Which CE-to-PE routing protocol implements the down bit as a loop prevention mechanism?

A.
RIPv2

B.
EIGRP

C.
IS-IS

D.
OSPF

E.
BGP

Explanation:

https://www.racf.bnl.gov/Facility/TechnologyMeeting/Archive/06-30-04-CISCO/Using-OSPF-inMPLS-VPNEnvironment.pdf