314 Chapter 7 MPLS VPNs and OSPF

Answers to Review Questions

1.B. 32 total processes are available. Connected, RIPv2, and BGP all use only one process each. OSPF uses a process for each individual VPN.

2.A. For standard OSPF, all areas must have a connection to Area 0, which is known as the backbone area.

3.B. Without the OSPF super-backbone, the service provider network looks like a standard BGP network. Therefore, PE routers are viewed as ASBRs.

4.C. The router ospf process_id vrf vpn_name command is used to configure OSPF for a VPN.

5.A. Intra-area routes from with the customer’s OSPF network are displayed as O in a customer router’s routing table.

6.B. Standard BGP rules still apply when redistributing OSPF routes into MP-BGP. The OSPF cost is carried in the BGP MED attribute.

7.B. Internal routes, when redistributed back into OSPF from MP-BGP, are LSA Type 3 routes and are displayed as O IA in a customer router’s global routing table.

8.B. Inter-area routes, when redistributed back into OSPF from MP-BGP, are LSA Type 3 routes and are displayed as O IA in a customer router’s global routing table.

9.C. External routes, when redistributed back into OSPF from MP-BGP, are LSA Type 5 routes and are displayed as O E2 in a customer router’s global routing table. The Cisco default is O E2 for external routes.

10.A. With the OSPF super-backbone, PE routers are not viewed as ASBRs but as ABRs.

11.B. The BGP AS number is mapped to the tag field. A PE router does not redistribute a route it learns if the tag field value is equal to its own AS number.

12.C. Routes with the routing bit set will not be displayed in the routing table and will not be sent to customer OSPF routers. The routing bit is used internally by a PE router to mark routes learned where the down bit is set. A route with the routing bit cleared is not displayed in the VRF routing table, even if it is the best OSPF route.

Answers to Review Questions 315

13.B. OSPF routes from one area to another are OSPF LSA Type 3.

14.D. OSPF routes from an external AS are OSPF LSA Type 5.

15.A. OSPF routes from the same area are OSPF LSA Type 1 or Type 2.

16.D. External routes, OSPF LSA Type 5, are flooded throughout the OSPF domain.

17.B. ASBR routers connect to an external AS and generate external, LSA Type 5, routes.

18.A. ABR routers connect to more than one OSPF area generate interarea, LSA Type 3, routes.

19.A, B. Both the down bit and the tag field are used to prevent routing loops.

20.C. When the routing bit is cleared, a route is not displayed in the global routing table, even if it is the best OSPF route.

Chapter Advanced MPLS Topics