4.1. Routing Table Components

💡 First Principle: When a packet arrives at a router, the router asks one question: "Where do I send this?" The routing table provides the answer. Think of it like GPS navigation—the router looks up the destination and gets directions (next hop and exit interface). Without a matching route, the packet gets dropped and connectivity fails.

Consider this troubleshooting scenario: Users report "I can't reach the server at 10.5.5.100." You SSH to the router and run show ip route 10.5.5.100. If it says "Routing entry for 10.5.0.0/16" with a valid next-hop, the router knows the path—look elsewhere. If it says "% Network not in table," you've found the problem: the router has no idea where 10.5.5.0/24 is. Add a static route or fix your routing protocol.

What happens with a bad routing table: Routes pointing to the wrong next-hop send traffic into black holes. Missing routes drop packets silently. Suboptimal routes create latency. The routing table is your first stop for any "can't reach" problem.

Routing Table Entry Structure

Each routing table entry contains:

O    172.16.0.0/16 [110/20] via 192.168.1.2, 00:05:23, GigabitEthernet0/1
│    │              │   │    │              │          │
│    │              │   │    │              │          └─ Exit interface
│    │              │   │    │              └─ Age (time since learned)
│    │              │   │    └─ Next-hop IP address
│    │              │   └─ Metric (OSPF cost)
│    │              └─ Administrative distance
│    └─ Destination network and prefix length
└─ Routing protocol code (O = OSPF)