Routing Table Components

The routing table is the most important data structure on a router. It contains all the information the router needs to forward packets to their destinations.

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

Route Entry Components

Each entry in the routing table contains:

Routing Protocol Codes

Administrative Distance (AD)

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

On the Exam: Memorize the AD values for Connected (0), Static (1), EIGRP (90), OSPF (110), and RIP (120). These are the most commonly tested. A route with AD 255 is considered unreachable and is never installed.

Example question scenario: A router learns about network 10.1.1.0/24 via both OSPF (AD 110) and a static route (AD 1). Which route is installed? The static route because AD 1 < AD 110.

Metric

The metric is used to compare routes learned from the same routing protocol. Different protocols use different metrics:

Key distinction: AD compares routes from different routing protocols. Metric compares routes from the same protocol.

Gateway of Last Resort (Default Route)

The gateway of last resort is the default route (0.0.0.0/0). When a packet's destination doesn't match any specific route in the routing table, the router forwards it to the default gateway.

Router(config)# ip route 0.0.0.0 0.0.0.0 10.0.0.1        ! Static default route

The 0.0.0.0/0 route matches every possible destination because it has 0 network bits—meaning all bits are host bits. However, it is always the least preferred match due to the longest prefix match rule.

Longest Prefix Match

When multiple routes match a destination, the router uses the longest prefix match (most specific route):

Example: A packet destined for 192.168.10.50. The routing table has:

• 0.0.0.0/0 → via 10.0.0.1 (matches — 0 bits)
• 192.168.10.0/24 → via 10.0.0.2 (matches — 24 bits)
• 192.168.10.48/28 → via 10.0.0.3 (matches — 28 bits)

The router uses 192.168.10.48/28 because /28 is the longest (most specific) match.

On the Exam: Longest prefix match is a fundamental concept. If a /32 route exists for a specific host, it will always be preferred over any less specific route for the same address. The routing decision process: longest prefix match first, then AD, then metric.