3.1 Routing Table Components

The routing table is the most important data structure on a router. It is the database the router consults for every single packet to decide where to forward it. On the CCNA 200-301 exam (roughly 100-120 items, 120 minutes, passing near 825/1000 on a 300-1000 scale), reading show ip route output fluently is a core skill tested in both multiple-choice and simulation items.

Viewing the Routing Table

Router# show ip route

Sample Routing Table Output

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, O - OSPF, IA - OSPF inter area
       * - candidate default

Gateway of last resort is 10.0.0.1 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 10.0.0.1
C     10.0.0.0/30 is directly connected, GigabitEthernet0/0
L     10.0.0.2/32 is directly connected, GigabitEthernet0/0
C     192.168.10.0/24 is directly connected, GigabitEthernet0/1
L     192.168.10.1/32 is directly connected, GigabitEthernet0/1
O     192.168.20.0/24 [110/2] via 10.0.0.1, 00:05:23, GigabitEthernet0/0
S     192.168.30.0/24 [1/0] via 10.0.0.1
D     192.168.40.0/24 [90/3072] via 10.0.0.1, 00:15:42, GigabitEthernet0/0

Notice every connected (C) route is paired with a local (L) /32 route — the L route is the router's own interface address. The L entry is how the router knows a packet is destined for itself.

Route Entry Components

Each entry packs several fields. In the bracket [110/2], the first number is administrative distance, the second is metric.

Routing Protocol Codes

Administrative Distance (AD)

Administrative distance measures the trustworthiness of a route source. When a router learns about the same destination prefix from multiple sources, it installs only the route with the lowest AD.

On the Exam: Memorize Connected (0), Static (1), EIGRP internal (90), OSPF (110), and RIP (120). A route with AD 255 is never installed. If a router learns 10.1.1.0/24 via OSPF (110) and a static route (1), the static route wins because 1 < 110.

Metric

The metric compares routes learned from the same protocol. AD compares routes from different protocols — keep these two roles separate.

Gateway of Last Resort and Longest Prefix Match

The gateway of last resort is the default route 0.0.0.0/0. Because it has zero network bits, it matches every destination but is always the least specific option. The router always applies longest prefix match first: the most specific (longest mask) matching route wins.

Example: A packet for 192.168.10.50 against these entries:

• 0.0.0.0/0 (matches — 0 bits)
• 192.168.10.0/24 (matches — 24 bits)
• 192.168.10.48/28 (matches — 28 bits)

The router forwards using 192.168.10.48/28 because /28 is the longest match. A host route (/32) always beats any shorter prefix for that address.

Decision order recap: longest prefix match first, then AD (across protocols), then metric (within a protocol).

Worked Example: Reading a Bracketed Entry

Consider the line O 192.168.20.0/24 [110/2] via 10.0.0.1, 00:05:23, GigabitEthernet0/0. Decode every field: the O means OSPF learned it; 192.168.20.0/24 is the destination prefix and mask; 110 inside the brackets is the administrative distance (OSPF's default), confirming the source; 2 is the OSPF cost metric; 10.0.0.1 is the next hop the router will forward to; 00:05:23 is how long the route has lived in the table; and GigabitEthernet0/0 is the egress interface.

If a second OSPF entry for the same prefix appeared with [110/2] via a different next hop, the router would install both and load-balance, because AD and metric tie.

Connected vs. Local Routes — A Common Trap

Exam items often show paired entries and ask which is which. A connected (C) route represents the whole subnet wired to an interface (for example C 192.168.10.0/24), installed the moment the interface comes up with a valid IP. The local (L) route is the host-specific /32 of the router's own address on that interface (L 192.168.10.1/32). The local route is how the router recognizes traffic addressed to itself versus traffic it must forward. Both vanish instantly if the interface goes down or loses its address — connected and local routes are never learned from a protocol, so they have no metric to compare.

Why AD and Metric Are Not Interchangeable

A frequent misconception is that a route with a lower metric always wins. It does not. Administrative distance is checked before metric, and only among routes of equal prefix length. A static route (AD 1, no real metric) beats an OSPF route (AD 110) with cost 1 to the same /24, even though OSPF's cost looks small — because AD is compared first. Metric only ever decides between two routes from the same protocol. Keep this separation crisp: AD picks the protocol you trust, metric picks the best path within it.