![]() ![]() But in our scenario, it does have a route back to R1 at 10.0.0.1. You go on to R1 to test this, and you ping PC3 at 10.1.2.10, but what's causing the actual issue is that R4 does not have a route back to PC1 at 10.0.1.10. So if you look at the scenario here, a user on PC1, over in the top right here, complains that they can't access PC3. The next thing to tell you is about the via extended ping command. For example, if it's being blocked by an Access Control List (ACL). Less common is, you can see U's, which means that it is unreachable, and that happens if the router discards the packet. So this could be because your router does not have a corresponding route or the destination IP address is not responding. If you see five dots, though, that is bad. ![]() So if we've got a success rate at 80%, four out of five pings are good, that would mean that we do have connectivity. It's quite normal for the first ping to fail if the router is updating its ARP cache at the time. In the example here, we've got one dot indicating the first ping failed and then four exclamation marks, which says that it succeeded. If the ping is successful on a Cisco router, you will see exclamation marks. It tests that the source can get to the destination and also that the reply can get back from the destination, back to the source again. That will get to R3, all being good on the network, and R3 will then send an ICMP echo reply back to R1. R1 will send out an ICMP echo request, using the exit interference as the source IP and wherever I configured it to send to as the destination IP. I'm going to do a ping from R1 to the 10.1.0.1 IP address on R3. ICMP stands for the Internet Control Message Protocol, and it's used for testing connectivity. Ping uses ICMP, which like TCP, UDP, IP, etc., makes up part of the TCP/IP suite. ![]()
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