Ch 7 - Fundamental OSPF Concepts Flashcards
A router has been configured with the commands router ospf 9, network 172.16.1.0 0.0.0.255 area 8 and network 172.16.0.0 0.0.255.255 area 9, in that order. No other OSPF-related commands have been configured. The answers list the IP addresses that could be assigned to this router’s Fa0/0 interface. Which answers list an IP address/prefix length that would cause the router to put Fa0/0 into area 9? (Choose two.)
a. 172.16.0.1/23
b. 172.16.1.1/26
c. 172.16.1.1/24
d. 172.16.0.255/23
e. None of the other answers is correct.
A and D. The wildcard mask is used for matching the prefix only, and not the prefix length. As such, 172.16.1.0 0.0.0.255 matches all addresses that begin with 172.16.1, and 172.16.0.0 0.0.255.255 matches all addresses that begin 172.16. Also, OSPF reviews the network command with the most specific wildcard masks (wildcard masks with the most binary 0s) first, so an interface IP address beginning with 172.16.1 matches the command that references area 8.
Which of the following is true about an OSPF area border router (ABR)?
a. The ABR must have multiple interfaces connected to the backbone area.
b. An ABR is a router with two interfaces, each connected to a different non-backbone area.
c. The only requirement to be considered an ABR is at least one interface connected to the backbone area.
d. An ABR must have at least one interface in the backbone area plus at least one other interface in a non-backbone area.
D. ABRs, by definition, connect the backbone area to one or more nonbackbone areas. To perform this function, a router must have at least one interface assigned to the backbone area and at least one interface assigned to a nonbackbone area.
Which of the following can either directly or indirectly identify all the interfaces for which 1) OSPF has been enabled and 2) OSPF is not passive? (Choose two.)
a. show ip ospf database
b. show ip ospf interface brief
c. show ip protocols
d. show ip route ospf
e. show ip ospf neighbors
B and C. First, for the two correct answers: show ip ospf interface brief explicitly lists all OSPF-enabled interfaces that are not passive. show ip protocols lists either the details of the configured network commands, or if configured using the ip ospf area command, it lists the interfaces on which OSPF is enabled. This command also lists the passive interfaces, so armed with interface IP address information, the list of OSPF-enabled non-passive interfaces could be derived.
Of the three wrong answers, show ip ospf database does not list enough detail to show the OSPF-enabled interfaces. show ip route ospf lists only routes learned with OSPF, so if no routes usea particular OSPF-enabled interface as an outgoing interface, this command would not indirectly identify the interface. Finally, an interface might be OSPF-enabled but with no neighbors reachable on the interface, so the show ip ospf neighbor command might not identify all OSPF-enabled interfaces.
Router R1 directly connects to subnet 10.1.1.0/24 with its Fa0/0 interface. R1 can ping four other working OSPF routers in that subnet. R1 is neither the designated router (DR) nor backup DR (BDR). OSPF is working correctly on all five routers. Which of the following are true on R1? (Choose two.)
a. The show ip ospf neighbors command lists two neighbors off Fa0/0.
b. The show ip ospf neighbors command lists four neighbors off Fa0/0.
c. The show ip ospf neighbors command lists two neighbors off Fa0/0 in the FULL state.
d. The show ip ospf neighbors command lists two neighbors off Fa0/0 in the DISCO state.
B and C. On a LAN, the non-DRs form fully adjacent neighborships with only the DR and BDR, giving R1 two neighbors in the FULL state. The other two neighbors settle into the 2-Way state.
Routers R1 and R2 are OSPF neighbors using their Fa0/0 interfaces, respectively, using default settings for all timers. An engineer adds the ip ospf hello-interval 6 command to R1’s Fa0/0 configuration. Which of the following are true regarding the results from this change? (Choose two.)
a. The show ip ospf neighbor command on R1 lists the revised Hello timer.
b. The show ip ospf interface brief command on R1 lists the revised Hello timer.
c. The R1-R2 neighborship fails because of Hello timer mismatch.
d. The show ip ospf interface command on R1 lists the revised Hello timer.
C and D. The show ip ospf interface command displays a router’s OSPF Hello Interval setting for each enabled interface. The other listed commands do not display the timer. Also, OSPF routers do need to have matching Hello timers to become neighbors, so the neighborship would fail.
