200-301 ocg

200-301 ocg.So far, the review of routing logic has focused on a single router from three different perspectives: a longer textual description of the process, a shorter textual description of the process, and a graphical description. But the logic on a single router does not allow for end-to-end delivery of packets over an IP network. End-to-end delivery requires that each router use the same logic, with IP routing telling each router to forward the packet to the next router, all the way down until the packet reaches its destination.Figure 4-4 shows the end-to-end impact of the packet routing process, focusing on the decapsulation (Step 2) and encapsulation (Step 4) processes on each router. Each router will drop the data link frame headers and tail frames of the upcoming frames, so each frame moves from just one host to one router, or between routers. Each router will add a new data link frame header and tail frame, and a new frame will be created each time the router forwards a packet. The address field of the data link frame header only affects the local data link.The diagram shows PC1 sending a packet to PC2, which contains a variety of different frames. At the top, PC1 sends an Ethernet frame to Rl, which decapsulates the packet and re-encapsulates it into an HDLC frame. r2 repeats the same process, except this time it encapsulates the packet in a frame relay. r3 also repeats the process, which encapsulates the packet in an Ethernet frame and sends the frame to PC2 O10.1.1.1 / Encapsulating IP packets in Ethernet|Eth |IP packets |Eth I168.1.1.1Figure 4.4 Encapsulation and decapsulation of packets during routing2.2 Basic Content 92Constructing a new data link frame header using ARP informationAnother important part of the routing process is how the router selects the data link address to be used in the new data link frame header. Typically, the data link frame header lists an address, such as the MAC address used in the Ethernet data link frame header. To encapsulate IP packets and deliver them to certain types of data links, the router must first know the data link addresses of other devices on the output link. For example, in Figure 4-4, router R3 must know the MAC address of PC2 before it can construct a unicast Ethernet frame and send it to PC2 at the bottom.