A2.3.4 Explain how static routing and dynamic routing move data across local area networks. (HL only)

Big Idea

In a local area network (LAN), data must traverse multiple interconnected devices—switches, routers and end-systems—to reach its destination. Routing is the process by which network devices decide the next hop for each data packet, ensuring efficient and reliable delivery. In this learning standard, two fundamental routing paradigms are distinguished: static routing, where network paths are manually configured and remain fixed, and dynamic routing, where routers exchange information to automatically build and update their forwarding tables .

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Static Routing

Process:

1. Manual Configuration
   • A network administrator enters routes into each router’s forwarding table (e.g., via commands such as ip route in Cisco IOS).
   • Each entry specifies a destination network, next-hop IP address or exit interface, and optionally an administrative metric.
2. Packet Forwarding
   • For each incoming packet, the router performs a longest‐prefix match lookup in the routing table and forwards the packet along the specified interface.

Advantages:

• Predictability & Control: Traffic always follows pre-defined paths, simplifying troubleshooting.
• Low Overhead: No CPU or bandwidth consumed by routing protocol messages.
• Security: With no protocol exchanges, there is minimal risk of malicious route injections.

Disadvantages:

• Scalability: Manual updates become infeasible as network size grows.
• Fault Tolerance: No automatic rerouting; a link failure can isolate parts of the network until the administrator intervenes.
• Maintenance Effort: Every topology change (new link, subnet, or router) requires manual reconfiguration on all affected devices.

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Dynamic Routing

Process:

1. Protocol Operation
   • Routers run a routing protocol (e.g., OSPF, RIP, EIGRP) to discover and share topology information.
   • They exchange periodic or event-triggered messages containing route updates or link-state advertisements.
2. Routing Table Computation
   • Each router applies a path-selection algorithm (e.g., Dijkstra’s for link-state, Bellman–Ford for distance-vector) to compute the best paths.
   • The routing table is automatically rebuilt whenever topology changes are detected.
3. Packet Forwarding
   • As with static routing, the router performs a longest-prefix match on each packet based on the dynamically maintained table.

Advantages:

• Adaptability & Resilience: Rapid convergence to new topology after failures or additions.
• Ease of Administration: Network changes propagate automatically; minimal manual intervention.
• Scalability: Protocols can manage large networks by aggregating routes and limiting update scope.

Disadvantages:

• Resource Usage: Routing updates consume CPU cycles, memory (to store neighbour and topology databases) and link bandwidth.
• Complexity: Protocol configuration and tuning (timers, metrics, authentication) require expertise.
• Convergence Time: There is a non-zero interval between a failure and full network reconvergence, during which packets may be lost or looped.

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Key Comparison Factors

Factor	Static Routing	Dynamic Routing
Configuration	Manual, per-router	Automatic via protocol exchanges
Maintenance	High human effort for every network change	Low—updates propagate automatically
Complexity	Simple to implement	Protocol setup and tuning adds complexity
Resource Usage	Minimal CPU & bandwidth	Periodic/event-driven messages consume CPU & link capacity
Convergence	Instantaneous (no protocol), but manual correction	Automatic, but finite convergence delay
Scalability	Poor beyond small topologies	Good—protocols support hierarchical designs and summarization
Network Size	Practical only for small, stable networks	Suitable for medium to large, dynamic environments

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In summary, static routing excels in small, stable LANs where simplicity and tight administrative control are paramount, but it does not scale and lacks fault-tolerance without manual updates. In contrast, dynamic routing offers adaptability and reduces manual overhead across larger, evolving networks at the cost of greater resource consumption and design complexity . Each approach must be selected in light of configuration effort, maintenance capacity, network size and desired resilience.