In Cisco networking, the Routing Information Base (RIB) and Forwarding Information Base (FIB) are two separate but related data structures used for managing the flow of data in a network. Both have different roles and are created and maintained for different reasons.
Routing Information Base (RIB): The RIB is a data structure used by routers to store all of the possible routes that are known by the router. The RIB is a dynamic database that maintains a list of all available routes and the quality of each route. This includes routes from static configuration as well as routes learned from dynamic routing protocols like OSPF, EIGRP, BGP, etc. When there are multiple routes to the same destination, the routing protocol’s algorithm decides the best path, and this path is placed into the routing table (a subset of the RIB). The RIB essentially helps make decisions about where to send traffic based on the best available route.
Forwarding Information Base (FIB): The FIB is another data structure, and it’s directly related to the RIB. After the router has determined the best path for each destination, these routes are placed into the FIB. Unlike the RIB, the FIB contains only the best routes from the RIB that the router will use to forward packets. In other words, the FIB is used to make forwarding decisions. When a packet arrives at a router, the router examines the destination IP address and then looks up the FIB to decide where to send the packet next. This process is typically faster than looking up the RIB because it contains fewer entries (only the best paths). This allows for very efficient, high-speed routing decisions.
In summary, the RIB is used for making routing decisions, i.e., determining the best path, while the FIB is used for making forwarding decisions, i.e., deciding where to send packets next based on those best paths.
WHAT ABOUT CEF?
Cisco Express Forwarding (CEF) is a network switching technology used in routers and switches to enable rapid packet forwarding. CEF relates directly to the Forwarding Information Base (FIB) and indirectly to the Routing Information Base (RIB). Here’s how:
CEF and FIB: CEF makes use of a data structure similar to the FIB to store the best path information. This information includes next-hop addresses for all known routes. The FIB in CEF is constantly updated and synchronized with the routing table (or RIB). It’s used for making extremely quick forwarding decisions. When a packet arrives at a router, CEF can quickly look up the FIB and make an immediate forwarding decision, instead of having to process the packet using the CPU (as would be required in process-switched or fast-switched environments). This significantly speeds up the packet forwarding process and reduces latency.
CEF and RIB: CEF indirectly relates to the RIB in the sense that changes to the routing table (RIB) cause updates to the FIB. When a best path is determined by the routing protocol and inserted into the routing table (RIB), CEF will automatically update the FIB with this new information, so that it always has the latest and most efficient routes for forwarding packets. In this way, the FIB under CEF acts as a “mirror” of the best paths in the RIB, optimized for quick lookup and forwarding.
So, CEF is essentially a more advanced and efficient method of forwarding packets than traditional methods. By keeping a FIB that’s synchronized with the routing table (RIB), CEF is able to make rapid forwarding decisions, increasing the overall performance and efficiency of packet forwarding within the network.
CONTROL PLANE VS DATA PLANE AND HOW IT RELATEDS TO THE RIB AND FIB
In the context of network architecture, the Control Plane and the Data Plane are crucial concepts that help explain how data is handled in a network, and they indeed relate to concepts such as Cisco Express Forwarding (CEF), the Routing Information Base (RIB), and the Forwarding Information Base (FIB).
Control Plane: This is responsible for how the network decides to route packets. Essentially, it’s where the network’s “brains” are, and it includes all the protocols that determine the best path for data to travel, like OSPF, EIGRP, BGP, etc. It also includes the process of building and maintaining routing tables (or RIB). The control plane dictates how to build the FIB by selecting the best routes from the routing table. It’s about making decisions on how data should flow.
Data Plane (or Forwarding Plane): Once the Control Plane has decided the best paths, the Data Plane is responsible for actually forwarding the packets of data based on these decisions. This involves the use of the FIB. The data plane doesn’t make any decisions about where data should go. It simply forwards the data according to the instructions given by the Control Plane. Cisco Express Forwarding (CEF) operates at this level by using the FIB and adjacency table for packet forwarding.
CEF, FIB, and RIB all play crucial roles in the operation of both the Control and Data Planes.
The Control Plane uses protocols and the RIB to make decisions about the best path for data to travel.
The Data Plane uses the FIB (populated by the Control Plane) and technologies like CEF to actually forward the data to its destination.
Remember that these planes are conceptual divisions and not necessarily distinct hardware or software components. They represent different types of operations that occur within a network device like a router or a switch.
To view the tables: RIB – show ip route FIB – show ip cef, show adjacency
To view specific protocol RIBs: OSPF – show ip ospf rib EIGRP – sh ip eigrp topology BGP – show ip bgp