A.A sham-link is created between any twoo VPN sites that belong to the same OSPF area and share an OSPF backdoor link
B.A sham-link is seen as an intra-area link between PE routers，an OSPF adjacency is created and data base is exchanged
C.A SHAM link is created between two directly connected PE routers so that it is preferred over the low speed backdoor door
D.A SHAM link is part of OSPF network of MPLS-VPN provider

A.AS numbers are assigned by the IANA
B.AS numbers are used by BGP （Border Gateway protocol ） to exchange IP routing information with neighboring AS
C.In the internet， an Autonomous System （AS） is a collection of IP networks and routers under the control
D.All of the Above