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CCNP Route FAQ: OSPF Virtual Links and Frame Reay Operations

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CCNP Route FAQ: OSPF Virtual Links and Frame Reay Operations


Figure: Connecting Two Area 0s with a Virtual Link

Q1. Which of the following answers can be verified as true based on the following command output from Router R1?

R1#show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 4.4.4.4 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface FastEthernet0/1, Cost of using 3

a. R1 is configured with an area 0 virtual-link 4.4.4.4 cost 3 command.
b. The ping 4.4.4.4 command on R1 must currently be successful.
c. R1’s Fa0/0 OSPF cost is 3.
d. 4.4.4.4 is known to R1 based on a Type 1 LSA in area 1.

Answer: D. The answer with area 0 virtual-link 4.4.4.4 cost 3 is incorrect because the show command output lists a transit area of 1, but the answer’s area parameter refers to area 0 as the transit area. (There is also no cost parameter on the area virtual-link command.) The RID of the router on the other end of the virtual link, 4.4.4.4 per the show command output, does not have to be pingable for the virtual link to work. The cost of the virtual link is 3, but that cost is calculated as the cost to reach the other router through the transit area, so the command output listed with the question can not be used to predict Fa0/0’s OSPF interface cost alone. However, because the output lists area 1 as the transit area, and because the neighbor RID is listed as 4.4.4.4, R1 will use the area 1 LSDB entries to calculate the cost to reach 4.4.4, a process that will include the area 1 Type 1 LSA for RID 4.4.4.4.

Q2. Several links have been broken so that for the next day or two, what was formerly a contiguous area 0 has been broken into two parts. However, both parts of area 0 have working links into area 1 using routers with RID 1.1.1.1 and 2.2.2.2. Which answers list the command on the router with RID 1.1.1.1 to create a virtual link to help solve this temporary problem?
a. area 0 virtual-link 2.2.2.2
b. area 1 virtual-link 2.2.2.2
c. area 0 source-rid 1.1.1.1 dest-rid 2.2.2.2
d. virtual-link transit-area 1 RID 2.2.2.2

Answer: B. The area virtual-link command defines the virtual link, with the transit area—the area through which the virtual link passes—listed as the first parameter. The other parameter is the RID of the other router. Two of the wrong answers are not IOS commands.

Q3. Router R1 connects to a Frame Relay cloud using a multipoint subinterface, with ten PVCs associated with the subinterface. What command would make this router not use a DR and require static OSPF neighbor definition?
a. ip ospf network broadcast
b. ip ospf network non-broadcast
c. ip ospf network point-to-multipoint
d. ip ospf network point-to-multipoint non-broadcast

Answer: D. Of the four types listed, only point-to-multipoint nonbroadcast does not use a DR but does require the static definition of neighbors.

Q4. Router R1 connects to a Frame Relay cloud using a multipoint subinterface, with ten PVCs associated with the subinterface. What command would make this router not use a DR, and dynamically discover OSPF neighbors?
a. ip ospf network broadcast
b. ip ospf network non-broadcast
c. ip ospf network point-to-multipoint
d. ip ospf network point-to-multipoint non-broadcast

Answer: C. Of the four types listed, only point-to-multipoint does not use a DR and dynamically discovers neighbors.

Q5. Ten routers, R1 through R10, connect in a partial mesh over Frame Relay. For the mesh, R1 and R2 have PVCs connected to all other routers, but Routers R3 through R10 act as branch routers, with only two PVCs–one to R1 and one to R2. The routers use IP subnet 10.1.1.0/24, with addresses 10.1.1.1, 10.1.1.2, and so on, through 10.1.1.10, respectively. The routers all use Inverse ARP to learn Frame Relay mapping information. All routers use a multipoint subinterface with network type point-tomultipoint nonbroadcast. A co-worker’s implementation plan lists lots of configuration commands related to this design. The design states that all hosts should be able to ping all other hosts. Which commands are required for proper functioning of OSPF in this case? (Choose two.)
a. frame-relay map commands on R3–R10 referencing the other routers in this group.
b. Nine OSPF neighbor commands on each router.
c. Nine OSPF neighbor commands on R1 and R2, with only two such commands on R3–R10.
d. R1 and R2 with ip ospf priority 1 commands to ensure they become DR and BDR.
e. R3–R10 with ip ospf priority 0 commands to ensure they do not become DR or BDR.

Answer: A and C. For routers to use their OSPF routes in a multipoint design, each router needs mapping to each other router in the Frame Relay subnet. In this case, R3–R10 all need frame-relay map commands to define the mapping to other routers with which they do not have a PVC. This network type requires static definition of neighbors, but the neighbor relationships match the PVC topology, so R3–R10 need only two neighbor commands. This OSPF network type does not use a DR, so the ip ospf priority commands have no effect and would be unnecessary.