The 15.7(3)M release is part of the "Extended Maintenance" train for Cisco IOS, offering stability and a wide range of enterprise capabilities.
The image represents a "sweet spot" for network simulation. It offers the feature-rich environment of IOS 15.x with the performance efficiency of IOU/IOL.
i86bi_LinuxL3-AdvEnterpriseK9-M2_157_3_May_2018.bin IOS Version: 15.7(3)M2 Release Date: May 3, 2018