Which of the following settings do not prevent two potential OSPF neighbors from becoming neighbors?
a. The interface used to connect to that neighbor being passive in the OSPF process
b. Duplicate OSPF router IDs
c. Mismatched Dead timers
d. IP addresses of 10.1.1.1/24 and 10.2.2.2/24
e. Mismatched OSPF process IDs
E. Table 7-5 in Chapter 7 lists the issues. For OSPF, Router IDs must be unique,
the interfaces must not be passive, the dead timers must match, and the primary IP addresses must be in the same subnet, with the same subnet mask. However, the process IDs, found in the router ospf process-id command, do not have to match.
A company has a Frame Relay WAN with one central-site router and 100 branch office routers. A partial mesh of PVCs exists: one PVC between the central site and each of the 100 branch routers. All routers use point-to-point sub-interfaces and one subnet per PVC. Which of the following is true about OSPF in this design?
a. The central-site router has 100 fully adjacent neighborships with the 100 branches.
b. The central-site router has neighborships with all branch routers, but fully adjacent neighborships with only two branches.
c. The central-site router has a neighborship with the Frame Relay switch.
d. None of the other answers is correct.
A. Frame Relay is a Layer 2 service and as such does not participate in customer routing protocols. Because the design uses a separate subnet per PVC, and one point-to-point subinterface per PVC/subnet, OSPF will use a point-to-point network type. That means that the two routers on either end of a PVC will become neighbors, and become fully adjacent, meaning that the central-site router will have 100 fully adjacent neighborships.
Which of the following answers can be verified as true based on the following com-
mand 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 Fa0/1 OSPF cost is 3.
d. 4.4.4.4 is known to R1 based on a Type 1 LSA in area 1.
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 cannot be used to predict Fa0/1’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.
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 answer lists 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
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 Cisco IOS commands.
T/F: Link State (LS) logic has 3 branches:
- neigh discovery
- topo database exchange - LSDB
- route computation - SPF algorithm
True.
LSDB contains RID, IP/mask/subnet, list of routers reachable by each interface
In OSPF, each router exists in only one area, except for _____ which bridge areas to the backbone.
ABR - Area Border Routers
ABRs advertise a much briefer summary inter-area than is exchanged intra-area by area routers.
T/F: In OSPF, packets that flow inter-area must flow through at least one backbone router.
True. ABRs bridge the backbone, Area 0, to the others.
T/F: ABRs keep only a summary copy of each LSDB they attach to and advertise only subnet/mask info inter-area.
False.
ABRs keep a copy of the entire LSDB for each area they attach to. It is true that they only advertise subnet/mask inter-area. To the other routers it appears that all the other subnets are attached directly to the ABR.
Name 3 benefits/reasons Areas are used in OSPF.
smaller topo databases reduced processing time reduced memory consumption simpler route calculation faster convergence
What is the LSDB?
data structure held by OSPF router for the purpose of storing topo data
What is an LSU? and what does it do?
Link State Update. These are OSPF packets that carry detailed topo info, specifically LSAs.
What is an LSA and what does it do?
Link State Advertisement. This is the name of a class of OSPF data structures that hold topo info. LSAs are held in memory in a LSDB and communicate over a network inside of LSU messages.
What is an ABR and what does it do?
Area Border Router. router with interfaces in at least two areas, one of which must be the backbone. ABRs hold topo data for each connected area along with the associated calculated routes, and advertise those routes inter-area.
What is the DR and what does it do?
Designated Router. On multi-access data links, like LANs, this router is electedon that link to perform special functions. Two of it’s main functions are generating LSA for the subnet and the DR plays a key role in the database exchange.
What are the required commands to configure a router for OSPF?
router ospf
network area
Note: IOS will order the network commands with the most specific wildcard mask first for over lapping ranges. If you sh run the network commands will appear in this order regardless of how you enter them.
Which of these two wildcard masks is more specific?
- 0.31.255
- 0.64.255
0.0.31.255 - More zeroes is more specific in wildcard masks.
T/F: The first network command to match an interface, per the order shown in the config, determines the OSPF area number associated with the interface.
True.
