I saw this question:
Suppose a university with address block 184.108.40.206/16 has a link connected to AT&T, where the AT&T router forwards packets destined to 220.127.116.11/16 to the university router. Suppose the university router has three forwarding entries: 18.104.22.168/24 out the link to the math department, 22.214.171.124/24 out the link to the CS department, and a “default route” for 0.0.0.0/0 pointing to the AT&T router. Suppose a host in the rest of the Internet sends a packet destined to 126.96.36.199. What would happen to that packet? What could be done to prevent it?
the answer is :
The packets would loop between the AT&T and university routers, because AT&T would forward the packet to the university (using the route for 188.8.131.52/16) and the university would forward the packet back to AT&T (using the default route 0.0.0.0/0). The university should configure a “null route” to drop all packets matching 184.108.40.206/16 to prevent this. See http://www.nanog.org/mtg-0602/gao.html for details about this issue, and the security vulnerabilities associated with it.
I can't understand what are the security vulnerabilities associated with it
all above taken from http://www.cs.princeton.edu/courses/archive/spring11/cos461/exams.html