| 1: | What command is used to manually determine which router on a LAN will become the DR? |
| A1: | The priority command is used to determine manually the DR. The higher the priority, the greater the likelihood is of success. Remember that the default=1 and p=0 means that the router cannot win. |
| 2: | What parameter is used to calculate the default metric of a route in OSPF on a Cisco router? |
| A2: | The bandwidth parameter configured on an interface is used to determine the default cost or the value of the path with the lowest cost. |
| 3: | It is possible to have more than one OSPF process on a router. How would you do this? |
| A3: | The router command creates the OSPF process with an ID number to identify it. To create another process on the same router, issue the same command again with a different ID number. It is possible to have more than one process, although it is rarely configured. The process ID in the command router ospf process-id not only starts the process, but also identifies the process; repeating the command with another ID number will create another process. One possible scenario for this configuration is a service provider that wants to separate its OSPF domain from its customer. |
| 4: | Explain the command ip ospf network non-broadcast . |
| A4: | The ip ospf network non-broadcast command is the RFC-compliant mode for NBMA. It is the default mode for interfaces and point-to-multipoint subinterfaces. It is used in a full or partial meshed network, and OSPF operates as if on a nonbroadcast network. It is necessary to define manually the DR to be a hub router that is connected to all the other routers. Neighbors must be defined manually. |
| 5: | In which of the NBMA configuration choices is it necessary to state the neighbors manually? Why is this necessary? |
| A5: | It is necessary to manually configure the neighbors in the industry-standard NBMA mode and in the Cisco point-to-multipoint nonbroadcast mode. You need to define the neighbors to the router because the router believes that it is a nonbroadcast medium, so it cannot send out the multicast traffic to ascertain the neighbors. |
| 6: | In a Frame Relay environment, which is fully meshed, which OSPF configurations might be chosen ? Give reasons for your choice. |
| A6: | The industry-standard NBMA configuration can be chosen in a fully meshed environment. It requires an additional manual configuration of the neighbors, but the network will elect the DR and the BDR. There might be some design concerns about running this mode in an unstable network, which could burden the CPU and the WAN links. It is possible to use point-to-point subinterfaces without worrying about the OSPF network type because they will become neighbors. The other alternative is the Cisco broadcast mode, which does not require the manual configuration of neighbors. |
| 7: | The Cisco solution point-to-point mode does not require the configuration of DR and BDR. Explain briefly why. |
| A7: | The Cisco solution point-to-point does not require the election of either a DR or a BDR because there are only two nodes on the network. They form an adjacency immediately. |
| 8: | The address 192.100.56.10/21 has been allocated to an interface on the router. This interface alone is to be included in the OSPF process. State the command that would start the process on this interface. |
| A8: | There are several ways to configure the process to include the interface. The command network network-number wildcard-mask area area-number would be a subcommand to the global command router ospf process-id . The network command is used in both possible solutions; the difference is in the wildcard mask. - - network 192.100.56.10 0.0.0.0 area 2 This will match every bit in the interface address.
- - network 192.100.56.10 0.0.7.255 area 2 This will also match the interface because it will resolve to the subnet assigned to the wire connected to the interface. This bit allocation was chosen merely to demonstrate the technique. The allocation assumed is the subnet mask of 255.255.248.0. Note that the wildcard mask is the inverse of the subnet mask, ensuring that the individual subnet is selected for the interface.
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| 9: | The metric used by OSPF is cost. How would you change the metric on an interface? |
| A9: | Underneath the appropriate interface, issue the command ip ospf cost . The value for cost is an unsigned integer value expressed as the link-state metric. It can be a value in the range 1 to 65,535. |
| 10: | If the command ip ospf network non-broadcast is used, what additional statement is necessary? |
| A10: | If the command ip ospf network non-broadcast is used, the additional statement that is required is the neighbor statement. Because the network is a nonbroadcast network that cannot see its neighbors, the neighbors are to be manually configured. |
| 11: | What command shows which router on a LAN is the BDR? |
| A11: | The show ip ospf neighbor command will show the DR and the backup router. Another command that will show the DRs is the show ip ospf interface command. |
| 12: | Explain briefly what show ip ospf database will reveal. |
| A12: | The command show ip ospf database shows the contents of the topology database and gives a status on the LSAs that have been sent and received, including how long it has been since the last LSA was received. |
| 13: | What command is used to show the state of adjacencies? |
| A13: | The command show ip ospf interface shows the adjacencies that exist with neighbors. |
| 14: | Which command is used to show OSPF packets being sent and received in real time? |
| A14: | The command debug ip packet shows OSPF packets being sent and received in real time. |
| 15: | How would you show the OSPF process ID of the router? |
| A15: | The commands show ip ospf, show ip ospf database , and show ip ospf interface all show the OSPF process ID on the router. |
| 16: | What makes debug a dangerous command for your router? |
| A16: | The debug command has the highest process priority and is therefore capable of consuming all the resources on the router, thus becoming the problem as opposed to helping to solve the problem. |
| 17: | What is the sequence number and where is it held? |
| A17: | The sequence number is used to ensure the LSA that has been received contains the most recent information about the network. This prevents any packets arriving out of sequence from resulting in a change in the network that is incorrect. |
| 18: | In the show ip ospf command, there is a field called the SPF schedule delay. What is the purpose of this field, and what is the default time? |
| A18: | The SPF schedule delay is the time between OSPF receiving a topology change and starting an SPF calculation. The delay can be an integer from 0 to 65,535. The default time is 5 seconds. If the value is set to 0, this means that the SPF calculation is started as soon as a valid LSA is received. There is a balance between responding to a topology change quickly and the use of CPU processing. |
| 19: | What is the advantage of the command show ip ospf interface in troubleshooting? |
| A19: | The show ip ospf interface command shows how the interface has been configured for OSPF. This allows for the immediate identification of typing errors that result in a mismatch between neighbors. |
