Overview
Routers use routing protocols to communicate and share information about network paths. This lesson covers interior gateway protocols (IGPs), including distance vector and link state protocols, which enable routers to maintain updated routing tables within autonomous systems.
Routing Protocol Categories
- Routing protocols enable routers to exchange information and discover optimal paths across networks.
- Two main categories exist: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs).
- IGPs operate within a single autonomous system; EGPs exchange data between independent autonomous systems.
- IGPs further divide into link state routing protocols and distance vector protocols.
Autonomous Systems
- An autonomous system consists of networks under control of a single network operator.
- Large corporations routing data between multiple office LANs represent typical autonomous systems.
- Internet service providers with national-scale router networks also operate as autonomous systems.
- Interior gateway protocols manage routing within these unified network collections.
Distance Vector Protocols
- Distance vector protocols represent an older routing standard still used in some networks.
- Routers send their complete routing table to all directly connected neighboring routers.
- The routing table lists every known network and hop count distances to reach them.
- Routers gain knowledge only of immediate neighbors, not the entire autonomous system state.
- Updates occur when neighboring routers share tables showing shorter paths to destination networks.
- Example: Router A learns from Router B that Network X is 2 hops away instead of 4 hops through Router C.
- Router A then updates its table to forward traffic through Router B, reducing total path to 3 hops.
- Distance vector protocols react slowly to network changes occurring far from the router's location.
Link State Protocols
- Link state protocols use more sophisticated methods to determine optimal network paths.
- Each router advertises the state of all its interfaces (connections to routers or networks).
- Information propagates across the entire autonomous system so every router knows complete network topology.
- Routers run complex algorithms against collected data to calculate best paths for routing table updates.
- Link state protocols require more memory to store comprehensive data and greater processing power for calculations.
- Decreasing hardware costs and increasing computer power have made link state protocols the modern standard.
- These protocols have largely replaced distance vector protocols in contemporary networking environments.
Protocol Comparison
| Characteristic | Distance Vector | Link State |
|---|
| Age | Older standard | Modern standard |
| Information shared | Routing table (networks + hop counts) | Interface states for all routers |
| Knowledge scope | Only immediate neighbors | Complete autonomous system topology |
| Algorithm complexity | Simple list exchange | Complex path calculation algorithms |
| Resource requirements | Lower memory and processing | Higher memory and processing power |
| Response to changes | Slower reaction to distant changes | Faster adaptation to network changes |
Key Terms & Definitions
- Routing Protocol: Special protocol routers use to communicate and share network path information.
- Interior Gateway Protocol (IGP): Protocol for sharing routing information within a single autonomous system.
- Exterior Gateway Protocol (EGP): Protocol for exchanging routing data between independent autonomous systems.
- Autonomous System: Collection of networks under control of a single network operator.
- Distance Vector Protocol: Routing protocol that shares routing tables listing networks and hop counts.
- Link State Protocol: Routing protocol that shares interface states and calculates paths using complex algorithms.
- Vector: In computer science, a list; routing tables sent as vectors in distance vector protocols.