IP Connectivity: Routing Fundamentals for CCNA

How Routers Forward Packets

A router examines the destination IP address of each incoming packet and consults its routing table to determine the best path. The routing table contains network prefixes, the next-hop address or exit interface, and the metric (cost) of each route.

Routing Table Sources

• Directly connected routes: Added automatically when an interface is configured with an IP address and is in an up/up state.
• Static routes: Manually configured by an administrator using the ip route command. A default static route (0.0.0.0/0) forwards packets with no more specific match.
• Dynamic routes: Learned automatically via routing protocols such as OSPF (Open Shortest Path First), EIGRP, or RIP.

Administrative Distance (AD)

When a router learns about the same destination from multiple sources, it uses Administrative Distance to select the most trusted source. Lower AD is preferred.

• Directly connected: AD 0
• Static route: AD 1
• OSPF: AD 110
• EIGRP: AD 90
• RIP: AD 120

Longest Prefix Match

When multiple routes match a destination, the router selects the route with the longest (most specific) prefix. For example, a /28 route is preferred over a /24 route for the same destination address.

OSPFv2 Basics (Single Area)

OSPF is a link-state routing protocol that uses cost (based on interface bandwidth) as its metric. Key points for CCNA:

• Router ID is the highest loopback IP or highest active interface IP if no loopback exists, or manually configured.
• Neighbors form adjacencies by exchanging Hello packets on the same subnet with matching Hello/Dead timers, area ID, and subnet mask.
• The network command with wildcard mask, or ip ospf interface command, enables OSPF on interfaces.
• Default reference bandwidth is 100 Mbps; FastEthernet and higher interfaces may share the same cost unless the reference bandwidth is adjusted.

Verifying IP Connectivity

• show ip route — displays the routing table.
• show ip ospf neighbor — verifies OSPF adjacencies.
• ping and traceroute — test reachability and identify hops.

Understanding how these components interact allows you to configure and troubleshoot routed networks, which is a core skill tested on the CCNA exam.