Distance Vector Routing (DVR)
An essential class of routing protocols for computer networks, distance-vector routing (DVR) protocols find the most effective way for data packets to move. The shortest path to every other router in the network is listed in the routing table that each router using these protocols keeps up to date. The Bellman-Ford algorithm or the outdated ARPANET routing algorithm are two historical names for DVR.
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Here’s an explanation of how Distance-Vector Protocols work and their key characteristics:
How Does Distance Vector Routing Work
Routing Tables: Routing tables are kept up to date by each router and usually contain two essential pieces of information for every known destination: an estimated time or delay (metric) to reach that destination and a preferred outgoing line (interface) to use. The quickest path to each other router is displayed in this table, frequently based on the number of hops.
Information Exchange with Neighbors: Routers may share all of their routing tables with their neighbors who are directly connected. This exchange occurs regularly, even if there have been modifications to the network structure.
Metric for Path Selection: Metric for Path Selection The “distance” is determined by a metric, which may be the number of hops (routers) a packet travels through or other variables such as load, reliability, bandwidth, or network latency. The best course is the one with the lowest metric.
Bellman-Ford Algorithm: The optimum route is determined and routing databases are updated by distance-vector protocols using the Bellman-Ford algorithm.
Table Updates: A router compares its own paths with those it receives from a neighbor when it receives a routing table. The router modifies its local table if connecting through the neighbor offers a cheaper option or a shorter route to a destination. By adding the cost of transportation to the neighbor and the distance the neighbor advertises for the destination, one can determine the distance to a destination via a neighbor.
Convergence: When all routers in the network have a stable and consistent representation of the network topology, this process of exchanging and updating tables is said to have reached convergence.
Key Characteristics
Routing by Rumour: Routers don’t have a complete map of the network topology, according to rumours. Rather, they only trust the information that their neighbors give them, trusting it without doing their own research. It is frequently referred to as “routing by rumour” for this reason.
Broadcast/Multicast Updates: All neighbors that are directly connected usually receive updates.
Simplicity: These protocols are generally simple to configure and manage compared to link-state protocols.
Example of Protocol Operation (Simplified)
Consider a network with three routers, A, B, and C, with specific weights (costs) between them (e.g., AB = 2, BC = 3, CA = 5).
Initial Exchange: Initially, each router (A, B, and C) gives its near neighbors access to its initial routing table. For example, B and C share the same table as A.
Update Process: The router changes its own routing table if it finds a cheaper route to a destination through a neighbor’s table than the one it currently knows. When a new, less expensive route via a neighbor is discovered, for instance, a router may change its cost to a destination from 4 to 3.
Final Tables: This iterative procedure creates updated tables that show the least expensive routes for every router until all routing tables converge.
Advantages
- Shortest Path: The shortest path for data to travel in a network is efficiently found via Distance-Vector Routing.
- Ease of Implementation: The approach is easy to set up and uses fewer resources.
- Usage: It can be used in wide-area, urban, and neighborhood networks.
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Distance Vector Routing Disadvantages
Slow Convergence: Slow convergence is a major disadvantage, which means that after a topology change (such as a link loss), it may take a long time for all routers to update their tables.
Count-to-Infinity Problem: Routing loops and inefficiencies can result from the “count-to-infinity” phenomenon, which affects distance-vector protocols and causes updates about network changes to spread slowly. When a neighbor’s path to the destination is broken, a router may erroneously think that the way is still open through that neighbor.
Increased Traffic: Because hop count changes need to be communicated to every router, frequently through recurring broadcasts, even in the absence of topology changes, they have the potential to generate more network traffic than link-state protocols.
Larger Routing Tables: Because each router must be aware of every other router, DVR may result in larger routing tables for larger networks, which could cause WAN connection congestion.
Loop Prevention Mechanisms
To mitigate issues like routing loops and the count-to-infinity problem, DVR protocols employ mechanisms such as:
Hold-down timers: When a route is retracted, hold-down timers are used to prevent updates for a while.
Split horizon: Stopping a router from signaling a return path to the neighbor from where it received the information.
Poison reverse updates: Using an infinite metric to advertise an inaccessible path back to the source in order to promptly notify it of a broken link is known as poison reverse updates.
Maximum hop count: Setting a cap on the number of hops that a route can make (such as RIP’s 15-hop restriction) to stop routes from continuously doing so.
Examples of Distance Vector Routing Protocols
Several protocols fall under the distance-vector category:
- Routing Information Protocol (RIP): One of the oldest and most basic IP distance-vector protocols.
- It uses hop count as its metric, with a maximum of 15 hops, limiting its use to smaller networks.
- RIPv1 is classful, while RIPv2 is classless and supports Variable Length Subnet Masking (VLSM).
- RIPng is the IPv6 version of RIP.
- Interior Gateway Routing Protocol (IGRP): A Cisco-proprietary protocol developed to improve upon RIP.
- It uses a composite metric of network delay, bandwidth, reliability, and load.
- Sends routing updates every 90 seconds.
- Enhanced Interior Gateway Routing Protocol (EIGRP): Often described as an advanced distance-vector or balanced-hybrid protocol because it combines features of both distance-vector and link-state protocols.
- Offers faster convergence and operational efficiency.
- Uses the Diffusing Update Algorithm (DUAL) for best path selection.
- Sends partial, triggered updates instead of full periodic updates.
- EIGRPv6 supports IPv6 networks.
- Border Gateway Protocol (BGP): A distance-vector protocol designed for Wide Area Networks (WANs) and used as an exterior gateway protocol on the Internet, particularly by ISPs. It determines paths based on various factors beyond just hops and can be configured for specific route preferences.
- Babel: Another example of a distance-vector routing protocol.
Finally, the Distance-Vector Routing protocols, which exchange routing table information with neighbors and compute pathways based on a distance metric, provide a straightforward and effective method of routing data. However, they encounter difficulties such as sluggish convergence and the count-to-infinity problem in large or dynamic networks. They perform best in networks that are flat and simpler, where quick convergence is not the main issue.
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