What is Basic Routing
Routing is the most fundamental element of a data communication network. Routing information is the path information that tells a device where to send a packet, and the process of routing is, in practice, the process of forwarding packets from source to destination across one or more Layer 3 devices.
"Basic routing" covers the foundations that every more advanced routing feature builds on: how a device stores and selects routes, how routes are categorized, and how an administrator can define paths manually using static routes and default routes. Dynamic routing protocols such as BGP, OSPF, and IS-IS all feed into the same underlying structures, so understanding the basics makes everything built on top of them easier to reason about.
How Basic Routing Works
At the heart of a router or Layer 3 switch is the RIB (Routing Information Base), a centralized database of routing information. It holds the routing table itself, along with routing-related supporting information such as route iteration, route sharing, and route extension data. Routes from every source — directly connected networks, static configuration, and dynamic routing protocols — all land in the RIB.
From there, the device selects the preferred route for each destination and installs it into the FIB (Forwarding Information Base). The FIB is what actually guides packet forwarding: each entry specifies the physical interface through which a packet bound for a given subnet or host should be sent, so that it reaches the next router along the path, or is delivered straight to the destination host if that host sits on a directly connected network. In short, the RIB decides, and the FIB forwards.
Routes can be classified in several ways. By source, there are three categories:
Directly connected routes: discovered by link-layer protocols and also known as interface routes, these exist automatically for networks attached to the device's own interfaces.
Static routes: configured manually by the network administrator. They are simple to set up and have low system requirements, which suits small networks with a simple, stable topology, but they do not adapt automatically to topology changes.
Dynamic routes: discovered by routing protocols, which run their own routing algorithms and can adapt automatically to topology changes. They suit networks with a meaningful number of Layer 3 devices, at the cost of more demanding configuration and higher consumption of network and system resources.
By destination, a route is either a segment route (the destination is a network with a prefix shorter than /32 for IPv4 or /128 for IPv6) or a host route (a /32 IPv4 or /128 IPv6 destination). And by reachability, a route is either directly connected (the destination network attaches straight to the router) or indirect (it does not, so the packet must pass through at least one more router).
A static route is defined by a destination address and prefix, an egress interface and/or next-hop address, and a priority. How the next hop is specified depends on the interface type. On point-to-point interfaces, naming the egress interface is enough, because the peer on the other end is implicitly the next hop. On NBMA interfaces, the next-hop IP address is configured, together with a link-layer address mapping. On broadcast interfaces such as Ethernet, the next-hop address must be specified, because multiple devices could be reachable through the same interface and the next hop cannot otherwise be determined uniquely.
Static route priority works on a "smaller number wins" basis. When multiple static routes to the same destination are configured with the same priority, they provide load sharing; when configured with different priorities, they provide route backup.
A default route is a special route with an all-zero destination and an all-zero prefix (0.0.0.0/0 for IPv4). It is used whenever no more specific entry in the routing table matches a packet's destination. If no default route exists and the destination is not in the routing table, the packet is discarded and an ICMP message is returned to the source reporting that the destination network is unreachable.
Why Basic Routing is Beneficial
Simple and predictable: Static and default routes are easy to configure and highly controllable, so the path traffic takes is exactly the path the administrator defined.
Low overhead: Static routes consume little bandwidth and use no CPU to calculate or exchange route updates, which leaves more resources available for forwarding.
Enough for simple networks: When the network structure is simple, static routes alone can be all it takes to make the network work properly.
A tool for steering traffic in complex networks: In more complicated environments, deliberately placed static routes can improve performance and guarantee bandwidth for critical applications.
Useful for VPN route management: Static routes can be used within VRFs (VPN instances), where they are mainly used for managing VPN routes.
A clean RIB/FIB separation: Because the RIB selects routes and the FIB forwards on them, route computation and packet forwarding are decoupled, which keeps forwarding fast regardless of how many routing sources feed the table.
The trade-off is that static routes do not update themselves. When a link fails or the topology changes, they must be reconfigured by hand, and adjusting static routes across a large, changing network quickly becomes difficult. That is why static routing is best suited to small or stable networks, or to specific, deliberate path control alongside dynamic routing protocols.