Huawei HCIE-Datacom H12-891_V1.0 Question # 76 Topic 8 Discussion

20This is a classicMPLS L3VPN with OSPF + Sham Linkscenario.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 advertises192.168.1.0/24to PE1.PE1 redistributes it into theVPNv4 BGProuting 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 nosham linkis used, the BGP-learned route is treated as aninter-arearoute by CE2, with a default OSPF cost of1(which would make CE2 prefer this over the real CE-CE backdoor link).Purpose of Sham Link:Sham linkis configured toequalizeoroverridethe cost of the BGP route so that CE2prefers the backdoor linkor 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 = also10 (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:Asham linkensures that the cost via the MPLS VPN path iscomparable to the real CE-to-CE OSPF backdoor path, preventing suboptimal routing.Without sham link: BGP redistributed routes appear asinter-area Type 3 LSAswith low cost (usually 1).With sham link: Route appears asintra-areaand OSPF calculates true cost based on interface metrics.