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Routing Information Protocol (RIP) was originally specified in RFC 1058. It has the following key characteristics:
- Hop count is used as the metric for path selection.
- If the hop count for a network is greater than 15, RIP cannot supply a route to that network.
- Routing updates are broadcast or multicast every 30 seconds, by default.
Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco. IGRP has the following key design characteristics:
- Bandwidth, delay, load and reliability are used to create a composite metric.
- Routing updates are broadcast every 90 seconds, by default.
- IGRP is the predecessor of EIGRP and is now obsolete.
Enhanced IGRP (EIGRP) is a Cisco proprietary distance vector routing protocol. EIGRP has these key characteristics:
- It can perform unequal cost load balancing.
- It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
- There are no periodic updates as with RIP and IGRP.
- Routing updates are sent only when there is a change in the topology.
As the name implies, distance vector means that routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface.
A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network. Instead the router knows only:
- The direction or interface in which packets should be forwarded and
- The distance or how far it is to the destination network
What is The Purpose of the Algorithm?
At the core of the distance vector protocol is the algorithm. The algorithm is used to calculate the best paths and then send that information to the neighbors.
An algorithm is a procedure for accomplishing a certain task, starting at a given initial state and terminating in a defined end state. Different routing protocols use different algorithms to install routes in the routing table, send updates to neighbors, and make path determination decisions.
What are advantages of distance vector routing?
Simple implementation and maintenance. The level of knowledge required to deploy and later maintain a network with distance vector protocol is not high.
Low resource requirements. Distance vector protocols typically do not need large amounts of memory to store the information. Nor do they require a powerful CPU. Depending of the network size and the IP addressing implemented they also typically do not require a high level of link bandwidth to send routing updates. However, this can become an issue if you deploy a distance vector protocol in a large network.
What are disadvantages of distance vector routing?
Slow convergence. The use of periodic updates can cause slower convergence. Even if some advanced techniques are used, like triggered updates which are discussed later, the overall convergence is still slower compared to link state routing protocols.
Limited scalability. Slow convergence may limit the size of the network because larger networks require more time to propagate routing information.
Routing loops. Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
what are some routing protocols characterstics?
Time to Convergence - Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol. Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
Scalability - Scalability defines how large a network can become based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be.
Classless (Use of VLSM) or Classful - Classless routing protocols include the subnet mask in the updates.
Resource Usage - Resource usage includes the requirements of a routing protocol such as memory space, CPU utilization, and link bandwidth utilization.
Implementation and Maintenance - Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.