PROJECTENGRAM

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:
Dx(y)=min⁡v{c(x,v)+Dv(y)}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

CONFIRMATION // 理解度の確認

ACTIVE RECALL VERIFICATION

Select an answer to verify conceptual comprehension and reveal the technical rationale.

Q1

What is the maximum allowable hop count for a valid route in the Routing Information Protocol (RIP)?

Q2

Which mechanism prevents a router from advertising a route back onto the same interface from which it was originally learned?

Q3

A routing table displays the entry: R 192.168.10.0/24 [120/4] via 10.1.1.2. What does the value 4 represent?

Q4

Which issue occurs in distance-vector networks when a link fails and routers repeatedly advertise increasing metrics to each other?

Q5

What is the primary purpose of Route Poisoning?

Q6

How do triggered updates improve distance-vector routing?

Q7

Which of the following best describes the "routing by rumor" concept?

Q8

What is the default Administrative Distance (AD) for RIP?

Q9

What is the role of a Hold-down Timer in routing protocols?

Q10

Poison Reverse goes a step further than Split Horizon by doing what?

Distance-Vector Routing and RIP | PROJECT ENGRAM