What is the difference between a RIB and a FIB?
In the context of Cisco networking, RIB (Routing Information Base) and FIB (Forwarding Information Base) are two important concepts used in the forwarding of IP packets.
The RIB is a software database that stores all the routing information learned by the router through different routing protocols such as OSPF, BGP, EIGRP, etc. It maintains a complete picture of the network topology and the available routes. The RIB contains all the routing information received from various routing protocols and applies administrative distance and other routing policies to select the best path to a destination network. RIB also calculates and stores the next-hop IP address for each destination network.
The FIB, on the other hand, is a hardware database that stores the forwarding information for each known destination network. It is generated from the RIB and used by the forwarding engine to make forwarding decisions for incoming IP packets. The FIB contains the next-hop IP address for each destination network and the outgoing interface to reach the next-hop.
When an IP packet arrives at a router, the router looks up the destination IP address in the FIB to determine the next-hop IP address and outgoing interface. The FIB is updated based on the information in the RIB, and any changes to the routing table are first made in the RIB and then propagated to the FIB.
To summarize, the main difference between the RIB and FIB is that the RIB is a software database that stores all the routing information learned by the router, while the FIB is a hardware database that contains the forwarding information for each known destination network. The FIB is generated from the RIB and is used by the forwarding engine to make forwarding decisions for incoming IP packets.