For some truly unknown reason, Cisco’s devices still have support for “small servers” or “simple services”. Examples of these include echo, chargen, daytime and discard. An attacker could possibly start a denial of service attack (DoS) against one or more network devices with those configured. In this case; echo and chargen are to blame by allowing an attacker to cause the chargen service to hit the echo services causing an endless loop of character generation and echo between the two hosts. To disable this, simply enter the following commands:
no service tcp-small-servers
no service udp-small-servers
These commands can be run on nearly all IOS based Cisco equipment.