| Lab 12.5.3 Preventing routing loops Estimated time: 45 min. Objectives: Understand methods of controlling routing loops including hold-down timers, defining a maximum hop count, counting to infinity, poison reverse and split-horizon. Adjust the RIP maximum hop count to control routing loops. Background: In the previous challenge lab, you saw how long it took to converge when a link went down. In this lab, your task is to find out how to prevent and control routing loops. The use of hold-down timers, defining a maximum hop count, counting to infinity, poison reverse and split-horizon are all methods of controlling routing loops. You will use the RIP hop count metric to control routing loops in this lab. You should have finished Lab 12.5.2 and have the 4th set of WAN cables connected from Lab-A Serial 1 to Lab-E Serial 0. To learn more about timers look at the worksheet answers "Understanding Timers". Tools / Preparation: Prior to starting this lab you will need to have the equipment for the standard 5-router lab available. The routers and workstations should be pre-configured by the instructor or lab assistant with the correct IP settings prior to starting the lab. Before beginning this lab you may want to review Chapters 11 in the Cisco Networking Academy First-Year Companion Guide and Semester 2 On-line Chapter 12. Resources Required: 5 PC workstations (min.) with Windows operating system and HyperTerminal installed. 5 Cisco Routers (model 1600 series or 2500 series with IOS 11.2 or later). 4 Ethernet hubs (10BASE-T with 4 to 8 ports). One Ethernet switch (Cisco Catalyst 1900 or comparable). 5 serial console cables to connect workstation to router console port (with RJ45 to DB9 converters). 4 Sets of V.35 WAN serial cables (DTE male/ DCE female) to connect from router to router. CAT5 Ethernet Cables wired straight through to connect routers and workstations to hubs and switches. AUI (DB15) to RJ45 Ethernet transceivers (Quantity depends on the number of routers with AUI ports) to convert router AUI interfaces to 10BASE-T RJ45. Websites Sites Required: Routing basics General information on routers 2500 series routers 1600 series routers Terms and acronyms IP routing protocol IOS command summary Notes: Step 1 – Turn on debugging. Working with router Lab-C, turn on debugging with the debug ip rip command. Step 2 – Shutdown Lab-A’s Ethernet 0 interface. Shutdown Lab-A’s Ethernet 0 interface. From Lab-C, watch the routing information and use the show ip route command to see how many routing updates it takes to flush out Lab-A’s Ethernet 0 network. 1. How many updates did it take to converge? Step 3 – Enable Lab-A’s Ethernet 0 interface. On Lab-A bring Ethernet 0 back up and allow enough time for the network to converge. Step 4 – Configure default metric, timers basic and split-horizon on Lab-C. There are other timers that can be modified to help avoid routing loops. This lab focuses on hop count. Change the RIP maximum hop count on router Lab-C to 10 (the default is 16), adjust the routing timers and split horizon using the following commands: Lab-C#conf t Lab-C(config)#router rip Lab-C(config-router)#default-metric 10 Lab-C(config-router)#timers basic 30 60 150 30 Lab-C(config-router)#exit Lab-C(config)#int s0 Lab-C(config-if)#ip split-horizon Lab-C(config-if)#int s1 Lab-C(config-if)#ip split-horizon Lab-C(config-if)#^Z Lab-C# Step 5 - Shutdown Lab-A's Ethernet 0 interface. Shutdown Lab-A's Ethernet 0 interface. From Lab-C, watch the routing information and use the show ip route command to see how many routing updates it takes to flush out Lab-A's Ethernet 0 network. 2. How many updates did it take to converge? 3. Compare question 1 and 2 and explain why the network converged faster after changing the default metric, timers and split horizon.