Super cool feature if you have GRE going through an Encryptor or other encryption/tunnels.
Simply type ping and press enter at your IOS prompt:
Switch#ping Protocol [ip]: Target IP address: 10.0.1.1 Repeat count [5]: 1 Datagram size [100]: Timeout in seconds [2]: Extended commands [n]: y Source address or interface: Type of service [0]: Set DF bit in IP header? [no]: y Validate reply data? [no]: Data pattern [0xABCD]: Loose, Strict, Record, Timestamp, Verbose[none]: Sweep range of sizes [n]: y Sweep min size [36]: 1300 Sweep max size [18024]: 1600 Sweep interval [1]: Type escape sequence to abort. Sending 300, [1300..1600]-byte ICMP Echos to 10.0.1.1, timeout is 2 seconds: Packet sent with the DF bit set !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!M.M.M.M....... Success rate is 86 percent (93/107), round-trip min/avg/max = 57/68/72 ms