What is routing, and how does a router decide where to forward a packet?

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Routing is the process of selecting the best path for network traffic to travel from a source to a destination across one or more networks. While switches forward traffic based on MAC addresses within a single network, routers operate at Layer 3 of the OSI model and forward packets between different networks using IP addresses.

When a router receives a packet, it examines the destination IP address in the packet header and consults its routing table to determine where to send the packet next. The routing table is essentially a map of known networks and the best path to reach each one. Each entry in the routing table contains the destination network and subnet mask, the next-hop IP address or exit interface, and a metric that indicates the cost or quality of that path.

The routing decision process follows these steps. First, the router extracts the destination IP from the packet. Then it compares this address against all entries in the routing table, looking for the most specific match. This is called longest prefix match. For example, if the routing table has entries for 10.0.0.0/8 and 10.1.0.0/16, a packet destined for 10.1.5.10 would match the /16 route because it is more specific. If no match is found, the router checks for a default route (0.0.0.0/0), which acts as a catch-all for traffic with no specific route. If there is no default route either, the packet is dropped and the router may send an ICMP Destination Unreachable message back to the sender.

Routing tables are populated in three ways. Connected routes are automatically added when a router interface is configured with an IP address and brought up. These represent the networks directly attached to the router. Static routes are manually configured by an administrator, specifying the destination network and next hop. Dynamic routes are learned from other routers through routing protocols like RIP, OSPF, EIGRP, or BGP.

Each routing source has an administrative distance (AD) that indicates its trustworthiness. Lower AD values are preferred. Connected routes have AD 0, static routes have AD 1, OSPF has AD 110, and RIP has AD 120. If a router learns about the same destination from multiple sources, it installs the route with the lowest AD.

To view the routing table on a Cisco router, use show ip route. This command shows all routes organized by source (C for connected, S for static, O for OSPF, and so on). Understanding how to read a routing table is one of the most fundamental skills for a network engineer.