OSPF Questions 4
Here you will find answers to OSPF Questions – Part 4
Question 1
Refer to the exhibit. Two routers are connected by Frame Relay and are running OSPF between them. Each router has been configured with the appropriate network statements under router ospf 1, but the routers are not forming an adjacency. Which of the following three commands could be configured on each router to correct this problem? (Choose three)
A.
RouterC(config-if )#ip ospf network broadcast
RouterD(config-if )#ip ospf network broadcast
B.
RouterC(config-if)#ip ospf network point-to-point
RouterD(config-if)#ip ospf network point-to-point
C.
RouterC(config-router)#neighbor 10.100.100.4
RouterD(config-router)#neighbor 10.100.100.3
D.
RouterC(config-router)#neighbor 10.255.255.254
RouterD(config-router)#neighbor 10.255.255.253
Answer: A B D
Explanation
By default, Frame Relay is classified as a non-broadcast network, meaning it doesn’t send any broadcasts/multicasts like RIP, OSPF or EIGRP updates across the network (hello packets of OSPF are multicast to 224.0.0.5). Hence, in NBMA network, the neighbors are not discovered automatically; they must be configured manually. There are two ways to simulate a broadcast model on an NBMA network:
+ Define the network type as broadcast with the “ip ospf network broadcast” interface sub-command
+ Configure the neighbor
Besides these two ways, another way for making OSPF work with Frame Relay is configuring the network as a Point-to-Point network (or Point-to-Multipoint, notice that OSPF treats Point-to-Multipoint network as a collective of point-to-point links).
Some information about Point-to-Multipoint (or Point-to-Point) network:
Note: Point-to-Multipoint networks do not maintain a DR/BDR relationship.
Question 2
Refer to the exhibit. What is the effect of the OSPF configuration on router B?
A. All interfaces will be in area 0.
B. The router will be an ABR with s1/0 in area 0 and 0/0 and 0/1 in area 1.
C. The router will be an ABR with s1/0 in area 0 and 0/0 and 0/1 in area 2.
D. The router will be an ABR with s1/0 in area 0, f0/0 in area 1, and f0/1 in area 2.
Answer: D
Question 3
OSPF is enabled on router A. You execute the following command on router A and receive the accompanying output:
| RouterA#ping 224.0.0.5 repeat 1 Type escape sequence to abort. Sending 1, 100-byte ICMP Echos to 224.0.0.5, timeout is 2 seconds: Reply to request 0 from 10.100.100.1, 4 ms |
10.100.100.1 is the IP address of a loopback interface on router A. What can you conclude about router A?
A. Only the router A loopback interface is participating in the OSPF routing process.
B. None of the router A interfaces are participating in the OSPF routing process.
C. Router A is using the loopback interface IP address as its OSPF router ID.
D. Router A does not have any reachable OSPF neighbors.
Answer: D
Explanation
The 224.0.0.5 address is the multicast for OSPF routers. Therefore when you ping to this address all OSPF routers will reply but from the output we learn that only the local router responded -> Router A does not have any reachable OSPF neighbors -> D is correct.
Question 4
Your network has a mixture of Fast Ethernet and Gigabit Ethernet links. What needs to be done to ensure optimal data routing when using OSPF?
A. Nothing. OSPF will determine the most optimal path for routing data by default.
B. Adjust the hello and dead timers for more rapid detection of link failures.
C. Increase the reference-bandwidth used to calculate the interface default metrics, on all routers in your network.
D. Set the priority values on every broadcast interface to ensure that the designated and backup designated routers are the routers with the most processor and memory resources.
Answer: C
Explanation
The default formula to calculate the cost for the OSPF metric is (108/BW). Therefore when using default reference bandwidth (100M) to calculate ospf, Gi and Fa interfaces could have the same cost (1). This problem can be fixed by configuring reference bandwidth to 1000M.
| Default Bandwidth (100M) | Configured Bandwidth (1000M) |
| Fa = 100M / 100M = 1 Gi = 100M / 1000M = 0.1 -> round up to 1 |
Fa = 1000M / 100M = 10 Gi = 1000M / 1000M = 1 |
This is how to configure reference bandwidth to 1000Mbps:
Router(config)#router ospf 1
Router(config-router)#auto-cost reference-bandwidth 1000
Question 5
| RouterA#debug ip ospf events OSPF events debugging is on RouterA# 04:43:16: OSPF: Rev pkt from 10.3.0.2, Ethernet0/1, area 0.0.0.1 mismatch area 0.0.0.2 in the header 04:43:19: OSPF: Rev hello from 10.100.100.3 area 0 from Ethernet0/0 10.1.0.1 04:43:19: OSPF: End of hello processing |
Refer to the exhibits. What can be done to fix the problem?
