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CISCO INTERNETWORK DESIGN EXAM OBJECTIVES COVERED IN THIS CHAPTER:
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.
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.
| 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:
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