Overview
This lecture covers RIP Version 2 (RIP-2), an extension of the original Routing Information Protocol, focusing on expanded information sharing, limitations addressed, and protocol enhancements like authentication.
RIP and Its Justification
- RIP is simple, low-overhead, and widely used, especially for small or moderate-sized networks.
- Despite being less advanced than OSPF and IS-IS, RIP persists due to ease of implementation and existing deployment.
- RIP-2 aims to enhance utility by addressing key limitations of RIP-1.
RIP-1 Limitations and Constraints
- RIP-1 lacks support for subnet masks, authentication, and autonomous systems (AS).
- Routers can't always distinguish subnet or host routes without subnet masks, leading to ambiguity.
- RIP is limited to networks with up to 15 hops and fixed metrics.
RIP Protocol and Distance Vector Algorithm
- RIP uses the Bellman-Ford (distance vector) algorithm for routing.
- Each router shares its routing table (distance to destinations) with neighbors periodically.
- Routing tables include destination address, next hop, metric (cost), timers, and change flags.
Refinements: Stability, Split Horizon, Triggered Updates
- Count-to-infinity problem is mitigated by setting unreachable networks to metric 16.
- Split horizon and split horizon with poisoned reverse reduce routing loops by not advertising learned routes back on the same interface.
- Triggered updates ensure rapid propagation of route changes.
- Timers control periodic updates (every 30s), route timeout (180s), and garbage collection (120s).
RIP Message Format and Fields
- RIP messages use UDP port 520, with commands for request or response.
- RIP-1 entries include only AFI, address, and metric.
- RIP-2 extends entries with route tag, subnet mask, and next hop fields, allowing for more precise routing and interoperability.
RIP-2 Protocol Extensions
- Authentication supports plain-text passwords for message security.
- Route Tag distinguishes internal from external routes (from other protocols).
- Subnet Mask in updates eliminates ambiguity of subnet/host routes.
- Next Hop optimizes path selection, preventing unnecessary routing hops.
- Multicasting (224.0.0.9) reduces unnecessary load by limiting the audience for updates.
Compatibility and Security
- RIP-2 maintains backward compatibility but requires configuration for RIP-1 interoperability.
- Compatibility switches allow interfaces to send/receive updates in RIP-1, RIP-2, or both.
- Authentication is optional; unconfigured routers accept both RIP-1 and unauthenticated RIP-2.
Key Terms & Definitions
- RIP (Routing Information Protocol) — A distance vector IGP used to exchange routing information.
- Distance Vector Algorithm — Routing algorithm where routers share distance estimates to destinations with neighbors.
- Split Horizon — Prevents a router from advertising a route back onto the interface it was learned from.
- Poisoned Reverse — Advertises routes as unreachable back to the source to break loops.
- Triggered Update — Immediate routing update sent when a route changes.
- Metric — Cost associated with reaching a destination; max valid is 15, 16 means unreachable.
- Route Tag — Field in RIP-2 to indicate route source (internal/external).
- Next Hop — Address to which packets should be sent next for a given route.
- Subnet Mask — Used to identify the network and host portions of an address.
- Authentication — Mechanism for verifying the validity of RIP messages.
Action Items / Next Steps
- Review the detailed RIP-2 message formats and protocol operations.
- Ensure understanding of split horizon, triggered updates, and compatibility controls.
- Practice interpreting and configuring RIP-2 in lab environments.