Hello Richard
The lesson describes a situation where a customer wants to run EIGRP on their internal networks, and thus also between their CE routers and the ISP’s PE routers. In the EIGRP between PE and CE routers section of the lesson, you can see this configuration using EIGRP.
In this MPLS topology, as in most MPLS implementations, OSPF is used as the IGP within the infrastructure of the MPLS network. Most MPLS networks are deployed using the MPLS/OSPF/BGP combination of technologies, even though MPLS/EIGRP/BGP is also possible.
The main reason OSPF is used is because it is a link state routing protocol. This is especially useful when using what is known as MPLS traffic engineering (MPLS-TE). You can find out more about MPLS-TE at the following link, although it is outside the scope of any current Cisco CCNP or CCIE Enterprise certifications.
IS-IS can also be used as the IGP within an MPLS backbone as it too is a link state routing protocol.
I hope this has been helpful!
Laz