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."