OSPF Questions 6
Here you will find answers to OSPF Questions – Part 6
Question 1
You are developing a verification plan for an upcoming OSPF implementation. Part of this plan is to verify the status of type 3 LSAs within the network. Which routers should you verify first to ensure that the configurations are correct for generating type 3 LSAs?
A. Internal routers within the backbone area (area 0)
B. Internal routers within the NSSAs
C. Internal routers within the stubby areas
D. ASBRs
E. ABRs
F. DRs and BDRs
Answer: E
Explanation
Type 3 LSA (Summary LSA) is advertised by the ABR of originating area to advertise network from other areas so we should check the ABRs first.
Question 2
To make OSPF area 4 totally stubby, the following command was issued on the ABR in router configuration mode:
area 4 stub
Which two things need to be done to finish making area 4 a totally stubby area? (Choose two)
A. Apply the area 4 stub command to all routers in the area.
B. On the ABR, use the area 4 stub command with the no-summary keyword.
C. On the ABR, specify a default cost for the area with the area default-cost command.
D. On the ABR, use the default-information originate command to inject a default route into area 4.
E. Use the auto-cost command on each router in the area to automatically determine the cost to other OSPF areas.
Answer: A B
Explanation
To make an area “totally stubby” we must apply the “area area-id stub no-summary” command on the ABR and area area-id stub” commands to all other routers in that area.
Note: The ABR in a totally stubby area does not create Type 3 summary LSA. It only creates a default route to outside destinations.
Question 3
Refer to the exhibit. Router C was configured so that it could form an adjacency with three OSPF neighbors, one connected to each of its three physical interfaces. Which statement is correct about router C?
A. It is configured and functioning correctly as an OSPF internal router.
B. It is configured and functioning correctly as an ABR attached to stub area 4.
C. It is configured and functioning correctly as an ASBR attached to external area 4.
D. It is configured and functioning correctly as an ABR attached to totally stubby area 4.
E. It is not configured correctly to function as specified.
Answer: D
Explanation
From the output of the “show ip ospf” command we notice 2 lines:
+ It is an area border router
+ It is a stub area, no summary LSA in this area (in Area 4)
Therefore we can conclude RouterC is an ABR and it is attached to a totally stubby area (with no summary LSA advertised) -> D is correct.
Question 4
Refer to the diagram. Which OSPF configuration command is required to configure Area 3 as a totally stubby area?
A. On the ABR
router ospf 10
area 3 nssa
B. On the ABR
router ospf 10
area 3 stub
C. On all Area 3 routers
router ospf 10
area 3 stub no-summary
D. On all Area 3 routers
router ospf 10
area 3 stub
E. On the ABR
router ospf 10
area 3 stub no-summary
Answer: E
Explanation
To make an area “totally stubby” we must apply the “area area-id stub no-summary” command on the ABR (the middle router in this case) and area area-id stub” commands to all other routers in that area.
Question 5
Which show command will display only the Type 5 LSAs in the OSPF topology database?
A. show ip route
B. show ip route ospf
C. show ip ospf database summary
D. show ip ospf database nssa-external
E. show ip ospf database external
Answer: E
Explanation
The “show ip ospf database external” command displays information only about external LSAs (Type 5 LSAs) -> E is correct.
Below is an example of the “show ip ospf database external” command
Notice the line “LS Type: AS External Link”, which means LSA Type 5. For your information, the “Link State ID: 143.105.0.0″ indicates the network being advertised; the “Advertising Router: 10.187.70.6″ indicates the router that originated this LSA.
Note:
+ The “show ip ospf database summary” command displays information only about the summary LSAs.
+ The “show ip ospf database nssa-external” command displays information only about the not so stubby area (NSSA) external LSAs.
Question 6
When learning a new route, if a LSA received is not found in the topological database, what will an internal OSPF router do?
