Routing Protocols

Static routes are configured directly, while dynamic routing protocols exchange reachability and topology information so routers can adapt. Dynamic learning reduces manual work but introduces protocol state, trust boundaries, timers, and failure modes that must be monitored.

Control Plane and Forwarding Plane

A routing protocol learns candidates in its own database. The router selects routes into a routing information base and installs usable next hops into a forwarding table. Hardware or the kernel then forwards packets from that table.

A protocol adjacency being established does not prove that the desired prefix was learned, selected, installed, or permitted by forwarding policy.

Interior and Exterior Routing

Interior gateway protocols operate within an administrative routing domain. Examples include RIP, OSPF, and IS-IS. BGP exchanges policy-controlled reachability within and between autonomous systems and is the Internet's exterior routing protocol.

Metrics have protocol-specific meaning. An OSPF cost, RIP hop count, and BGP attribute set cannot be compared as if they shared one universal numerical scale. Implementations use route preference or administrative distance to choose between sources before or alongside protocol-specific selection.

Distance Vector and Link State

Distance-vector protocols advertise reachability and distance through neighbors, deriving paths from neighbor reports. Link-state protocols form adjacencies, flood link-state information through a scope, build a topology database, and calculate shortest-path trees. Modern protocols include refinements that make simple category summaries incomplete.

Convergence

After a topology or policy change, routers detect it, propagate control information, calculate paths, and update forwarding state. Convergence is the period and outcome in which the network reaches stable, mutually usable routing for the affected destinations. It does not require every router to have an identical full table; roles and policies can intentionally differ.

During convergence, transient loss, loops, or black holes can occur. Measure detection, propagation, calculation, and installation separately and verify with data-plane probes.

Summary

You can now place dynamic routing information in the path from protocol exchange to forwarding.

1. Separate learned candidates, selected routes, and forwarding entries.
2. Distinguish interior routing from BGP policy exchange.
3. Compare metrics only within their protocol semantics.
4. Verify convergence in both control and data planes.