200-301 odom

200-301 odom.To help learn the MAC addresses of other devices on the LAN, IPv4 uses the Address Resolution Protocol (ARP).ARP defines two messages (an ARP request and an ARP reply). The sending host and router will use the ARP request to generate a request like this: If this is your IP address, please tell me your MAC address. The second host sends back an ARP reply listing their IPv4 address and Ethernet MAC address. For example, Figure 4-5 shows the same network as Figure 4-4, showing the ARP process that occurs before R3 forwards the packet (encapsulated in a frame) to the MAC address of PC2.The CCENT and CCNA issues contain just one LAN data link protocol (Ethernet) and three WAN data link protocols (HDLC, PPP, and FrameRelay) Ethernet uses ARP, as shown in Figure 4-5, and HDLC and PPP are used on serial links in a point-to-point topologydo not require ARP-like functionality. Frame Relay, on the other hand, uses a similar feature known as reverse ARP, which is discussed in the Chapter 13 section of this book.168.1.1.1 0200.2222.2222Figure 4-5 Simple ARP ProcessIn summary, hosts forward packets to their default routers when the destination address of the packet is located on another subnet. Each router will decapsulate the packet, choosing where to forward it and re-encapsulating the packet into a new data link frame. Only IP packets will flow from the source to the destination host, but each data link frame header/tail frame will be used as one of the ways in which packets move from host to router, or router to router, or router to host.Now that you understand how hosts and routers forward IPv4 packets, the next two topics will discuss how to isolate the root cause of the problem using the ping command and the traceroute command. These commands eliminate the need for you to consider the IPv4 routing process portion, as these commands determine that it is working properly.4.2.2 Using the ping Command to Isolate ProblemsSomeone sends you an email or short phone message requesting that you take a look at a user's network problem. You have taken SSH measures on the router and the ping command is working fine. Can you rule out some possible causes of the problem? And what is the most fundamental cause?You then execute a ping command in another location, but this time the ping command does not have the desired effect. What does the failure of the ping command tell you this time? Which part of IPv4 is likely to be the problem, or do you now know which part is causing the problem?