source: http://www.securityfocus.com/bid/2684/info

Hot Standby Routing Protocol is an Internet Protocol based routing protocol implemented by Cisco Systems. It is designed to offer traffic rerouting services to networks when one router within a pool ceases to operate, and users of the network segment aren't using ICMP Router Discovery Protocol to find the new router handling traffic for their segment.

A problem in the Cisco Hot Standby Routing Protocol (HSRP) makes it possible to deny service to users of network resources. By eavesdropping on HSRP management messages sent over the network, it is possible to create a spoofed message that will reroute all network traffic to a particular system. By doing so, it is possible to prevent traffic from entering or leaving that network.

This problem makes it possible for system local to the network to deny service to legitmate users of that network segment. 

https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/20821.tgz