Overview
This lecture explains the Routing Information Protocol (RIP), a foundational distance-vector routing protocol, its mechanisms, versions, limitations, message exchange, and its place among other routing protocols.
Introduction to RIP
- RIP is one of the oldest distance-vector routing protocols, using hop count as its routing metric.
- The maximum hop count in RIP is 15; any route requiring more is considered unreachable.
- RIP employs mechanisms like split horizon, route poisoning, and holddown to avoid routing loops.
- RIP uses UDP as its transport protocol, specifically on port 520.
RIP Operation and Limitations
- RIPv1 routers broadcast their routing tables every 30 seconds, leading to potential traffic bursts as networks grow.
- RIP’s time to converge and scalability are poor compared to protocols like OSPF and EIGRP, but it is easy to configure.
- RIPv1 uses classful routing and does not support variable length subnet masks (VLSM) or router authentication.
- RIP is susceptible to "count to infinity" problems and has slow convergence.
Evolution and Versions of RIP
- RIP is based on the Bellman-Ford and Ford-Fulkerson algorithms.
- There are three standardized versions: RIPv1 (IPv4), RIPv2 (IPv4), and RIPng (IPv6).
- RIPv2 introduced support for subnet information (CIDR), multicast updates, authentication (MD5), and route tags.
- RIPng extends RIPv2 for IPv6 but relies on IPsec for authentication and uses multicast group ff02::9 on UDP port 521.
RIP Message Exchange & Timers
- RIP uses two message types: Request (asks neighbors for routing tables) and Response (carries routing table data).
- Four main timers manage routing information:
- Update Timer: Sends routing updates every 30 seconds.
- Invalid Timer: Marks routes as invalid after 180 seconds with no update.
- Flush Timer: Removes unreachable routes after 240 seconds.
- Holddown Timer: Prevents route changes for 180 seconds when hop count increases.
- RIPv1 can be configured in silent mode to avoid unnecessary broadcasts.
Implementations & Related Protocols
- RIP is implemented in various systems (Cisco IOS, Junos, Windows Server, Quagga, BIRD, FreeBSD/NetBSD/OpenBSD, Netgear, Huawei).
- Cisco replaced its proprietary IGRP with EIGRP, a more advanced distance-vector protocol.
Key Terms & Definitions
- Distance-vector routing protocol — Routes are advertised as vectors of distance and direction (hop count).
- Hop count — The number of routers a packet needs to traverse to reach its destination.
- Split horizon — Prevents routing information from being sent back in the direction from which it came.
- Route poisoning — Advertises a failed route with an infinite metric (hop count 16).
- Holddown — Temporarily prevents route changes when network instability is detected.
- Classful routing — Routing that does not include subnet mask information, restricting subnetting flexibility.
- Classless Inter-Domain Routing (CIDR) — Allows variable-length subnet masks for more efficient IP address allocation.
- UDP port 520/521 — Transport ports used by RIP (520 for IPv4, 521 for IPv6).
Action Items / Next Steps
- Review the differences between RIPv1, RIPv2, and RIPng.
- Study examples of RIP timers and their effects on route updates.
- Compare RIP with OSPF and EIGRP regarding convergence and scalability.