Classfull IP Addressing

When the ARPANET was commissioned in 1969, no one anticipated that the Internet would explode out of the humble beginnings of this research project. By 1989, ARPANET had been transformed into what we now call the Internet. Over the next decade, the number of hosts on the Internet grew exponentially, from 159,000 in October 1989, to over 72 million by the end of the millennium. As of January 2007, there were over 433 million hosts on the Internet.

Without the introduction of VLSM and CIDR notation in 1993 (RFC 1519), Name Address Translation (NAT) in 1994 (RFC 1631), and private addressing in 1996 (RFC 1918), the IPv4 32-bit address space would now be exhausted.

RIP Historical Impact

RIP is the oldest of the distance vector routing protocols. Although RIP lacks the sophistication of more advanced routing protocols, its simplicity and continued widespread use is a testament to its longevity. RIP is not a protocol "on the way out." In fact, an IPv6 form of RIP called RIPng (next generation) is now available.

Click the dates in the figure to compare RIP and network protocol development over time.

RIP evolved from an earlier protocol developed at Xerox, called Gateway Information Protocol (GWINFO). With the development of Xerox Network System (XNS), GWINFO evolved into RIP. It later gained popularity because it was implemented in the Berkeley Software Distribution (BSD) as a daemon named routed (pronounced "route-dee", not "rout-ed"). Various other vendors made their own, slightly different implementations of RIP. Recognizing the need for standardization of the protocol, Charles Hedrick wrote RFC 1058 in 1988, in which he documented the existing protocol and specified some improvements. Since then, RIP has been improved with RIPv2 in 1994 and with RIPng in 1997.

Enchaned Interior Gateway Routing Protocol

Enhanced IGRP (EIGRP) was developed from IGRP, another distance vector protocol. EIGRP is a classless, distance vector routing protocol with features found in link-state routing protocols. However, unlike RIP or OSPF, EIGRP is a proprietary protocol developed by Cisco and only runs on Cisco routers.

EIGRP features include:

• Triggered updates (EIGRP has no periodic updates).
• Use of a topology table to maintain all the routes received from neighbors (not only the best paths).
• Establishment of adjacencies with neighboring routers using the EIGRP hello protocol.
• Support for VLSM and manual route summarization. These allow EIGRP to create hierarchically structured large networks.

Advantages of EIGRP:
Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path rather than hop count.
Fast convergence due to Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers.
Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes.
EIGRP supports multiple network layer protocols through Protocol Dependent Modules, which include support for IP, IPX, and AppleTalk.

Routing Information Protokol (RIP)

Over the years, RIP has evolved from a classful routing protocol (RIPv1) to a classless routing protocol (RIPv2). RIPv2 is a standardized routing protocol that works in a mixed vendor router environment. Routers made by different companies can communicate using RIP. It is one of the easiest routing protocols to configure, making it a good choice for small networks. However, RIPv2 still has limitations. Both RIPv1 and RIPv2 have a route metric that is based only on hop count and which is limited to 15 hops.

Features of RIP:
Supports split horizon and split horizon with poison reverse to prevents loops.
Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.

RIPv2 introduced the following improvements to RIPv1:
Includes the subnet mask in the routing updates, making it a classless routing protocol.
Has authentication mechanism to secure routing table updates.
Supports variable length subnet mask (VLSM).
Uses multicast addresses instead of broadcast.
Supports manual route summarization.

What are the Implications of Routing Loops?

A routing loop can have a devastating effect on a network, resulting in degraded network performance or even a network downtime.

A routing loop can create the following conditions:

• Link bandwidth will be used for traffic looping back and forth between the routers in a loop.
• A router's CPU will be strained due to looping packets.
• A router's CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network.
• Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse.
• Packets may get lost in "black holes."

What is routing loop

A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid path to an unreachable destination exists.

The loop may be a result of:

• Incorrectly configured static routes
• Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol and is discussed in CCNP-level courses)
• Inconsistent routing tables not being updated due to slow convergence in a changing network
• Incorrectly configured or installed discard routes

Distance vector routing protocols are simple in their operations. Their simplicity results in protocol drawbacks like routing loops. Routing loops are less of a problem with link-state routing protocols but can occur under certain circumstances.

Distance vektor routing protokol

Dynamic routing is the most common choice for large networks like the one shown. Distance vector routing protocols include RIP, IGRP, and EIGRP.

RIP

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.

IGRP

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.

EIGRP

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.
-here are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology.

Purpose of Administrative Distance

Administrative distance (AD) defines the preference of a routing source. Each routing source - including specific routing protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when it learns about the same destination network from two or more different routing sources.

Administrative distance is an integer value from 0 to 255. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed.

It is possible to modify the administrative distance for static routes and dynamic routing protocols. This is discussed in CCNP.

An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table.

Purpose of metric

To select the best path, the routing protocol must be able to evaluate and differentiate between the available paths. For this purpose a metric is used. A metric is a value used by routing protocols to assign costs to reach remote networks. The metric is used to determine which path is most preferable when there are multiple paths to the same remote network.
For example, RIP uses hop count, EIGRP uses a combination of bandwidth and delay, and Cisco's implementation of OSPF uses bandwidth. Hop count is the easiest metric to envision. The hop count refers to the number of routers a packet must cross to reach the destination network. For R3 in the figure, network 172.16.3.0 is two hops, or two routers away.

The Evolution of Dynamic Routing Protocols

Dynamic routing protocols have been used in networks since the early 1980s. The first version of RIP was released in 1982, but some of the basic algorithms within the protocol were used on the ARPANET as early as 1969.

One of the earliest routing protocols was Routing Information Protocol (RIP). RIP has evolved into a newer version RIPv2. However, the newer version of RIP still does not scale to larger network implementations. 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 Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), which also scales well in larger network implementations.