Chapter 6

     

"Do I Know This Already?" Quiz

A1:	d
A2:	b
A3:	c
A4:	d
A5:	c
A6:	b
A7:	b
A8:	a
A9:	a
A10:	b
A11:	c
A12:	d

Q&A

1:	What information is held in the topology table?
A1:	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?
A2:	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?
A3:	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.
A4:	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?
A5:	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?
A6:	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.
A7:	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?
A8:	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?
A9:	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.
A10:	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.
11:	What is used to elect the DR when the election is dynamic?
A11:	When selected dynamically, the DR is elected arbitrarily. The election is made on the basis of the highest router ID or IP address present on the network segment. It is wise to be aware that the highest IP address is the numerically highest number, not the class ranking of the addresses. Therefore, a remote, small router with a Class C address might end up as a DR.
12:	When a new router joins the OSPF network, will it learn about the rest of the OSPF network through the flooding method or the exchange method?
A12:	When a new router connects to a network, it will find a neighbor using the Hello protocol and will exchange routing information.
13:	If an LSA is received that is present in the OSPF database, and the receiving LSA is older than the one currently held by the router, what action is taken?
A13:	The receiving router will send a copy of the LSA it holds in its database to the source of the old LSA and then discard the old LSA it received.
14:	A router has made a neighbor relationship with another router and exchanged DDP. Having compared the routing information from its neighbor, the router realizes that its topology database is incomplete. Name the different stages or states that a router goes through to update its topology database.
A14:	When the router has received the DDPs from the neighboring router, it compares the received network information with that in its topology table. In the case of a new router, such as the 2500, all the DDPs are new. Remember that the DDPs are simply a summary of the routes about which the neighbor knows . If there is a discrepancy between the information in the received DDPs and the router's topology database, then the router requests more detailed information from its neighbor on those routes of which it was unaware. The different stages or states that the router goes through gathering routing information to update the topology database from a neighbor are shown in the following list: - The loading state If the receiving router, the 2500, requires more information, it will request that particular link in more detail using the LSR packet. - The LSR will prompt the master router to send the LSU packet. This is the same as an LSA used to flood the network with routing information. While the 2500 is awaiting the LSUs from its neighbor, it is in the loading state. - The full state When these LSRs are received and the databases are updated and synchronized, the neighbors are fully adjacent.
15:	How many equal-cost paths will Cisco enter into the routing table?
A15:	The RFC 2328 that defines OSPF does not state the number of equal-cost paths that can be entered into the routing table. Cisco has defined this to be four paths by default, which can be configured to contain up to six equal-cost paths.
16:	An LSA is received by a router, and when checked against the topology database, it finds the LSA is new or a change in the status of an existing route that has been received. What action will the receiving router take?
A16:	The LSA is flooded out of all the interfaces, excepting the interface through which it was received. The LSA is copied into the topology database, replacing the original LSA if it existed. The received LSA is acknowledged . The SPF algorithm is run to update the routing table.
17:	Which NBMA configuration options are Cisco-specific?
A17:	The configuration options proprietary to Cisco for NBMA are: - Point-to-multipoint nonbroadcast - Broadcast - Point-to-point
18:	What is the difference between a point-to-point interface and a point-to-multipoint interface?
A18:	In a point-to-point network, the concept of broadcast is not relevant because the communication is direct to another router. There is very little network overhead. An IP subnet is required for each point-to point link. In point-to-multipoint connections, OSPF simulates a broadcast, the network traffic is replicated and sent down each physical link and uses multicast addressing.
19:	What is the default network type for serial interfaces with HDLC encapsulation, and how often is the hello packet sent?
A19:	The default network type for serial interfaces with HDLC encapsulation is point-to-point and the hello packet is sent out every 10 seconds.
20:	On a multiaccess link, what role does the BDR play?
A20:	The BDR listens to all the OSPF network traffic, which is addressed to both the designated and BDRs. All the routers on the medium have an adjacency with both DRs. The difference is that the BDR listens but does not respond. If the DR fails, the BDR becomes the DR.