Smurf attack

The Smurf attack is a distributed denial-of-service (DDpS) attack in which large numbers of ICMP (Internet Control Message Protocol) packets with the intended victim’s spoofed source IP are broadcast to a computer network using an IP broadcast address. Most devices on a network will, by default, send a reply to the source IP address. If the number of machines on the network that receive and respond to these packets is very large, the victim’s computer will be flooded with traffic. This can slow down the victim’s computer to the point where it becomes impossible to work on.

In past, default value for routers was to pass such packets, but now it is changed. To disable this value use:
Router(config-if)# no ip directed-broadcast
On this way network can be protected from incoming attack and prevent attack on other networks from inside.

A fraggle attack is a variation of a Smurf attack. In this case an attacker sends a large amount of UDP traffic to ports 7 (echo) and 19 (chargen) to an IP Broadcast Address.

Read more:
Understanding a Smurf attack is the first step toward thwarting one