Cisco IOS 15.0 and 15.1 and IOS XE 3.x do not properly handle the "set mpls experimental imposition" command, which allows remote attackers to cause a denial of service (device crash) via network traffic that triggers (1) fragmentation or (2) reassembly, aka Bug ID CSCtr56576.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.