200-301 online

200-301 online.172.16.2.2 0200.0202.0202~0200.0202.0202 172.16.2.2sw2: tfea tableMACifcht output0200.2222.2222 F0/20200.0202.0202 F0/10Figure 4-10 ARP and MAC Address TablesAs you saw on the previous pages, using the ping command strategy from a nearby problem source can eliminate many of the root causes of problems between two hosts-assuming the ping command is successful. Nevertheless, the ping command here is not exactly the same as the ping command on the actual host. To overcome the pitfalls of issuing ping commands from nearby routers The next few examples show strategies for testing alternative paths between two hosts that may be problematic. Using extended ping to test reverse routingThe ping command from the default router, as discussed on the previous pages, sometimes fails to test IP routing comprehensively. In particular, it cannot test for reverse routes back to the original host.For example, going back to the interconnected network in Figure 4-7, notice that the reverse route does not point to an address in the host A subnet. When R1 processes the ping 172.16.2.101 command, R1 must select a source IP address to be used in response to the request, and the router will select the IP address of the output interface. The response request from R1 to Host B will flow with the source IP address 172.16.4.1 (R1's S0/0/0 IP address). The response reply will flow back to the same address (172.16.4.1).