OSPF Q & A in CCNP

1: What information is held in the topology table?

** The topology table holds a map of every link in the area. Every topology table in the area is the same. This is sometimes referred to as the link-state database.

2: What command is used to determine manually which router on a LAN will become the DR?

** The ip ospf priority number command is used to determine the DR manually. The higher the priority, the greater the likelihood is of success.

3: How many subnets are required in an OSPF configuration over a point-to-point network that has multiple connections?

** It is necessary to have one subnet per connection. Thus, if there are four point-to-point links, four subnets are required.

4: State the different types of packets used to build a routing table for the first time.

** Five packets are used to build the routing table for the first time:

- The hello packet— This is used to find neighbors and to determine the designated and BDR. The continued propagation of the hello packet maintains the transmitting router in the topology database of those that hear the message.
- The database descriptor— This is used to send summary information to neighbors to synchronize topology databases.
- The LSR— This is a request for more detailed information, which is sent when the router receives a database descriptor that contains new information.
- The LSU— This is the LSA packet issued in response to the request for database information in the LSR packet.
- The link-state acknowledgement— This acknowledges the LSU.


5: In creating an adjacency, what is the exstart state?

** The exstart state is a stage in the forming of an adjacency between neighbors. This stage is the stage when the DR and the BDR have been elected. The master/slave relationship has been established, as has the initial sequence number of the DDP packets.

6: What is the database descriptor and when is it used?

** Referred to as DBDs or database descriptor packets (DDPs), these are packets exchanged between neighbors during the exchange state. The DDPs contain summary information taken from the LSAs, which describe the links of every router in the neighbor's topology table.

7: Explain the difference between an LSR and an LSA.

** A link-state advertisement (LSA) is a packet describing a router's links and the state of those links. There are different types of LSAs to describe the different types of links.
An LSR is a link-state request, which is used when the router receives a DDP complete with summary information taken from the LSA. It compares the LSA against the topological database. If either the LSA entry is not present or the entry is older than the DDP, it will request further information via an LSR.

8: What packet is used to maintain the neighbor table?

** The hello packet is used to maintain the neighbor table. Whenever a hello is heard, the source address in the hello packet is used to reset the hello interval timer. This shows that the neighbor is still active.

9: What is the metric used by OSPF standards? Is this the same metric that Cisco uses?

** OSPF defines cost as the OSPF metric, but does not define what cost represents. Thus, any determinant could be used and defined manually as cost. Cisco has set a default metric to be the inverse of bandwidth, making the fastest link the most preferred link. This default can be overridden by manual configuration.

10: Explain the meaning of the letters BDR.

** BDR stands for backup designated router. This router acts as the backup to the DR in case the DR fails. The BDR performs none of the DR functions while the DR is operating correctly.