See CiscoMPLS for MPLS and OSPF backdoor issues including Sham-link.
- Due to redistribbution.
- Two autonomous systems, connected with dual links, redistributing routes.
- Router through R1 learned from R2.
OSPF notes http://bradhedlund.com/notes/ospf/
OSPF dual distribution example http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080531fd2.shtml#singlered
OSPF - EIGRP route tag and filter http://shawnmoorecisco.blogspot.com/2010/06/preventing-redistributed-routing-loops.html