Which two statements are correct about a sham link? (Choose two.)

A sham link is a logical, unnumbered point-to-point link configured between two Provider Edge (PE) routers within a specific VRF instance. Its primary purpose in an MPLS L3VPN environment is to make the path over the MPLS backbone appear as an OSPF intra-area link. This is necessary when a customer has a direct "backdoor" link between two sites that are also connected via the VPN. By creating an OSPF adjacency over the sham link, the backbone path is advertised using Type 1 LSAs, which OSPF prefers over the inter-area path (Type 3 LSAs) that would otherwise be advertised. This ensures traffic correctly favors the MPLS backbone over the backdoor link.

B. A sham link is an OSPF-specific feature used for PE-CE routing. It does not create or modify BGP neighborships, which are handled separately by MP-BGP between PEs.

D. This statement is the inverse of the sham link's function. The sham link is specifically created to force the use of Type 1 LSAs instead of Type 3 LSAs for the backbone path.

1. Juniper Networks TechLibrary

"Understanding OSPF Sham Links for PE-to-PE Router Connectivity in VPNs": "To make the PE router advertise the routes as intra-area routes (type 1 LSAs)

you can configure a sham link between the PE routers. A sham link is an unnumbered point-to-point intra-area link between two PE routers." This source confirms that the sham link's purpose is to use Type 1 LSAs (Option C) and that it creates a link between two PE routers for OSPF (Option A).

2. RFC 4577

"OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)"

Section 4.2.7

"The Sham Link": "To make the path over the VPN backbone an intra-area path

a 'sham link' is created between the PEs... The sham link is advertised as a point-to-point link in a Type-1 LSA for the area." This RFC

which defines the standard

directly supports that the link is between PEs (Option A) and is advertised via a Type 1 LSA (Option C). The adjacency is formed over the MPLS core

which is inherently a multihop transport path.