A. The sequence numbers are checked, and if the LSA is valid it is entered into the topology database.
B. The LSA is placed in the topological database and an acknowledgement is sent to the transmitting router.
C. The LSA is dropped and a message is sent to the transmitting router.
D. The LSA is flooded immediately out of all the OSPF interfaces, except the interface from which the LSA was received.
Answer: D
Question 7
Refer to the exhibit. What two statements are true? (Choose two)
A. Interface FastEthernet 0/0 was configured with the ipv6 ospf 1 area 1 command.
B. OSPF version 2 has been enabled to support IPv6.
C. The IP address of the backup designated router (BDR) is FE80::205:5FFF:FED3:5808.
D. The output was generated by the show ip interface command.
E. The router was configured with the commands:
router ospf 1
network 172.16.6.0 0.0.0.255 area 1
F. This is the designated router (DR) on the FastEthernet 0/0 link.
Answer: A C
Question 8
Refer to the exhibit. OSPF has been configured on all routers in the network. However, router R1 does not receive a default route to router R2 as intended. Which configuration change would ensure that R1 would receive a default route from R2?
A. Add the area 1 stub command on routers R1.
B. Add the always keyword to the default-information originate configuration command on router R2.
C. Remove the default information originate configuration command from router R2 and place it on router R1.
D. Add the ip route 5.0.0.0 255.255.255.0 0.0.0.0 command to router R2.
Answer: B
Explanation
The default-information originate command advertises a default route into a normal area, provided the advertising router already has a default route. If we want the router to advertise the default route even if it does not have a default route then we can append the keyword always after the above command.
In this question, notice that R2 does not have a default route (something like this: ip route 0.0.0.0 0.0.0.0 6.0.0.2) so the “default-information originate” will not advertise this default route to R1 -> We need to add the keyword “always” or declare a default route -> B is correct.
Question 9
Refer to the exhibit. OSPF is configured on all routers in the network. Area 5 is configured as an NSSA area. The RIPv2 routes are redistributed into the OSPF domain on router R5. What two types of LSAs will be originated by router R5? (Choose two)
A. type 1 Router LSA
B. type 2 Network LSA
C. type 3 Network Summary LSA
D. type 4 ASBR Summary LSA
E. type 5 AS External LSA
F. type 7 NSSA External LSA
Answer: A F
Explanation
Each router creates Type 1 LSA to describe itself, its interfaces, and neighbors -> A is correct.
Notice that LSA Type 7 is generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into the NSSA -> F is correct.
For your information, the main difference between LSA Type 7 and Type 5 is Type 7 is generated inside a NSSA while Type 5 is generated inside a standard area.
Question 10
Refer to the exhibit. Based on this command output, what can we conclude about R3?
A. R3 is an ABR.
B. R3 is not connected to the backbone.
C. R3 has four neighbors.
D. R3 is the DR for area 2.
Answer: A
Explanation
From the output we learned that this router is getting Type 3 LSAs (Summary Net Link States) for both Area 0 and Area 3 -> It is an ABR between Area 0 & Area 3 -> A is correct; B is not correct.
From the LSA Type 1 we learn that R3 only has 1 neighbor, which is 192.168.0.4 -> C is not correct.
To find the DR for an area, we need to look at the LSA Type 2 of that area. In this case we find out 116.16.35.5 is the DR for area 2, not R3 -> D is not correct.
To help you understand more clearly about the “show ip ospf database” command, we want to explain more about the fields in the output:
+ Link ID is the OSPF Router-ID of a router in the area for LSA Type 1 & 2 but it is can be the Router-ID or the network address for LSA Type 3, 5 & 7.
+ The ADV Router is the ID of the router that sent the LSA (Advertising Router) into the area
+ Age: The max age of the link state
+ Seq# and Checksum: these fields are used to verify link-state integrity.
For example, from the Router Link States (Area 0.0.0.0):
The first “Link ID” 172.16.1.1 is the Router-ID of the local router R3 (because it is the same as “OSPF Router with ID”). This “Link ID” is, of course, advertised by itself so the ADV Router has the same value.
The second “Link ID” is the Router-ID of 192.168.0.4, which is advertised by 192.168.0.4 so it is directly connected to R3.
Notice that these 2 routers belong to Area 0.
From the Net Link States:
We learn that the “Link ID” 116.16.34.4 is advertised from the neighbor 192.168.0.4, which is directly connected to R3. This router (116.16.34.4) also belongs to Area 0 and it is the DR of that segment.
The Summary Net Link States gives us information about LSA Type 3 (advertised by the ABR of area 0. Recall that ABRs generate a Type 3 LSA for each subnet in one area, and advertises each Type 3 LSA into the other areas)
From the output we learn that the subnet 116.16.35.0 is advertised via 172.16.1.1 and four routers (in fact, interfaces) are advertised by 192.168.0.4. Notice that these routers (interfaces) can belong to other areas.
