Content 9.1 Examining the Routing Table 9.1.3 Determining route source and destination For traffic going through a network cloud, path determination occurs at the network layer. The path determination function enables a router to evaluate the available paths to a destination and to establish the preferred handling of a packet. Routing services use network topology information when evaluating network paths. This information can be configured by the network administrator or collected through dynamic processes running in the network. The network layer provides best-effort, end-to-end, packet delivery across interconnected networks. The network layer uses the IP routing table to send packets from the source network to the destination network. After the router determines which path to use, it takes the packet from one interface and forwards it to another interface or port that reflects the best path to the packet's destination. Web Links Using the Extended ping and Extended traceroute Commands http://www.cisco.com/en/US/tech/ tk826/tk365/ technologies_ tech_ note09186a0080093f22.shtml
Content 9.1 Examining the Routing Table 9.1.4 Determining L2 and L3 addresses While network layer addresses are used to get packets from source to destination, it is important to understand that a different type of address is used to get packets from one router to the next. For a packet to get from the source to the destination, both Layer 2 and Layer 3 addresses are used. As shown in Figure , at each interface, as the packet moves across the network, the routing table is examined and the router determines the next hop. The packet is then forwarded using the MAC address of that next hop. The IP source and destination headers do not change, at any time. The Layer 3 address is used to route the packet from the source network to the destination network. The source and destination IP addresses remain the same. The MAC address changes at each hop or router. A data-link layer address is necessary because delivery within the network is determined by the address in the Layer 2 frame header, not the Layer 3 packet header. Interactive Media Activity Drag and Drop: L2 and L3 Address After completing this activity, the student will be able to identify L2 and L3 addresses. Web Links Using the Extended ping and Extended traceroute Commands http://www.cisco.com/en/US/tech/ tk826/tk365/ technologies_tech_ note09186a0080093f22.shtml
Content 9.1 Examining the Routing Table 9.1.5 Determining the route administrative distance A router can discover routes using dynamic routing protocols, or routes can be configured manually on the router by an administrator. After the routes are discovered or configured, the router must choose which routes are the best routes to given networks.The administrative distance of the route is the key information that the router uses in deciding which is the best path to a particular destination. The administrative distance is a number that measures the trustworthiness of the source of the route information. The lower the administrative distance, the more trustworthy the source. Different routing protocols have different default administrative distances. If a path has the lowest administrative distance it is installed in the routing table. A route is not installed in the routing table if the administrative distance from another source is lower. Lab Activity e-Lab Activity: Administrative Distance In this lab, the students will analyze the effects of using two routing protocols. Web Links What Is Administrative Distance? http://www.cisco.com/en/US/tech/ tk826/tk365/ technologies_tech_ note09186a0080094195.shtml
Content 9.1 Examining the Routing Table 9.1.6 Determining the route metric Routing protocols use metrics to determine the best route to a destination. The metric is a value that measures the desirability of a route. Some routing protocols use only one factor to calculate a metric. For example, RIP version 1 (RIP v1) uses hop count as the only factor to determine the metric of a route. Other protocols base their metric on hop count, bandwidth, delay, load, reliability, ticks delay, maximum transmission unit (MTU), and cost. Each routing algorithm interprets what is best in its own way. The algorithm generates a number, called the metric value, for each path through the network. Typically, the smaller the metric number, the better the path. Factors such as bandwidth and delay are static because they remain the same for each interface until the router is reconfigured or the network is redesigned. Factors such as load and reliability are dynamic because they are calculated for each interface in real-time by the router. The more factors that make up a metric, the greater the flexibility to tailor network operations to meet specific needs. By default, IGRP uses the static factors bandwidth and delay to calculate a metric value. These two factors can be configured manually, allowing precise control over what routes a router chooses. IGRP may also be configured to include the dynamic factors, load and reliability, in the metric calculation. By using dynamic factors, IGRP routers can make decisions based on current conditions. If a link becomes heavily loaded or unreliable, IGRP will increase the metric of routes using that link. Alternate routes may present a lower metric than the downgraded route and would be used instead. IGRP calculates the metric by adding the weighted values of different characteristics of the link to the network in question. In the following example the values bandwidth, bandwidth divided by load, and delay, are weighted with the constants K1, K2, and K3. Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) + K3 * Delay The default constant values are K1=K3=1 and K2=K4=K5=0 so: Metric=Bandwidth + Delay Interactive Media Activity Drag and Drop: Route Metric After completing this activity, the student will be able to understand route metric. Web Links Behavior of RIP and IGRP When Sending and Receiving Updates http://www.cisco.com/en/US/tech/index.html
Content 9.1 Examining the Routing Table 9.1.7 Determining the route next hop Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router. This router represents the next hop on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop. Web Links Using the Extended ping and Extended traceroute Commands http://www.cisco.com/en/US/tech/ index.html
Content 9.1 Examining the Routing Table 9.1.8 Determining the last routing update Use the following commands to find the last routing update: Lab Activity Lab Exercise: Last Route Update In this lab, the student will gather information about routing updates and routing protocols. Web Links Missing RIP Routes? Popular Troubleshooting Techniques http://www.cisco.com/en/US/about/ac123/ ac114/ac173/ac169/ about_cisco_packet_ enterprise_ solution09186a00800a3453.html
Content 9.1 Examining the Routing Table 9.1.9 Observing multiple paths to destination Some routing protocols support multiple paths to the same destination. Unlike single path algorithms, these multi-path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable.Rt1 has two routes to network 192.168.30.0. The variance command will be set on Rt1 to ensure that both paths to network 192.168.30.0 are utilized. Figure shows the output from show ip