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3.1 Routing Table Components

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Cisco Certified Network Associate CCNA

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1 Introduction

About the CCNA 200-301 Exam Domains and Weights Study Strategies and Tips

2 Chapter 1: Network Fundamentals (20%)

1.1 Network Components 1.2 Network Topology Architectures 1.3 Physical Interfaces and Cabling 1.4 TCP vs. UDP 1.5 IPv4 Addressing and Subnetting 1.6 IPv6 Addressing and Prefix 1.7 Wireless Principles 1.8 Virtualization Fundamentals 1.9 Switching Concepts 1.10 OSI and TCP/IP Reference Models 1.11 Verifying IP Parameters on Client Operating Systems

3 Chapter 2: Network Access (20%)

2.1 VLANs — Configuration and Verification 2.2 Trunking and 802.1Q 2.3 Layer 2 Discovery Protocols (CDP and LLDP) 2.4 EtherChannel (Link Aggregation) 2.5 Spanning Tree Protocol (STP) and RSTP 2.6 Wireless Architectures and AP Modes 2.7 Network Access Troubleshooting 2.8 Inter-VLAN Routing — Deep Dive

4 Chapter 3: IP Connectivity (25%)

3.1 Routing Table Components 3.2 Router Forwarding Decisions 3.3 IPv4 and IPv6 Static Routing 3.4 Single-Area OSPFv2 — Concepts 3.5 Single-Area OSPFv2 — Configuration 3.6 First Hop Redundancy Protocols (FHRP) 3.7 Troubleshooting IP Connectivity 3.8 OSPF Troubleshooting Scenarios

5 Chapter 4: IP Services (10%)

4.1 NAT (Network Address Translation) 4.2 DHCP and DNS 4.3 NTP, SNMP, and Syslog 4.4 QoS Fundamentals 4.5 TFTP/FTP and Remote Access 4.6 IP Services — Comprehensive Review

6 Chapter 5: Security Fundamentals (15%)

5.1 Key Security Concepts 5.2 Device Access Control and Password Policies 5.3 VPN Fundamentals 5.4 Access Control Lists (ACLs) 5.5 Layer 2 Security Features 5.6 Network Security Best Practices and Hardening 5.7 Threat Mitigation and Incident Response

7 Chapter 6: Automation and Programmability (10%)

6.1 Impact of Automation on Network Management 6.2 Controller-Based and Software-Defined Architectures 6.3 AI and Machine Learning in Network Operations 6.4 REST-Based APIs 6.5 Configuration Management Tools 6.6 JSON-Encoded Data 6.7 Automation and Programmability — Comprehensive Review

3.1 Routing Table Components

Key Takeaways Explain This Topic AI

The routing table contains prefix/mask, next-hop IP, outgoing interface, administrative distance, and metric.

Administrative distance (AD) determines which routing source is trusted most (lower AD = more trusted).

Directly connected routes have AD 0, static routes have AD 1, OSPF has AD 110, EIGRP internal has AD 90.

The gateway of last resort (default route 0.0.0.0/0) is used when no specific route matches.

Routing protocol codes: C=connected, L=local, S=static, O=OSPF, D=EIGRP, R=RIP, B=BGP.

Last updated: June 2026

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.

  • Component Example Purpose
  • Route source code O (OSPF) How the route was learned
  • Destination prefix 192.168.20.0 Network being reached
  • Prefix length /24 Defines the network boundary
  • Administrative distance [110/...] Trustworthiness of the source
  • Metric [.../2] Cost within the routing protocol
  • Next-hop address via 10.0.0.1 Where to send the packet
  • Route age 00:05:23 How long ago the route was learned
  • Outgoing interface GigabitEthernet0/0 Interface to send the packet out
  • Routing Protocol Codes
  • Code Protocol Description
  • C Connected Directly connected networks
  • L Local The router's own interface /32 addresses
  • S Static Manually configured routes
  • S * Static default Default route (0.0.0.0/0) marked candidate default
  • R RIP Routing Information Protocol
  • D EIGRP Enhanced Interior Gateway Routing Protocol
  • O OSPF Open Shortest Path First
  • O IA OSPF Inter-Area OSPF route from a different area
  • B BGP Border Gateway Protocol
  • 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 .

  • Route Source Default AD
  • Connected 0
  • Static 1
  • EIGRP summary 5
  • eBGP 20
  • EIGRP (internal) 90
  • OSPF 110
  • IS-IS 115
  • RIP 120
  • EIGRP (external) 170
  • iBGP 200
  • Unreachable 255 (never installed)

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.

  • Protocol Metric What It Measures
  • RIP Hop count Routers to destination (max 15)
  • OSPF Cost Based on bandwidth (lower = better)
  • EIGRP Composite Bandwidth + delay by default
  • BGP Path attributes AS-path length, local preference, MED
  • 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.