RIP-2 Protocol Overview

Overview

RFC 2453 defines RIP Version 2 (RIP-2), extending RIP-1 to include subnet masks, route tags, next-hop, multicasting, and authentication, while preserving backward compatibility.

Background and Justification

• RIP remains useful in small/moderate networks due to low overhead and simplicity.
• RIP-1 lacks subnetting, authentication, and AS/IGP/EGP awareness; RIP-2 addresses these gaps.
• RIP uses distance vector (Bellman-Ford) algorithm and is suited as an IGP within an AS.

Distance Vector Basics

• Each router maintains routes: destination, next hop, interface, metric, timer.
• Metric: additive positive integers; typical hop-count with max valid 15; 16 means infinity.
• Update process:
  • Periodic full-table sharing with neighbors.
  • On receipt: add incoming interface cost; adopt better routes; always accept updates from current next hop.

Handling Topology Changes and Stability

• Timeouts: if no update from next hop in 180s, mark route invalid (metric 16), start garbage-collection.
• Counting to infinity: slow convergence when routes vanish; choose small infinity (16) to limit duration.
• Split horizon:
  • Simple: omit routes back to the neighbor learned from.
  • Poisoned reverse: advertise those routes with metric 16 for faster loop breakage.
• Triggered updates:
  • Send immediate updates on route metric change; rate-limit with 1–5s random delay; may still race with regular updates.

RIP-2 Protocol Specification

• Metric 1–15; admin-configurable per interface; typical cost 1.
• Routing table entries include destination, metric, next hop, change flag, timers, and subnet mask.
• All routers in the AS must participate; leaks needed between multiple IGPs.

Message Format

• Transport: UDP port 520 for both RIP-1 and RIP-2.
• Header: command (1=Request, 2=Response), version, MBZ fields zero.
• RIP-1 RTEs: AFI, IPv4 address, metric; zeros in unused fields.
• RIP-2 RTEs: AFI, Route Tag, IP Address, Subnet Mask, Next Hop, Metric.

Addressing and Route Types

• Supports routes to networks, subnets, hosts, and default (0.0.0.0).
• Longest-match selection: host routes, then subnet/network, then default.
• RIP-1 ambiguity without masks; avoid advertising subnets where masks are unknown.
• Border routers:
  • Inside subnetted network: advertise subnets internally.
  • Outside: advertise only the aggregated network entry; suppress internal host routes.
• Default routes: 0.0.0.0 entries configured by admin; scope should not exceed AS.

Timers and Processing

• Periodic updates: every 30s; add jitter or use stable clock to avoid synchronization.
• Per-route:
  • Timeout: 180s since last update; then mark invalid and trigger update.
  • Garbage-collection: 120s after invalidation; then remove route; cancel if a valid new route appears.
• Request handling:
  • Special single-entry (AFI=0, metric=16) means “send entire table.”
  • Specific lookups return exact table values without split horizon.
• Response handling:
  • Validate source (RIP port, directly connected, not self).
  • Validate entries (valid unicast, metric 1–16).
  • metric = min(received metric + incoming link cost, 16).
  • Add or update routes; reinit timers; trigger update on change.
  • Optional heuristic: if equal metric but route is aging, switch to fresher equal-cost next hop.

Output and Triggered Updates

• Send one Response per directly-connected network: unicast over p2p and broadcast/multicast over LANs.
• Triggered updates:
  • Include at least changed routes (route change flag set); apply split horizon.
  • Suppress if a regular update is imminent; clear change flags after sending.
  • Rate-limit with random 1–5s timer; coalesce multiple changes.

RIP-2 Extensions

• Authentication:
  • First RTE with AFI=0xFFFF indicates authentication.
  • Type 2: simple password; 16-octet left-justified, null-padded.
• Route Tag:
  • Preserved attribute to distinguish internal vs external routes (e.g., AS number for imported routes).
• Subnet Mask:
  • Eliminates subnet/host ambiguity; zero means no mask included.
  • Interop rules near RIP-1: do not leak internal details, avoid advertising more-specifics seen as host routes, and avoid supernets to RIP-1 neighbors.
• Next Hop:
  • Advises immediate next hop; 0.0.0.0 means sender.
  • Must be directly reachable on the receiving interface; else treat as 0.0.0.0.
• Multicasting:
  • Use 224.0.0.9 for periodic RIP-2 updates; configurable per-interface; accept multicast responses on NBMA if received.
• Queries:
  • RIP-2 router responds to RIP-1 Requests with RIP-1 Responses unless configured to send only RIP-2.

Compatibility and Mixed Environments

• Version handling: discard v0; discard v1 with non-zero MBZ; accept versions >1 despite MBZ content.
• Compatibility switches per-interface:
  • Send: RIP-1, RIP-1-compatible (RIP-2 via broadcast), RIP-2 (multicast), none.
  • Receive: RIP-1 only, RIP-2 only, both, none.
• Authentication policy:
  • If not authenticating: accept RIP-1 and unauthenticated RIP-2; drop authenticated RIP-2.
  • If authenticating: accept RIP-2 that pass; drop unauthenticated and failed-auth RIP-2; ignore RIP-1 for maximum security.
• Infinity cannot be increased without breaking compatibility.
• Addressless links are not supported.
• Mixed v1/v2: use a single subnet mask across the network; disable auto-summarization; otherwise risk blackholes or excess routes.

Example Use of Next Hop (Conceptual)

• Internal routers speak RIP-2; external routers speak another IGP.
• External router adjacent to internal domain advertises routes with Next Hop set to optimal external egress.
• Internal routers forward directly to that Next Hop, avoiding unnecessary extra hops via the advertising external router.

Key Terms & Definitions

• Distance Vector: Routing algorithm exchanging vector of distances to destinations with neighbors.
• Metric: Additive cost; RIP uses 1–15; 16 is infinity (unreachable).
• Split Horizon: Do not advertise a route back on the interface it was learned; poisoned reverse advertises with metric 16.
• Triggered Update: Immediate update sent upon route change; rate-limited.
• Route Tag: Attribute marking external vs internal routes; preserved in re-advertisement.
• Next Hop: Immediate next hop IP for a specific route; advisory to optimize forwarding.
• Default Route: 0.0.0.0 entry used when no specific match exists.
• Timeout: 180s without refresh marks route invalid; garbage collection removes after 120s.

Structured Details

Action Items / Next Steps

• Configure per-interface send/receive modes (RIP-1, RIP-2, compatibility, none) as appropriate.
• Enable split horizon; prefer poisoned reverse; allow disabling if needed to reduce overhead.
• Implement and enforce triggered update rate-limiting (1–5s random).
• Include subnet masks and next hop in RIP-2; suppress suboptimal next hops not directly reachable.
• Use multicast 224.0.0.9 for RIP-2 where supported; fall back to broadcast/unicast as configured.
• In mixed v1/v2 networks, standardize a single subnet mask and disable auto-summarization.
• If using authentication, deploy Type 2 simple password consistently; consider ignoring RIP-1 for security.
• Ensure timers (30s, 180s, 120s) and route change flags are correctly maintained and processed.