How does the RIB differ from the FIB?
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The RIB (Routing Information Base) and the FIB (Forwarding Information Base) are two important components of the routing table in a router. The main difference between the two lies in their functions and the way they store information.
The RIB is a list of all the routes known to the router, along with their attributes such as the next hop, metric, and administrative distance. It is essentially a database that holds all the routing information learned through various sources such as static routes, dynamic routing protocols like OSPF and BGP, and other sources like policy-based routing and tunneling protocols. The RIB is also responsible for selecting the best route among the many possible routes to the same destination prefix based on various metrics such as bandwidth, delay, and reliability. Once the best route is selected, it is then installed into the FIB for forwarding packets.
On the other hand, the FIB is a subset of the RIB that contains only the best routes to each destination prefix. The FIB is created by the router's forwarding engine, which uses the information in the RIB to build a table that maps destination prefixes to outgoing interfaces and next-hop addresses. The FIB is used by the router to make forwarding decisions quickly and efficiently, without having to consult the RIB every time a packet needs to be forwarded.
To summarize, the RIB is a database that contains all the routing information known to the router, and it is responsible for selecting the best route to a destination prefix. The FIB is a table that contains only the best routes to each destination prefix, and it is used by the router's forwarding engine to make fast and efficient forwarding decisions.