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Cisco Networking Academy's Introduction to Routing Dynamically

Mar 24, 2014

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Dynamic Routing Protocols (3.1)

Dynamic routing protocols play an important role in today’s networks. The following sections describe several important benefits that dynamic routing protocols provide. In many networks, dynamic routing protocols are typically used with static routes.

The Evolution of Dynamic Routing Protocols (3.1.1.1)

Dynamic routing protocols have been used in networks since the late 1980s. One of the first routing protocols was Routing Information Protocol (RIP). RIP version 1 (RIPv1) was released in 1988, but some of the basic algorithms within the protocol were used on the Advanced Research Projects Agency Network (ARPANET) as early as 1969.

As networks evolved and became more complex, new routing protocols emerged. The RIP routing protocol was updated to accommodate growth in the network environment, into RIPv2. However, the newer version of RIP still does not scale to the larger network implementations of today. To address the needs of larger networks, two advanced routing protocols were developed: Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). Cisco developed the Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), which also scales well in larger network implementations.

Additionally, there was the need to connect different internetworks and provide routing between them. The Border Gateway Protocol (BGP) is now used between Internet service providers (ISPs). BGP is also used between ISPs and their larger private clients to exchange routing information.

Table 3-1 classifies the protocols.

Table 3-1 Routing Protocol Classification

	Interior Gateway Protocols				Exterior Gateway Protocols
	Distance Vector		Link-State		Path Vector
IPv4	RIPv2	EIGRP	OSPFv2	IS-IS	BGP-4
IPv6	RIPng	EIGRP for IPv6	OSPFv3	IS-IS for IPv6	MBGP

With the advent of numerous consumer devices using IP, the IPv4 addressing space is nearly exhausted; thus, IPv6 has emerged. To support the communication based on IPv6, newer versions of the IP routing protocols have been developed, as shown by the IPv6 row in Table 3-1.

RIP is the simplest of dynamic routing protocols and is used in this section to provide a basic level of routing protocol understanding.

Purpose of Dynamic Routing Protocols (3.1.1.2)

Routing protocols are used to facilitate the exchange of routing information between routers. A routing protocol is a set of processes, algorithms, and messages that are used to exchange routing information and populate the routing table with the routing protocol’s choice of best paths. The purpose of dynamic routing protocols includes:

Discovery of remote networks
Maintaining up-to-date routing information
Choosing the best path to destination networks
Ability to find a new best path if the current path is no longer available

The main components of dynamic routing protocols include:

Data structures: Routing protocols typically use tables or databases for their operations. This information is kept in RAM.
Routing protocol messages: Routing protocols use various types of messages to discover neighboring routers, exchange routing information, and perform other tasks to learn and maintain accurate information about the network.
Algorithm: An algorithm is a finite list of steps used to accomplish a task. Routing protocols use algorithms for facilitating routing information and for best path determination.

Figure 3-1 highlights the data structures, routing protocol messages, and routing algorithm used by EIGRP.

Figure 3-1 Components of Routing Protocols

The Role of Dynamic Routing Protocols (3.1.1.3)

Routing protocols allow routers to dynamically share information about remote networks and automatically add this information to their own routing tables.

Routing protocols determine the best path, or route, to each network. That route is then added to the routing table. A primary benefit of dynamic routing protocols is that routers exchange routing information when there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths when there is a link failure to a current network.

Compared to static routing, dynamic routing protocols require less administrative overhead. However, the expense of using dynamic routing protocols is dedicating part of a router’s resources for protocol operation, including CPU time and network link bandwidth. Despite the benefits of dynamic routing, static routing still has its place. There are times when static routing is more appropriate and other times when dynamic routing is the better choice. Networks with moderate levels of complexity may have both static and dynamic routing configured.

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