IOU L3 can act as a DMVPN hub or spoke, supporting NHRP, IPsec, and GRE. Perfect for CCIE Security or Enterprise labbing.
Here is everything you need to know about why IOU L3 still matters and how to make it sing in GNS3. Cisco IOU L3 - GNS3
GNS3 needs an "IOU Mapping" file ( i86bi_linux_l2-adventerprisek9-ms ). This tells GNS3 which binary to use for which virtual device. You can usually find a default mapping file online. IOU L3 can act as a DMVPN hub
GNS3 is a graphical network emulator that allows you to drag, drop, and connect virtual devices. By integrating IOU L3 with GNS3, you get: Cisco IOU L3 - GNS3