I was configuring some VRF lite and I had two VRFs that I was configuring on 4 Routers in my isp. I was trying to figure out how the other side would know how to separate traffic from the different VRFs. At first I thought well maybe if the other side gets traffic from a VRF it would know. Now that I think of it there would be nothing in the Ethernet/IP header to tell the router anything. The answer is 802.1q which is not obvious because when you think of routers you don’t think of 802.1q unless there is a switch on the other side(router on a stick). To be explicit, you 802.1q encapsulate traffic on sub interfaces on the neighboring routers to transport traffic in different vrfs between routers if you are only using one interface. Networking is awesome 
R1
interface GigabitEthernet0/2.1
encapsulation dot1Q 10
ip vrf forwarding Gandalf
ip address 192.168.2.1 255.255.255.0
end
R2
interface GigabitEthernet0/0.1
encapsulation dot1Q 10
ip vrf forwarding Gandalf
ip address 192.168.2.2 255.255.255.0
end