CISCO CCNA Exam – Q158

What are two benefits of using a single OSPF area network design? (Choose two.)

A. It is less CPU intensive for routers in the single area.
B. It reduces the types of LSAs that are generated.
C. It removes the need for virtual links.
D. It increases LSA response times.
E. It reduces the number of required OSPF neighbor adjacencies.

Correct Answers: B, C

Explanation:
OSPF uses a LSDB (link state database) and fills this with LSAs (link state advertisement). The link types are as follows:

  • LSA Type 1: Router LSA
  • LSA Type 2: Network LSA
  • LSA Type 3: Summary LSA
  • LSA Type 4: Summary ASBR LSA
  • LSA Type 5: Autonomous system external LSA
  • LSA Type 6: Multicast OSPF LSA
  • LSA Type 7: Not-so-stubby area LSA
  • LSA Type 8: External attribute LSA for BGP

If all routers are in the same area, then many of these LSA types (Summary ASBR LSA, external LSA, etc) will not be used and will not be generated by any router.
All areas in an Open Shortest Path First (OSPF) autonomous system must be physically connected to the backbone area (Area 0). In some cases, where this is not possible, you can use a virtual link to connect to the backbone through a non-backbone area. You can also use virtual links to connect two parts of a partitioned backbone through a nonbackbone area. The area through which you configure the virtual link, known as a transit area, must have full routing information. The transit area cannot be a stub area. Virtual links are not ideal and should really only be used for temporary network solutions or migrations. However, if all locations are in a single OSPF area this is not needed.

CISCO CCNA Exam – Q157

When a router undergoes the exchange protocol within OSPF, in what order does it pass through each state?

A. exstart state > loading state > exchange state > full state
B. exstart state > exchange state > loading state > full state
C. exstart state > full state > loading state > exchange state
D. loading state > exchange state > full state > exstart state

Correct Answer: B

Explanation:
When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. Those states are defined in the OSPF RFC 2328, section 10.1. The states are (in order) Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full.

Reference: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13685-13.html

CISCO CCNA Exam – Q156

What are two enhancements that OSPFv3 supports over OSPFv2? (Choose two.)

A. It requires the use of ARP.
B. It can support multiple IPv6 subnets on a single link.
C. It supports up to 2 instances of OSPFv3 over a common link.
D. It routes over links rather than over networks.

Correct Answers: B, D

CISCO CCNA Exam – Q155

What are two enhancements that OSPFv3 supports over OSPFv2? (Choose two.)

A. It requires the use of ARP.
B. It can support multiple IPv6 subnets on a single link.
C. It supports up to 2 instances of OSPFv3 over a common link.
D. It routes over links rather than over networks.

Correct Answers: B, D

Explanation:
Here is a list of the differences between OSPFv2 and OSPFv3:

  • They use different address families (OSPFv2 is for IPv4-only, OSPFv3 can be used for IPv6-only or both protocols
  • OSPFv3 introduces new LSA types
  • OSPFv3 has different packet format
  • OSPFv3 uses different flooding scope bits (U/S2/S1)
  • OSPFv3 adjacencies are formed over link-local IPv6 communications
  • OSPFv3 runs per-link rather than per-subnet
  • OSPFv3 supports multiple instances on a single link, Interfaces can have multiple IPv6 addresses
  • OSPFv3 uses multicast addresses FF02::5 (all OSPF routers), FF02::6 (all OSPF DRs)
  • OSPFv3 Neighbor Authentication done with IPsec (AH)
  • OSPFv2 Router ID (RID) must be manually configured, still a 32-bit number

CISCO CCNA Exam – Q154

What command sequence will configure a router to run OSPF and add network 10.1.1.0 /24 to area 0?

A.
router ospf area 0
network 10.1.1.0 255.255.255.0 area 0
B.
router ospf
network 10.1.1.0 0.0.0.255
C.
router ospf 1
network 10.1.1.0 0.0.0.255 area 0
D.
router ospf area 0
network 10.1.1.0 0.0.0.255 area 0
E.
router ospf
network 10.1.1.0 255.255.255.0 area 0
F.
router ospf 1
network 10.1.1.0 0.0.0.255

Correct Answer: C

Explanation:
Enabling OSPF

SUMMARY STEPS
1. enable
2. configure terminal
3. router ospf process-id
4. network ip-address wildcard-mask area area-id
5. end

DETAILED STEPS
CISCO CCNA Exam – Q154

CISCO CCNA Exam – Q153

Refer to the exhibit.
CISCO CCNA Exam – Q153

If the router Cisco returns the given output and has not had its router ID set manually, what value will OSPF use as its router ID?

A. 192.168.1.1
B. 172.16.1.1
C. 1.1.1.1
D. 2.2.2.2

Correct Answer: D

Explanation:
If a router-id is not configured manually in the OSPF routing process the router will automatically configure a router-id determined from the highest IP address of a logical interface (loopback interface) or the highest IP address of an active interface. If more than one loopback interfaces are configured, the router will compare the IP addresses of each of the interfaces and choose the highest IP address from the loopbacks.

CISCO CCNA Exam – Q152

What OSPF command, when configured, will include all interfaces into area 0?

A. network 0.0.0.0 255.255.255.255 area 0
B. network 0.0.0.0 0.0.0.0 area 0
C. network 255.255.255.255 0.0.0.0 area 0
D. network all-interfaces area 0

Correct Answer: A

Explanation:
Example 3-1 displays OSPF with a process ID of 1 and places all interfaces configured with an IP address in area 0. The network command network 0.0.0.0 255.255.255.255 area 0 dictates that you do not care (255.255.255.255) what the IP address is, but if an IP address is enabled on any interface, place it in area 0.

CISCO CCNA Exam – Q151

Which statement describes the process ID that is used to run OSPF on a router?

A. It is globally significant and is used to represent the AS number.
B. It is locally significant and is used to identify an instance of the OSPF database.
C. It is globally significant and is used to identify OSPF stub areas.
D. It is locally significant and must be the same throughout an area.

Correct Answer: B

Explanation:
The Process ID for OSPF on a router is only locally significant and you can use the same number on each router, or each router can have a different number-it just doesn’t matter. The numbers you can use are from 1 to 65,535. Don’t get this confused with area numbers, which can be from 0 to 4.2 billion.

CISCO CCNA Exam – Q150

Refer to the exhibit.
CISCO CCNA Exam – Q150

All of the routers in the network are configured with the ip subnet-zero command. Which network addresses should be used for Link A and Network A? (Choose two.)
A. Network A – 172.16.3.48/26
B. Network A – 172.16.3.128/25
C. Network A – 172.16.3.192/26
D. Link A – 172.16.3.0/30
E. Link A – 172.16.3.40/30
F. Link A – 172.16.3.112/30

Correct Answers: B, D

Explanation:
Only a /30 is needed for the point to point link and sine the use of the ip subnet-zero was used, 172.16.3.0/30 is valid. Also, a /25 is required for 120 hosts and again 172.16.3.128/25 is the best, valid option.

CISCO CCNA Exam – Q149

The network administrator needs to address seven LANs. RIP version 1 is the only routing protocol in use on the network and subnet 0 is not being used. What is the maximum number of usable IP addresses that can be supported on each LAN if the organization is using one class C address block?

A. 8
B. 6
C. 30
D. 32
E. 14
F. 16

Correct Answer: C

Explanation:
Since there is one class C network that means 256 total IP addresses. Since we need 7 LAN blocks and we cannot use the first one (subnet 0) we take 256/8=32 hosts.
However, since we need to reserve the network and broadcast addresses for each of these subnets, only 30 total IP addresses are usable.