H12-891_V1.0 Exam Dumps : HCIE-Datacom V1.0

20This is a classic MPLS L3VPN with OSPF + Sham Link scenario.Key Components:CE1 to PE1 (GE0/0/0): OSPF cost = 10CE2 to PE2 (GE0/0/0): OSPF cost = 10PE1 to PE2 via Sham Link (over loopback interfaces)What happens:CE1 advertises 192.168.1.0/24 to PE1.PE1 redistributes it into the VPNv4 BGP routing table.PE2 learns this route and redistributes it back into OSPF for CE2.Route Options Seen by CE2:Backdoor link (intra-area):CE1 ↔ CE2 directly, via GE0/0/1 on each CE → OSPF cost = 10 + 10 = 20Sham Link (inter-area route): By default, if no sham link is used, the BGP-learned route is treated as an inter-area route by CE2, with a default OSPF cost of 1 (which would make CE2 prefer this over the real CE-CE backdoor link).Purpose of Sham Link:Sham link is configured to equalize or override the cost of the BGP route so that CE2 prefers the backdoor link or sees both paths equally.In this case, Huawei defaults will cause CE2 to compute the total cost as:PE1–CE1 = 10Sham link route via PE1–PE2–CE2 = also 10 (CE1 to PE1) + 10 (PE2 to CE2) = 20Final OSPF Cost Calculation at CE2:Route learned via sham link path: 10 (CE1 to PE1) + 10 (PE2 to CE2) = 20Correct answer: 20In Huawei MPLS L3VPN OSPF deployment:A sham link ensures that the cost via the MPLS VPN path is comparable to the real CE-to-CE OSPF backdoor path, preventing suboptimal routing.Without sham link: BGP redistributed routes appear as inter-area Type 3 LSAs with low cost (usually 1).With sham link: Route appears as intra-area and OSPF calculates true cost based on interface metrics.