The ping command is a very common method for testing network connectivity of devices. It uses ICMP Echo messages to determine whether a remote host is active, the round trip time (RTT) for communicating with the host, and the packet loss.
ICMP Type Values
| ICMP Type | Meaning and Code Values |
|---|
| 0 | Echo-reply |
| 3 | Destination unreachable code 0 = net unreachable 1 = host unreachable 2 = protocol unreachable 3 = port unreachable 4 = fragmentation needed and DF set 5 = source route failed |
| 4 | Source-quench |
| 5 | Redirect code 0 = redirect datagrams for the network 1 = redirect datagrams for the host 2 = redirect datagrams for the type of service and network 3 = redirect datagrams for the type of service and host |
| 6 | Alternate-address |
| 8 | Echo |
| 9 | Router-advertisement |
| 10 | Router-solicitation |
| 11 | Time-exceeded code 0 = time to live exceeded in transit 1 = fragment reassembly time exceeded |
Cisco IOS ping Output
| Character | Description |
|---|
| ! | Each exclamation point indicates receipt of a reply. |
| . | Each period indicates that the device timed out while waiting for a reply. |
| U | A destination unreachable error protocol data unit (PDU) was received. |
| Q | Source quench (destination too busy). |
| M | Could not fragment. |
| ? | Unknown packet type. |
| & | Packet lifetime exceeded. |