There is no command
ip vrf forwarding frvf/ivrf
But there is concept of using tunnel or source of tunnel in vrf.
Let assume you have hub and spoke
In spoke you use default route to connect to ISP and in same time we receive defualt router from hub
If you use one routing table then either you will lost connect to isp or to hub
Solution is put each one
Tunnel in ivrf
Source of tunnel in fvrf
This solve the issue of conflict between two routing table
