Content

| 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.

This module focuses on IGPs. A comprehensive discussion of EGPs, particularly BGP4, will be presented in Module 8. These protocols can be further categorized as either distance vector 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 IPX and 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 because of a past investment in protocol-specific printers, software, and servers. Although Cisco EIGRP offers comprehensive support for both IPX and AppleTalk, it is important to be familiar with three proprietary routing protocols. These three protocols are IPX or Novell RIP, NetWare Link Services Protocol (NLSP), and AppleTalk Routing Table Maintenance Protocol (RTMP). Implementation of AppleTalk and Novell proprietary routing protocols is beyond the scope of this curriculum.

IP routing protocols and the routing table
Cisco IOS commands enable dynamic routing based 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. Notice that most organizations would not normally use more than one or two routing protocols.

Figure examines 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 [administrative distance/metric]. For example, [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. Hop count is the only factor that IP RIP uses to determine the metric. In the sample entry shown in Figure , the number 3 indicates that the destination network is three hops away.

Each routing protocol calculates its metrics differently. EIGRP uses a complex combination of factors that include bandwidth and reliability to calculate its metric. Using the default settings, the EIGRP 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, the router will use the administrative distance metric to compare the two. This process is similar to comparing three apples to over three million oranges.

When a router receives updates from different routing protocols about the same network, it cannot use dissimilar metrics to evaluate a route. The router will use administrative distance as the deciding metric for route validation. Cisco IOS assigns a default administrative distance to every routing protocol. A lower value signifies a more trustworthy routing protocol. A complete list of administrative distances can be found in Module 8, Route Optimization.

| Interactive Media Activity Drag and Drop: Routing Table Entry In this media activity, the student will identify the different fields in a routing table entry.