BGP Routing Tables

Routers running the BGP protocol use different routing tables for different purposes. The main IP routing table contains routes obtained through IGP routing processes, such as RIP or Open Shortest Path First (OSPF), static, or directly connected networks. There are also three other conceptual BGP tab les, referred to as Routing Information Bases (RIBs), which contain only BGP-specific routing information. The BGP tables are used to store information about BGP paths. This information includes the best path to each destination network (used for local routing), the information to send to other BGP peers, and information obtained from other BGP peers. After BGP has selected the best path to a n etw ork, that path is added to the main IP routing table.

BGP uses two different routing tables to store incoming and outgoing network advertisements: Adj-RIB^n and Adj-RIB-Out. These tables store information received from other BGP speakers and information to be passed to other BGP peers. Each BGP speaker maintains one Adj-RIB-In and one Adj-RIB-Out per BGP peer relationship. The Adj-RIB-In table stores unprocessed BGP information learned from other BGP peers. The information contained in this table is used to determine the best path to the destination network, based on its BGP attributes, using the BGP route selection process. Information from this table is processed by the BGP finite-state machine (the local BGP routing process) and sent to the local BGP table. Information in the Adj-RIB-Out table is sent to other BGP peers.

After the local BGP decision process has finished selecting the best path to each destination network, information is stored in the local BGP table, which is known as the Loc-RIB. Loc-RIB stores information about paths that conform to the locally configured BGP policies. The BGP speaker learns these routes by means of local BGP configuration, or by ieatning the route through a BGP session with another BGP speaking router. Unlike the other two BGP routing tables, there is only one Loc-RIB per router (for IPv4 BGP routing). Each path in the Loc-RIB is accompanied by the following rout i ng data: the next -hop IF3 address used to reach the networks, the metric applied to the network path, the local preference of the path, the weight assigned to the path, the A S path used to retch ea ch of these networka, vahethem t he path was learned through an interior or exterior BGP process or undetermined network origin. If a local router is able to verify that the next hop to ehe route i s reachable— using an IGP route from the local routing table, a static route, or from a d irectly connected network—me BGe process se lecrs the come and stores i t in the main IIs rodting t ab^ Figu re 7-k Wustrates how the BGP routing tables noe used durin g the ex ch ange of1 BGP routes between two BGP peers, the Apples and Oranges routers, w h i l et exchangi ng rou res.

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