For question no. 4 I think C is the correct answer, because if apply the command only on ABR there will be no adjacency between the ABR and other routers in Area 3 because of stub Flag mismatch, applying the command to all routers in area 3 will solve this.
(Area 3 stub no-summary = area 3 stub) on internal routers
What do you think ??!!
@Q4: In fact, stub flag is only a bit in the Hello packet. It is set on all interfaces which belong to stub area. A totally stub area is an extension of stub area so the characteristics of this area are still the same -> no mismatch.
Q4 is right…no where in question did it mention stub was already configured on the other routers.
I think C is correct : from my book by Wendell Odom CCNP Route Official Cert Guide Copyright 2010 a note is added in shaded on p. 241
“For totally stubby areas, only ABR’s must have the no-summary keyword on area [area-id] stub no-summary command. However, including the keyword on internal routers does not cause a problem.”
What I get from this statement is that if you put this on all Area 3 routers it becomes a totally stubby area, regardless of the no-summary command on the internal routers. No where does it say that the internal Area 3 routers had a Area stub command on them already and putting no-summary command on ABR alone does not make the area a totally stubby area.
What do you think Digital Tut? Thanks for all the help you are providing us, you are the best.
Totally agree with you saini, answer C would definitely do the job.
So why not d is the answer ? D should be answer !!
Hello Friends,
I just started to study the ROUTE test after taking almost two years off from passing the CCNA. I am planning to take the ROUTE test in two months. Are the Sims, Drag and Drop, Questions, and everything else on this site still valid for the ROUTE test?
Thank you all!
I think Answer B is Correct.
If the entry is not in LS DB then steps are:
Add to DB >> Send ACK to originator >> Flood LSA >> run SPF to update routing table
Sir CCNP
Yes, that’s correct! All Sims, Drag and Drop, Questions, on this site are still valid for the ROUTE test. I took the test last summer and I passed. It seemed easier than the CCNA test. Make sure that you understand thoroughly all the sims. That’s very, very important. Good luck bud!
Q 10:why 116.16.34.4 is the DR? can someone explain ? Thanks.
@johncc42:
You can read carefully the 9tut’s explain. Pay attention on the LSA types 2 that is advertised by the DR by definition.
I think Answer B is Correct.
If the entry is not in LS DB then steps are:
Add to DB >> Send ACK to originator >> Flood LSA >> run SPF to update routing table
I think what digitaltut answer is correct.
You can’t check sequence number for non-exist LSA. For this check, you must compare exist LSA sequence number with recieved.
In question said “if LSA received is not found int the topological database”
So, B is correct
In question said “if LSA received is not found int the topological database”
Re MTU msitamch: Some routers cannot receive oversized packets, so the OSPF flooding process might randomly hang up. DBD (initial database exchange) packets thus contain MTU field which allows the routers to verify whether they can safely communicate or not. If the MTU in the received packet is larger than the interface MTU, the router drops the packet and the neighbors stay stuck in EXSTART state (= I’ve seen the neighbor and I’ve sent him my first DBD packet) until the timeout expires and the quarter-baked adjacency is cleared.
Saini is right on Q4. Answer should be C.
q4.
I agree with saini525. The answer should be C, and the only way thats wrong is if they made the question a “choose 2″. Then you could choose D and E for the correct answer. But since you can only choose one, you have to go with C and the Odem book quote mentioned validates that. It can not be assumed that the stub command already exists on the internal routers based on the given info, so therefore I do believe you would indeed get a mismatch flag.
Regarding Q4:
+1 for C
I tried E on packet tracer and as expected the area3 router and it’s ABR had the stub area flag mismatched and they couldn’t recognize themselves as neighbors
..
as mentioned in the official certification guide: for a totally stubby area configuration you need to configure the no-summary option on the routers but keeping in mind that the ABR is the only router who is concerned with summarization and you only need to configure it with the “no-summary” option -but if you had it on all area routers that definitely won’t be wrong-
conclusion:
C is not wrong
E cannot be correct
and by the way the question didn’t mention any thing at all about configuring the “area 3 stub” command on internal routers
Q9.
don’t u think that answers A,C also is right.
question 10 is critical for practical work what i suggest is to draw a topology in gns3 or anyother simulator and then compare the output of that command and play with it to get the real understanding. When you reach to that stage that you can build the topology from the output shown, then i guess you will be fine. Its my opinion about understanding please comment!
thanks
Q10 has an error.
