DISTANCE-VECTOR ROUTING AND RIP

Tags: #◇Computer Science #◇NSCOM02 #◇Conceptual #◇Distance-Vector Routing

◇Overview

Distance-vector routing protocols are fundamental dynamic routing protocols that operate on a "routing by rumor" principle. Each router calculates the distance (metric) to a destination and determines the vector (next-hop router). The Routing Information Protocol (RIP) is the quintessential example of a distance-vector protocol, utilizing hop count as its primary metric. These protocols are generally suited for small, non-complex networks but suffer from slow convergence and scalability issues.

◇Key Definitions

• Distance-Vector: A routing algorithm where routers determine the distance (cost) and direction (vector/next-hop) to destination networks.
• RIP (Routing Information Protocol): A standard distance-vector protocol using hop counts.
• Hop Count: The number of routers a packet must pass through. In RIP, a hop count of 16 is considered infinite (unreachable).
• Administrative Distance (AD): A measure of the trustworthiness of a routing protocol. RIP's AD is 120.
• Count-to-Infinity (Bouncing Effect): A routing loop issue where routers continuously increment metrics for a failed route.

◇Theoretical Framework / Core Concepts

Bellman-Ford Algorithm

Distance-vector protocols utilize the Bellman-Ford equation to calculate the shortest path. Each node receives the cost arrays from its neighbors, computes the cost to all destinations, and updates its routing table:
$D_x(y) = \min_v \{ c(x, v) + D_v(y) \}$

RIP Characteristics

• Metric: Hop count. Maximum valid hops = 15.
• Updates: Full routing table updates are broadcast periodically (typically every 30 seconds).
• Administrative Distance: 120.

Loop Prevention Mechanisms

To combat the count-to-infinity problem, several mechanisms are deployed:

1. Split Horizon: Prevents a router from advertising a route back out of the interface it was learned from.
2. Route Poisoning: Immediately advertises a failed route with an infinite metric (16 in RIP).
3. Poison Reverse: Overrides Split Horizon; sends the route back on the learned interface but marked as unreachable.
4. Hold-down Timers: Prevents a router from accepting new, potentially bad routing information for a set period after learning of a route failure.
5. Triggered Updates: Sends immediate updates when a topology change occurs, rather than waiting for the periodic timer.

◇Worked Examples / Protocol Walkthroughs / Calculations

Routing Table Entry Breakdown

R 192.168.10.0/24 [120/4] via 10.1.1.2

• R: Indicates the route was learned via RIP.
• 192.168.10.0/24: The destination network.
• [120/4]:
  • 120 is the Administrative Distance.
  • 4 is the metric (hop count).
• via 10.1.1.2: The next-hop router's IP address.

◇Summary & Key Takeaways

• Distance-vector protocols rely on neighbors for routing information ("routing by rumor").
• RIP is limited to 15 hops; 16 implies the network is unreachable.
• Loop prevention is critical and handled via Split Horizon, Route Poisoning, Hold-down timers, and Triggered Updates.

◇References & Navigation

• Next Topic: ◇Link-State Routing and OSPF
• Related Notes: ◇Link-State Routing and OSPF, ◇Network Layer