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OSPF Questions 2

May 6th, 2014 in ROUTE 642-902 Go to comments

Here you will find answers to OSPF Questions – Part 2

Question 1

Into which two types of areas would an area border router (ABR) inject a default route? (Choose two)

A. the autonomous system of a different interior gateway protocol (IGP)
B. area 0
C. totally stubby
D. NSSA
E. stub
F. the autonomous system of an exterior gateway protocol (EGP)

 

Answer: C E

Explanation

Both stub area & totally stubby area allow an ABR to inject a default route. The main difference between these 2 types of areas is:

+ Stub area replaces LSA Type 5 (External LSA – created by an ASBR to advertise network from another autonomous system) with a default route
+ Totally stubby area replaces both LSA Type 5 and LSA Type 3 (Summary LSA – created by an ABR to advertise network from other areas, but still within the AS, sometimes called interarea routes) with a default route.

Below summarizes the LSA Types allowed and not allowed in area types:

Area Type Type 1 & 2 (within area) Type 3 (from other areas) Type 4 Type 5 Type 7
Standard & backbone Yes Yes Yes Yes No
Stub Yes Yes No No No
Totally stubby Yes No No No No
NSSA Yes Yes No No Yes
Totally stubby NSSA Yes No No No Yes

Question 2

Which three restrictions apply to OSPF stub areas? (Choose three)

A. No virtual links are allowed.
B. The area cannot be a backbone area.
C. Redistribution is not allowed unless the packet is changed to a type 7 packet.
D. The area has no more than 10 routers.
E. No autonomous system border routers are allowed.
F. Interarea routes are suppressed.

 

Answer: A B E

Question 3

Refer to the partial configurations in the exhibit. What address is utilized for DR and BDR identification on Router1?

Router1#show run

**** output omitted ******

interface serial1/1
ipv6 address 2001:410:FFFE:1::64/64
ipv6 ospf 100 area 0
!
interface serial2/0
ipv6 address 3FFF:B00:FFFF:1::2/64
ipv6 ospf 100 area 0
!
ipv6 router ospf
router-id 10.1.1.3

A. the serial 1/1 address
B. the serial 2/0 address
C. a randomly generated internal address
D. the configured router-id address

 

Answer: D

Explanation

In OSPFv3 and OSPF version 2, the router uses the 32-bit IPv4 address to select the router ID for an OSPF process. The router ID selection process for OSPFv3 is described below (same as OSPF version 2):

1. The router ID is used if explicitly configured with the router-id command.
2. Otherwise, the highest IPv4 loopback address is used.
3. Otherwise, the highest active IPv4 address.
4. Otherwise, the router ID must be explicitly configured.

In this case the router ID 10.1.1.3 is explicitly configured -> D is correct.

Question 4

By default, which statement is correct regarding the redistribution of routes from other routing protocols into OSPF?

A. They will appear in the OSPF routing table as type E1 routes.
B. They will appear in the OSPF routing table as type E2 routes.
C. Summarized routes are not accepted.
D. All imported routes will be automatically summarized when possible.
E. Only routes with lower administrative distances will be imported.

 

Answer: B

Explanation

Type E1 external routes calculate the cost by adding the external cost to the internal cost of each link that the packet crosses while the external cost of E2 packet routes is always the external cost only. E2 is useful if you do not want internal routing to determine the path. E1 is useful when internal routing should be included in path selection. E2 is the default external metric when redistributing routes from other routing protocols into OSPF -> B is correct.

Question 5

Which statement is true about OSPF Network LSAs?

A. They are originated by every router in the OSPF network. They include all routers on the link, interfaces, the cost of the link, and any known neighbor on the link.
B. They are originated by the DR on every multi-access network. They include all attached routers including the DR itself.
C. They are originated by Area Border Routers and are sent into a single area to advertise destinations outside that area.
D. They are originated by Area Border Router and are sent into a single area to advertise an Autonomous System Border Router.

 

Answer: B

Explanation

Popular LSA Types are listed below:

LSA Type Description Details
1 Router LSA Generated by all routers in an area to describe their directly attached links
2 Network LSA Advertised by the DR of the broadcast network (does not cross ABR)
3 Summary LSA Advertised by the ABR of originating area
4 Summary LSA Generated by the ABR of the originating area to advertise an ASBR to all other areas in the autonomous system
5 AS external LSA Used by the ASBR to advertise networks from other autonomous systems
7 Defined for NSSAs Generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into the NSSA

Question 6

Refer to the exhibit. OSPF is configured on all routers in the network. On the basis of the show ip ospf neighbor output, what prevents R1 from establishing a full adjacency with R2?

show_ip_ospf_neighbor.jpg

A. Router R1 will only establish full adjacency with the DR and BDR on broadcast multiaccess networks.
B. Router R2 has been elected as a DR for the broadcast multiaccess network in OSPF area
C. Routers R1 and R2 are configured as stub routers for OSPF area 1 and OSPF area 2.
D. Router R1 and R2 are configured for a virtual link between OSPF area 1 and OSPF area 2.
E. The Hello parameters on routers R1 and R2 do not match.

