Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 49 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 449 Tuesday May 20 2003 2 53 PM Routed Routable and Routing Protocols 449 Figure 8-20 Encapsulation E-Mail Message Data Data Data Segment Packet Network Header Data Frame Header Network Header Data Frame Trailer 101101010001101110100011010111010010 Frame Medium Dependent IP determines the form of the IP packet header which includes addressing and other control information but does not concern itself with the actual data. It accepts whatever is passed down from the higher layers as shown in Figure 8-21. Figure 8-21 IP Header IP Header Information Data From Upper Layers Packet Propagation and Switching Within a Router As a packet travels through an internetwork to get to its final destination the frame s headers and trailers are stripped and replaced at every router Layer 3 device as shown in Figure 8-22. The reason for this is that Layer 2 units frames are for local addressing and Layer 3 units packets are for end-to-end addressing. Layer 2 Ethernet frames are meant to work within a broadcast domain and to work with the MAC addresses that are assigned to the physical devices. Other Layer 2 frame types include point-to-point serial links and Frame Relay WAN connections that use their own Layer 2 addressing scheme. The main point is that no matter what type of Layer 2 addressing is used it is designed to stay within a Layer 2 broadcast domain. As the data crosses a Layer 3 device the Layer 2 information changes. Page 450 Tuesday May 20 2003 2 53 PM 450 Chapter 8 Routing Fundamentals and Subnets Figure 8-22 Network Layer Device Data Flow X Y Application ABC Application Presentation Presentation Session Session Transport Transport Network Network Network Network Network Data Link Data Link Data Link Data Link Data Link Physical Physical Physical Physical Physical The exact process that is performed at a Layer 3 device is detailed in Figure 8-23 and the following paragraph. As a frame is accepted at a router interface the destination MAC address