Area 3 dont exist in the Based command output. Only Area 0 & area 2.
Q6. From official ROUTE Guide : “Each routing device stores the LSA, forwards the LSA to all neighboring devices (within an area; areas are described in the next section, “OSPF Area Structure”) and updates its link-state database (LSDB). This flooding of the LSA ensures that all routing devices can update their databases and then update their routing tables to reflect the new topology.”
I think, D is more correct.
Q6. Addition. From official ROUTE Guide:
“As shown in Figure 3-7, when each router receives the LSU, it does the following:
■ If the LSA entry does not already exist, the router adds the entry to its LSDB, sends
back a link-state acknowledgment (LSAck), floods the information to other routers,
runs SPF, and updates its routing table.”
in Figure 3-7: “Is entry in link-state database? –> N0 –> Add to Database –> Send LSAck –> Flood LSA –> Run SPF to calculate new routing table –> END”
In D : “The LSA is flooded immediately”. But here “Add to Database” and “Send LSAck” before “Flood LSA “.
In B : “topological database” But in ROUTE Gide: “OSPF topology table = OSPF topology database = LSDB” . “topological database” and “topology database” are the same or not?
Help ))))))
Great post! Thanks.
The choices of the question always distinguish the difference between ip & ipv6. When the answers are both A & C, C reads as follows:
“C. The IP address of the backup designated router (BDR) is FE80::205:5FFF:FED3:5808.”
The correction should read “The ipv6 address of the …” would have been correct.
Thank you to the Author for this site!!
Wow what a brilliant lecture digitaltut.com,i learnt a lot on this thanx so much for helping us pass our exams.I couldnt understand Q10 till i got thru your explanation,now i can see the light….
johncc42 re Q10 – why 116.16.34.4 is the DR? can someone explain ?
Because the link ID relates to an interface on a segment. net link states are related to DR advertisements so the only interface (link) that could be referenced in the network LSA is that of the DR.
Q.9 if the router is an ASBR wouldnt it generate a type 4 LSA?
Q4 should be a 2 answer question, D and E.
Area 3 routers –> area 3 stub
ABR –> area 3 stub no-summary
any update about Q4?? I think correct answer should be C or, if it was a two answers question, it should be D and E…. Will answer E be correct for the exam??
Macro is correct. but for exam E is correct.
Question 4
correct answer is C
The correct answer for Q4 is E. The question asks what command is REQUIRED to configure Area 3 as a TOTALLY STUBBY area. The only choice on the list that is REQUIRED is answer E. The question does not ask what is required to make it work correctly, it only asks, what command (from the list of choices) is required to make area 3 totally stubby.
So A is wrong, B is wrong, C is wrong because all routers in area 3 are not REQUIRED to have the no-summary keyword to make it totally stubby, D is wrong because the ABR needs the area 3 stub no-summary command string to make area 3 totally stubby. So that leaves answer E. For area 3 to be a totally stubby area you need the following commands on the ABR
router ospf 10
area 3 stub no-summary
Q4 is one of these questions that can be annoying. Cisco is looking for the E answer. Most of us can guess that. But, the question is phrased incorrectly. We all know that we need to apply D to all routers in the area, except the ABR and E at the ABR prompt. I don’t see the point of Cisco trying to make the exam more difficult than it already is!
So, in the exam, try to read Cisco’s mind. Follow mimmo’s advice above.
I meant: Follow Jay’s advice above…
About Question 6 (my interpretation):
Notice that in the sentence says: “…what will an internal OSPF router do?”.The key word here is “internal”.
So, although B and D both seems correct, there is one more correct than the other (i.e. more or less general; in this case, they ask us about one less general).
Answer B (-> LSA placed in topology database and LSAck sended) is an action taken by any router that receives an LSA.Is a more general answer.
Answer D (-> LSA flooded all out OSPF interfaces) is an action taken only by internal routers.Notice that they say “all OSPF interfaces”.And if the receiving router is a border router? It will send the LSA out ALL OSPF interfaces, or only to the interfaces to a determined area? This is a less general answer, so more correct then B.
Q.9 I think D and F are correct. I know 1 is also correct, but I think they are looking for D and F. Please advice!
Q4: I agree with ROAD_WARRIOR:
key word is “REQUIRED” -> it means Answer E.
(Cisco seems like to be political sometimes, not technical)