Routing Tables

• The routing table is a data structure stored in router memory that contains information about network destinations and how to reach them
• Functions as the router’s “roadmap” - determines the best path for forwarding packets based on destination IP addresses
• Built and maintained through static routes (manually configured) and dynamic routing protocols (automatically learned)
• Each entry contains destination network, next-hop IP, outgoing interface, and routing metrics

How Routing Tables Work

• Router examines destination IP in packet header and performs longest prefix match against routing table entries
• Longest prefix match wins - most specific route (highest subnet mask) takes precedence over less specific routes
• If no match found, packet is dropped (unless default route exists)
• Default route (0.0.0.0/0) acts as “gateway of last resort” when no specific match exists

Routing Table Components

Administrative Distance vs Metric

• Administrative Distance (AD) = “Which routing source should I trust?” (lower is better)
• Metric = “Which path is best within this routing protocol?” (varies by protocol)
• AD determines route selection between different protocols (OSPF vs EIGRP)
• Metric determines route selection within same protocol (multiple OSPF paths)

Route Types and Codes

• Connected (C) - Directly attached networks (interfaces with IP addresses)
• Static (S) - Manually configured routes using ip route command
• Dynamic - Learned through routing protocols (OSPF, EIGRP, RIP)
• Default - Gateway of last resort, shown as S* 0.0.0.0/0

For example: C 192.168.1.0/24 is directly connected, FastEthernet0/0 indicates a connected network

Longest Prefix Match Example

Given destination 192.168.1.50 and these routes:

• 192.168.0.0/16 (/16 = less specific)
• 192.168.1.0/24 (/24 = more specific)
• 0.0.0.0/0 (/0 = least specific, default route)

Router selects 192.168.1.0/24 because /24 provides longest match (most specific)

Vocabulary

Administrative Distance (AD) - Numerical value (0-255) indicating trustworthiness of routing information source; lower values preferred

Metric - Protocol-specific value used to determine best path; hop count for RIP, bandwidth for OSPF

Longest Prefix Match - Algorithm selecting most specific route (highest subnet mask bits) matching destination

Next Hop - IP address of next router in path toward destination network

Convergence - Process of all routers learning about network topology changes and updating routing tables

Gateway of Last Resort - Default route used when no specific route matches destination

Notes

• Use show ip route to display routing table contents on Cisco devices
• Routing table size impacts router performance - summarization reduces entries
• Static routes remain until manually removed or interface goes down (unless tracking configured)
• Floating static routes use higher AD than dynamic protocols for backup purposes
• Connected routes automatically appear when interface configured with IP address and comes up
• Router must know route to destination and source networks for bidirectional communication
• Load balancing occurs automatically when multiple equal-cost paths exist (up to 4 by default in OSPF/EIGRP)
• Routing loops prevented through split horizon, poison reverse, and hold-down timers in distance-vector protocols