Cisco IOS On Linux (IOL) Containers
Most virtual Cisco routers are available as virtual machines (VM). One exception are the IOS On Linux (IOL) routers. These are Cisco IOS-XE routers running the latest IOS-XE version that are supplied as containers.
Containers have far less overhead than VMs so this is an advantage for labs. The containers require less CPU power and memory compared to the router VMs. They also boot noticeable faster.
Most features are supported. L3 routing is no problem, L2 switching is not supported. Some L2 encapsulation options like EoMPLS and L2TPv3 are supported though.
As of writing, RESTCONF and NETCONF are not supported. EEM Cisco Embedded Event Manager is also not supported.
L1 signaling also does not exist. In practice, this means that from within the container, an interface will always show as UP.
Because these containers are so lightweight and quick to start, it's a good idea to use them in your labs. They will work fine with general "routing" labs such as OSPF, EIGRP, BGP, etc.
If you get stuck or think a feature doesn't work, you can always try a VM instead.
Cisco CML comes with these IOL containers. Since Cisco CML 2.8, you can download it for free: