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Chapter 4
The IP Routing Protocols


ü Choose the appropriate IP routing protocol and features based
ü on convergence, overhead, and topology.
ü Identify IP routing pathologies and issues and how to avoid them.
ü Use modular design and summarization features to design
ü scalable Open Shortest Path First (OSPF) internetworks.
ü Allocate IP addresses in contiguous blocks so that OSPF
ü summarization can be used.
ü Determine IGRP convergence time for various internetwork configurations.
ü Use IGRP for path determination in IP internetworks.
ü Use Enhanced IGRP for path determination in internetworks that support IP, IPX, and AppleTalk.

With the explosive growth of the Internet, the IP protocol has become a de facto standard for virtually all networks. As such, the protocol is continuing to undergo rapid development, and that development includes enhancements in terms of routing protocol features and general network design. This chapter will focus specifically on the IP routing protocols and how to consider each for integration into a network design.

Readers will likely note a number of recurrent themes in this presentation—the features of each protocol and the convergence time characteristics. Whenever a network topology changes, it is the job of the routing protocol to reroute traffic and determine the new best paths for data flow on the inter-network. (The amount of time required to complete this process in the event of any change is referred to as convergence time.) These are two of the most significant factors in selecting a routing protocol. Additional factors include familiarity, support, and availability.

IP Routing Protocols

In the previous chapter, the Internet Protocol (IP) and the criteria for designing networks using IP were addressed. This chapter will build upon those concepts by adding the dynamic IP routing protocols including RIP, RIP version 2, IGRP, EIGRP, OSPF, ODR, BGP, and IS-IS.

Dynamic routing protocols were developed to circumvent the deficits found in static routing. This chapter will present network design with static routes, in addition to the IP routing protocols listed in Table 4.1. Please note that each of these protocols will be presented in greater detail later in this chapter—Table 4.1 is primarily concerned with providing an overview of the routing protocols that will be discussed.

TABLE 4 .1 Comparison of the IP Routing Protocols

Protocol Characteristics

RIP The Routing Information Protocol (RIP) supports IP and is still a reasonable choice for small networks that do not require variable-length subnets. It is supported by most vendors and is interoperable with servers and workstations. Unfortunately, RIP uses hops only to determine the path, and the hop count is limited to 15. In addition, updates are sent every 30 seconds and incorporate the entire routing table.
RIP v2 Version 2 of RIP builds upon the success of the original protocol. However, it is still limited by hop count, sends its complete routing table every 30 seconds, and is limited by a 15-hop network diameter. Version 2 also adds VLSM (variable-length subnet mask) support and authentication.
IGRP Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary, distance-vector, routing protocol. It uses a composite metric of 24 bits and offers faster convergence when compared to RIP. However, it does not support VLSM and sends its entire routing table every 90 seconds.
EIGRP Enhanced IGRP (EIGRP) is built upon IGRP, and thus the protocol is also proprietary to Cisco. It was designed for easy migration from existing IGRP networks and adds a number of features to the routing process. These enhancements include support for VLSM, fast convergence, incremental updates, compound metrics, and additional support for IPX and AppleTalk, which are not supported in IGRP.
OSPF The Open Shortest Path First (OSPF) routing protocol will typically be selected by designers looking for an open standards-based routing protocol that compares with EIGRP. Updates are based on a link-state database, which is shared by all routers in the network area.
IS-IS The Intermediate System-to-Intermediate System (IS-IS) protocol is also an open standards-based routing process that provides fast convergence. In addition, updates contain only changes. IS-IS uses a hello-based system (hello-based systems confirm the operation of the adjacent router with hello packets) and supports variable-length subnet masks; however, it has a limited metric and some topology restrictions. Updates are based on links, not routes.
ODR On-demand routing (ODR) makes use of data in the proprietary Cisco Discovery Protocol (CDP) function in the Cisco IOS (Internet Operating System). CDP packets typically provide diagnostic information only about other Cisco routers; however, the ODR process can use this information to develop a routing table. It is a very limited routing function, but it provides many of the benefits of static routes without incurring the over-head of a routing protocol.
BGP The Border Gateway Protocol (BGP) is the de facto protocol of the Internet backbone. Technically a path-vector protocol, the external version (eBGP) is primarily concerned with the relationships between autonomous systems (AS). One benefit to BGP is its use of persistent TCP sessions for the exchange of routing information.

Chapter 3 defined path determination as an overhead activity for the router. This factor directly impacts the selection of a routing protocol. Designers should consider the different resources that are needed to implement a routing protocol, including router CPU, router memory, link bandwidth, support staff familiarity, and protocol features, which include support for VLSM, summarization, and convergence.

Designers should ask themselves the following questions when selecting a routing protocol:

  Under what conditions are routing updates sent?
  This relates to timers, events, or both.
  What is transmitted during a routing update?
  Some protocols send only the changes to the routing table during an update. Other protocols send the entire routing table.
  How are routing updates propagated?
  Some routing protocols send updates and information only to adjacent neighbors, while others send information to a cluster of routers (an area) or to an autonomous system.
  How long does the convergence process take?
  The time required to converge all routing tables in the internet-work depends upon many factors. Re-convergence occurs when a path that is used suddenly becomes unavailable. Dynamic routing protocols make every effort to locate an alternative route to the destination. Some protocols, like EIGRP, calculate alternative paths before the failure, which facilitates rapid convergence. Other protocols require significant amounts of time to distribute information regarding the failure and calculate the alternative path.

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