 

Answer: A

Explanation

From the output, we learn that R4 is the DR and R3 is the BDR so other routers will only establish full adjacency with these routers. All other routers have the two-way adjacency established -> A is correct.

Question 7

Refer to the exhibit. On the basis of the configuration provided, how are the Hello packets sent by R2 handled by R5 in OSPF area 5?

OSPF_Hello_packets.jpg

A. The Hello packets will be exchanged and adjacency will be established between routers R2 and R5.
B. The Hello packets will be exchanged but the routers R2 and R5 will become neighbors only.
C. The Hello packets will be dropped and no adjacency will be established between routers R2 and R5.
D. The Hello packets will be dropped but the routers R2 and R5 will become neighbors.

 

Answer: C

Explanation

Recall that in OSPF, two routers will become neighbors when they agree on the following: Area-id, Authentication, Hello and Dead Intervals, Stub area flag.

We must specify Area 5 as a stub area on the ABR (R2) and all the routers in that area (R5 in this case). But from the output, we learn that only R2 has been configured as a stub for Area 5. This will drop down the neighbor relationship between R2 and R5 because the stub flag is not matched in the Hello packets of these routers.

Question 8

When an OSPF design is planned, which implementation can help a router not have memory resource issues?

A. Have a backbone area (area 0) with 40 routers and use default routes to reach external destinations.
B. Have a backbone area (area 0) with 4 routers and 30,000 external routes injected into OSPF.
C. Have less OSPF areas to reduce the need for interarea route summarizations.
D. Have multiple OSPF processes on each OSPF router. Example, router ospf 1, router ospf 2

 

Answer: A

Question 9

When verifying the OSPF link state database, which type of LSAs should you expect to see within the different OSPF area types? (Choose three)

A. All OSPF routers in stubby areas can have type 3 LSAs in their database.
B. All OSPF routers in stubby areas can have type 7 LSAs in their database.
C. All OSPF routers in totally stubby areas can have type 3 LSAs in their database.
D. All OSPF routers in totally stubby areas can have type 7 LSAs in their database.
E. All OSPF routers in NSSA areas can have type 3 LSAs in their database.
F. All OSPF routers in NSSA areas can have type 7 LSAs in their database.

 

Answer: A E F

Explanation

Below summarizes the LSA Types allowed and not allowed in area types:

Area Type Type 1 & 2 (within area) Type 3 (from other areas) Type 4 Type 5 Type 7
Standard & backbone Yes Yes Yes Yes No
Stub Yes Yes No No No
Totally stubby Yes No No No No
NSSA Yes Yes No No Yes
Totally stubby NSSA Yes No No No Yes

Popular LSA Types are listed below:

LSA Type Description Details
1 Router LSA Generated by all routers in an area to describe their directly attached links
2 Network LSA Advertised by the DR of the broadcast network (does not cross ABR)
3 Summary LSA Advertised by the ABR of originating area
4 Summary LSA Generated by the ABR of the originating area to advertise an ASBR to all other areas in the autonomous system
5 AS external LSA Used by the ASBR to advertise networks from other autonomous systems
7 Defined for NSSAs Generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into the NSSA

Question 10

You are troubleshooting an OSPF problem where external routes are not showing up in the OSPF database. Which two options are valid checks that should be performed first to verify proper OSPF operation? (Choose two)

A. Are the ASBRs trying to redistribute the external routes into a totally stubby area?
B. Are the ABRs configured with stubby areas?
C. Is the subnets keyword being used with the redistribution command?
D. Is backbone area (area 0) contiguous?
E. Is the CPU utilization of the routers high?

 

Answer: A C

Explanation

A totally stubby stubby area cannot have an ASBR so it will discard this type of LSA (LSA Type 5) -> A is a valid check.

Each stubby area needs an ABR to communicate with other areas so it is normal -> B is not a valid check.

When pulling routes into OSPF, we need to use the keyword “subnets” so that subnets will be redistributed too. For example, if we redistribute these EIGRP routes into OSPF:

+ 10.0.0.0/8
+ 10.10.0.0/16
+ 10.10.1.0/24

without the keyword “subnets”

router ospf 1
redistribute eigrp 1

Then only 10.0.0.0/8 network will be redistributed because other routes are not classful routes, they are subnets. To redistribute subnets we must use the keyword “subnets”

router ospf 1
redistribute eigrp 1 subnets

-> C is a valid check.

We don’t need to care if area 0 is contiguous or not -> D is not a valid check.

CPU utilization cannot be the cause for this problem -> E is not a valid check.

For this question, please read our tutorial about OSPF LSA Types for more detail.

Comments
  1. Anonymous
    October 31st, 2018

    please share update files, I want to try in 15.11.2018.

  2. nicck
    February 8th, 2019

    thanks for the post..very helpful

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