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Cisco Networking Academy's Introduction to Routing Dynamically

Mar 24, 2014

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Routing Protocols Companion Guide

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Distance Vector Dynamic Routing (3.2)

This section describes the characteristics, operations, and functionality of distance vector routing protocols. Understanding the operation of distance vector routing is critical to enabling, verifying, and troubleshooting these protocols.

Distance Vector Technologies (3.2.1.1)

Distance vector routing protocols share updates between neighbors. Neighbors are routers that share a link and are configured to use the same routing protocol. The router is only aware of the network addresses of its own interfaces and the remote network addresses it can reach through its neighbors. Routers using distance vector routing are not aware of the network topology.

Some distance vector routing protocols send periodic updates. For example, RIP sends a periodic update to all of its neighbors every 30 seconds. RIP does this even if the topology has not changed; it continues to send updates. RIPv1 reaches all of its neighbors by sending updates to the all-hosts IPv4 address of 255.255.255.255, a broadcast.

The broadcasting of periodic updates is inefficient because the updates consume bandwidth and consume network device CPU resources. Every network device has to process a broadcast message. RIPv2 and EIGRP, instead, use multicast addresses so that only neighbors that need updates will receive them. EIGRP can also send a unicast message to only the affected neighbor. Additionally, EIGRP only sends an update when needed, instead of periodically.

As shown in Figure 3-21, the two modern IPv4 distance vector routing protocols are RIPv2 and EIGRP. RIPv1 and IGRP are listed only for historical accuracy.

Figure 3-21 Distance Vector Routing Protocols

Distance Vector Algorithm (3.2.1.2)

At the core of the distance vector protocol is the routing algorithm. The algorithm is used to calculate the best paths and then send that information to the neighbors.

The algorithm used for the routing protocols defines the following processes:

Mechanism for sending and receiving routing information
Mechanism for calculating the best paths and installing routes in the routing table
Mechanism for detecting and reacting to topology changes

In the animation in the online course, R1 and R2 are configured with the RIP routing protocol. The algorithm sends and receives updates. Both R1 and R2 then glean new information from the update. In this case, each router learns about a new network. The algorithm on each router makes its calculations independently and updates the routing table with the new information. When the LAN on R2 goes down, the algorithm constructs a triggered update and sends it to R1. R1 then removes the network from the routing table.

Different routing protocols use different algorithms to install routes in the routing table, send updates to neighbors, and make path determination decisions. For example:

RIP uses the Bellman-Ford algorithm as its routing algorithm. It is based on two algorithms developed in 1958 and 1956 by Richard Bellman and Lester Ford, Jr.
IGRP and EIGRP use the Diffusing Update Algorithm (DUAL) routing algorithm developed by Dr. J.J. Garcia-Luna-Aceves at SRI International.

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