3.1 Routing
3.1.3 Configuring dynamic routing
Dynamic routing of TCP/IP can be implemented using one or more protocols. These protocols are often grouped according to where they are used. Routing protocols designed to work inside an autonomous system are categorized as interior gateway protocols (IGPs), and protocols that work between autonomous systems are classified as exterior gateway protocols (EGPs). Figure lists widely supported EGPs and IGPs for TCP/IP routing.

A comprehensive discussion of EGPs, in particular BGP4, can be found in Chapter 8, BGP. Chapter 3 focuses on IGPs. As shown in Figure , you can further categorize these protocols as either distance-vector routing protocols or link-state routing protocols, depending on their method of operation.

Routing protocols for IPX and AppleTalk
Despite the dominance of IP, a significant number of organizations continue to support legacy protocols, such as Novell's IPX and Apple's AppleTalk. A legacy technology is one that is supported because of a significant past investment or deployment. Many organizations continue to support IPX and AppleTalk to leverage a past investment in protocol-specific printers, software, and servers. Although Cisco's EIGRP offers comprehensive support for both IPX and AppleTalk, it is important to be familiar with the names of the following proprietary routing protocols: IPX RIP (or Novell RIP), NetWare Link Services Protocol (NLSP), and AppleTalk's Routing Table Maintenance Protocol (RTMP).

Implementing these Apple and Novell proprietary routing protocols is beyond the scope of this curriculum.

IP routing protocols and the routing table
The Cisco IOS commands to enable dynamic routing vary depending on the routing protocol used. Figure displays the routing table of a router configured to use four IP routing protocols: RIP, IGRP, EIGRP, and OSPF. Note that most organizations would not normally use more than one or two routing protocols. This example is provided to show different types of routing table entries.

Figure dissects the specific table entry for 192.168.1.0/24. Routes in the routing table that are not directly connected include two numbers offset by brackets, in the form [administrative distance/metric]. Therefore, [120/3] means that the administrative distance is 120 and the metric is 3. Routers base their evaluations of routes on these two numbers. Since this is a RIP route, the metric represents hop count.

Routers use metrics to evaluate, or measure, routes. When multiple routes to the same network exist and the routes are from the same routing protocol, the route with the lowest metric is considered the best. IP RIP uses only one factor to determine the metric: hop count. In the sample entry shown in Figure , the number 3 indicates that the destination network is three hops away, which is a better metric than four hops.

Each routing protocol calculates its metrics differently. For example, EIGRP uses a sophisticated combination of factors that typically includes bandwidth and reliability to calculate a metric. With default settings, EIGRP's metric for the same route to 192.168.1.0 is 3,219,456! If RTA receives a RIP update and an EIGRP update for this same network, how can the router compare what is, in effect, three apples against more than 3 million oranges? That is where administrative distance comes in.

When a router receives updates from different routing protocols about the same network, it can not use dissimilar metrics to evaluate a route. It uses administrative distance to decide which protocol to believe. The Cisco IOS assigns a default administrative distance to every routing protocol; the lower the value, the more trustworthy the routing protocol. A complete list of administrative distances can be found in Chapter 7, Route Optimization.