGP Route Determination and the BGP Decision Process
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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:
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