Cisco Systems CCNA 2 Determining route source and destination, Determining L2 and L3 addresses

An administrator can use two different commands to configure default routes:

•ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address exit-interface]

•ip default-network

The ip default-networkcommand establishes a default route in networks with dynamic routing protocols. Discuss the important concept of the gateway of last resort and the two commands used to configure default routes. Have the students brainstorm and present their ideas on why routers do not maintain routes to every possible destination to the class. One thing the students should know is that the ip route 0.0.0.0 0.0.0.0 command is referred to as the “quad zero route”.

9.1.3 Determining route source and destination

Path determination occurs at the network layer, which is Layer 3. The path determination function enables a router to evaluate the available paths to a destination and to establish the preferred way to handle a packet. The network layer provides best-effort, end-to-end packet delivery. The network layer uses the IP routing table to send packets from the source network to the destination network. Discuss Layer 3 of the OSI model as a review. This should be a concept that the students have already mastered.

9.1.4 Determining L2 and L3 addresses

While network layer addresses are used to get packets from their 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. Explain how important this concept is for the students to understand. The Layer 3 address is used to route a 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 the source host must have a way to address the next-hop router to which the packets are being forwarded.

For students to understand the routing process, the Layer 2 and 3 addresses must be understood. Be sure to review the various names for IP addresses, such as a Layer 3 address, a network layer address, or a logical address. Review the same concept for a MAC address, that it is also called a Layer 2 address, a data link address, or a physical address. It is also important to understand that the MAC will change while the IP will remain the same. Remind the students that the packet will remain intact from the source host to destination host. At each hop along the path a new frame is created and addressed in the frame to the next hop.

9.1.5 Determining route administrative distance

One of the most intriguing aspects of Cisco routers is how the router chooses which route is the best. As each routing process receives updates and other information, it chooses the best path to any given destination and attempts to add this path to the routing table. The router decides whether or not to add the routes presented by the routing processes based on the administrative distance of each route. The route with the lowest administrative distance is considered the best route.

103 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright ♥ 2004, Cisco Systems, Inc.