Content Overview A router learns about routes to destination networks using a dynamic routing protocol, or by manually configured static routes. The router very likely will use a combination of dynamic and static routing to discover routes. Regardless of the method used, when a router determines that a route is the best path to a destination, it installs that route in its routing table. This module will describe methods for examining and interpreting the contents of the routing table.Network testing and troubleshooting are perhaps the most time consuming components of every network administrator’s job. Efficient testing and troubleshooting must be done in a logical, orderly, and well-documented fashion. Otherwise, the same problems will reoccur, and the network administrator will never truly understand the network. This module describes a structured approach to network troubleshooting and provides some tools to use in the troubleshooting process. Routing problems are among the most common and difficult for network administrators to diagnose. Identifying and solving routing problems may not be simple, but many tools exist that can make the task easier. This module will introduce several of the most important of these tools and provide practice in their use. Students completing this module should be able to:
Content 9.1 Examining the Routing Table 9.1.1 The show ip route command One of the primary functions of a router is to determine the best path to a given destination. A router learns paths, also called routes, from an administrator's configuration or from other routers by way of routing protocols. Routers store this routing information in routing tables using on-board dynamic random access memory (DRAM). A routing table contains a list of the best available routes. Routers use the routing table to make packet forwarding decisions.The show ip route command displays the contents of the IP routing table. This table contains entries for all known networks and subnetworks, as well as a code that indicates how that information was learned. The following are some additional commands that can be used with the show ip route command: A routing table maps network prefixes to an outbound interface. When RTA receives a packet destined for 192.168.4.46, it looks for the prefix 192.168.4.0/24 in its table. RTA then forwards the packet out an interface (Ethernet0) based on the routing table entry. If RTA receives a packet destined for 10.3.21.5, it sends that packet out Serial 0/0. The example routing table shows four routes for directly connected networks. These routes, labeled with a C, are available to directly connected networks. RTA drops any packet destined for a network that is not listed in the routing table. In order to forward to other destinations the routing table for RTA will have to include more routes. These new routes may be added using one of two methods: Administratively defined routes are said to be static because they do not change until a network administrator manually programs the changes. Routes learned from other routers are dynamic because they can change automatically as neighboring routers update each other with new information. Each method has fundamental advantages and disadvantages. Lab Activity Lab Exercise: Using Show IP Route to Examine Routing Tables In this lab, students will set up an IP addressing scheme using class B networks. Web Links show ip route Command http://www.cisco.com/en/US/products/sw/iosswrel
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Content 9.1 Examining the Routing Table 9.1.2 Determining the gateway of last resort It is not feasible, or even desirable, for a router to maintain routes to every possible destination. Instead, routers keep a default route, or a gateway of last resort. Default routes are used when the router is unable to match a destination network with a more specific entry in the routing table. The router uses this default route to reach the gateway of last resort in an effort to forward the packet. A key scalability feature is that default routes keep routing tables as lean as possible. They make it possible for routers to forward packets destined to any Internet host without having to maintain a table entry for every Internet network. Default routes can be statically entered by an administrator or dynamically learned using a routing protocol. Default routing begins with the administrator. Before routers can dynamically exchange information, an administrator must configure at least one router with a default route. Depending on the desired results, an administrator can use either of the following commands to statically configure a default route: ip default-network
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ip route 0.0.0.0 0.0.0.0 The ip default-network command establishes a default route in networks using dynamic routing protocols. The global command ip default-network 192.168.17.0 defines the Class C network 192.168.17.0 as the destination path for packets that have no routing table entries. For every network configured with ip default-network, if a router has a route to the network, that route is flagged as a candidate for the default route. Creating an ip route to 0.0.0.0/0 is another way to configure a default route. Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address | exit-interface] After configuring a default route or default network, the command show ip route will show the following: Gateway of last resort is 172.16.1.2 to network 0.0.0.0 Lab Activity Lab Exercise: Gateway of Last Resort In this lab, students will configure RIP routing and default routes (gateways) on the routers. Web Links ip default-network Command http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/ products_command_reference_ chapter09186a008007ff84.html#1050101