A. Change router B E0/1 interface to area 0.
B. Change router A interface E0/1 to area 0.0.0.2.
C. Configure the E0/1 interfaces of router A and router B to be in area 0.
D. Shut down the E0/1 interfaces in router A and router B as OSPF does not allow “back doors” between areas.
E. Remove the E0/1 interfaces in router A and router B from the OSPF process. Use static routes to route data directly from router A to router B to avoid passing data through router C in area 0.
Answer: C
Question 6
Which two statements are true of the OSPF link-state routing protocol? (Choose two)
A. Using the Bellman-Ford algorithm, each OSPF router independently calculates its best paths to all destinations in the network.
B. Using the DUAL algorithm, each OSPF router independently calculates its best paths to all destinations in the network.
C. OSPF sends summaries of individual link-state entries every 30 minutes to ensure LSDB synchronization.
D. OSPF sends triggered updates when a network change occurs.
E. OSPF sends updates every 10 seconds.
F. When a link changes state, the router that detected the change creates a link-state advertisement (LSA) and propagates it to all OSPF devices using the 224.0.0.6 multicast address.
Answer: C D
Question 7
Given the following partial configuration for Router A:
|
interface serial 0 router ospf 7 |
Which two statements are correct? (Choose two)
A. DR/BDR elections do not take place.
B. The router is restricted to a hub and spoke topology.
C. The area 0 NBMA cloud is configured as more than one subnet.
D. OSPF neighbor statements are not necessary.
Answer: A D
Explanation
The command “ip ospf network point-to-multipoint” configures this interface as point-to-multipoint for broadcast media. In broadcast networks, there is no need to specify neighbors. No DR or BDR is elected in this network type.
Note: If we configure this interface as a point-to-point non-broadcast (with the “ip ospf network point-to-multipoint non-broadcast” command) then we need to manually declare the OSPF neighbors.
Question 8
What are two Cisco IOS commands that can be used to view neighbor adjacencies? (Choose two)
A. show ip ospf database
B. show ip ospf neighbors
C. show ip ospf protocols
D. show ip ospf interfaces
Answer: B D
Explanation
The output of these commands are shown below:
Notice that for the “show ip ospf interface” command, the “Neighbor Count” is the number of OSPF neighbors discovered on this interface while the “Adjacent neighbor count” is the number of routers running OSPF that are fully adjacent with this router. Adjacent means that their databases are fully synchronized. In this example, this router has one neighbor on its Ethernet0 interface.
Question 9
Refer to the exhibit. What additional commands should be used to configure OSPF area 5 as a Totally Stubby area?
A. area 0 stub on routers R4 and R5
B. area 5 stub on routers R4 and R5
C. area 5 stub no-summary on routers R4 and R5
D. area 0 stub no-summary on router R4 and area 5 stub no-summary on router R5
E. area 5 stub no-summary on router R4 and area 5 stub on router R5
Answer: E
Explanation
To define a totally stub area, use the area area-id stub no-summary command on the ABR (in OSPF router configuration) and the area area-id stub on the totally stub router. The ABR will inject a default route into the area so routers in this type of area only see routing information local to their area, plus a default route pointing to the ABR, from which they can reach all other areas and all other networks
Question 10
According to RFC 2328, what is the stateful order in which an OSPF router transitions to a full adjacency with a neighbor router?
A. Down, Init, 2-Way, Exstart, Exchange, Loading, and Full
B. Down, Init, 2-Way, Exchange, Exstart, Loading, and Full
C. Down, 2-Way, Init; Loading, Exstart, Exchange, and Full
D. Down, 2-Way, Init, Exchange, Exstart, Loading, and Full
E. Down, Init, 2-Way, Loading, Exstart, Exchange, and Full
F. Down, 2-Way, Init, Exstart, Exchange, Loading, and Full
Answer: A
Explanation
When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below:
Down: no information (hellos) has been received from this neighbor.
Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval.
Init: specifies that the router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet
2-Way: indicates bi-directional communication has been established between two routers.
Exstart: Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR.
Exchange: OSPF routers exchange database descriptor (DBD) packets
Loading: In this state, the actual exchange of link state information occurs
Full: routers are fully adjacent with each other
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.shtml)
Q2 WHY would someone configure area 2 network that way? I don’t get it. Someone pl explain.
@aza. Why not? They have used wildcard mask of 0.0.1.255 for 10.100.4.0 range of which is 10.100.4.0-10.100.5.255. And F0/1′s IP falls in that range.
It just seems silly. Why not just do it consistently across all interfaces? Guess they’re just having fun with it.
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!
Q3: D is right. Why isn’t C right as well?
Hello every body, don’t worry, i will take the ROUTE exam next Month, February 4, 2012, and i’ll give you guys an update about the exams :-)
THank you and God Bless
Q5: Answer is B
Q5: In my opinion both B & C will work, it all depends on what area you want the e0/1 interfaces of A&B to be in.
I’ve recreated this in GNS3 and they both work, but i would go with option C on this one because A is ABR for area 1 and it just seems silly to use 0.0.0.2 as an area :)
@gogo:
Q3: D is right. Why isn’t C right as well?
—–
because it doesnt say anything about router-id, it could be some id issued by router-id command, wich beats ‘lo if’ by priority.
@Imr
I assumed, that reply to ping is carrying router-ID, instead it is carrying IP address of responding interface. There is not enough evidence for C, D must be right. Thanks
Hi all.
In question 6 – why A is not correct?
Which two statements are true of the OSPF link-state routing protocol? (Choose two)
A. Using the Bellman-Ford algorithm, each OSPF router independently calculates its best paths to all destinations in the network.
Cause OSPF uses Dijkstra’s SPF algorithm. Bellman-ford is used by distance vector protocols
Hi,
for @Alam it’s Dijkstra SPF algorithm :) not Bellman-ford
for Q5: There is Area 0 notification at the inner top of a triangle so all inner interfaces should belong to area 0.
Gabriel:
Could you explain Q5 . where is the Area notification?
Q2: If you execute the last ospf network command, both the 10.100.4.0 & 10.100.5.0 would be added to area 2 but 10.100.4.0 would still remain in area 1 also; either option C or D could be taken as correct.
Q1. There are no neighbor statement for configuring frame-relay over point to point in OSPF. Also, why would you use both point-to-point and broadcast? Can someone explain the reason for the answers chosen?
IPEXPERT:
http://blog.ipexpert.com/2009/08/08/ospf-over-frame-relay-part-3-point-to-point/
I figured out that you can statically assign neighbors but I’m still confused to why you would use both broadcast and point-to-point network-type!
Q3: 10.100.100.1 is the IP address of a loopback interface on router A
**????** A. Only the router A loopback interface is participating in the OSPF routing process. —> BUT, In my GNS3 i test with only a Loopback interface, OSPF is running and reply the ping of 224.0.0.5
**NOT** B. None of the router A interfaces are participating in the OSPF routing process. —> NOT
**NOT** C. Router A is using the loopback interface IP address as its OSPF router ID. —> MORE INFORMATION IS NEEDED
**YES**D. Router A does not have any reachable OSPF neighbors. —> IS TRUE
Q 1, why not C ? its loop back address
question 2
Should the display showing the network statements be .3 .4 .5 networks,
instead of 3. 4. 4.
Otherwise I dont see how .4 is activated on Fa1/0 since the only network connected to fa1/0 is
10.100.5.0 /24. Could the question be referring to a loopback not shown? Seems to me answer based on the info given is none of the above?
Q2.
Some tests in gns3 and we have what more specific mask have priority.
Ok, for example. Put command in this order:
network 10.100.4.0 0.0.3.255 area 0 – interface in area0
network 10.100.4.0 0.0.0.255 area 1 – change – int in area 1
network 10.100.4.0 0.0.1.255 area 0 – no changes int still in area 1
network 10.100.4.0 0.0.0.3 area 0 – change – now int in area 0 .
for Q2 we have:
network 10.100.4.0 0.0.0.255 area 1 – net 10.100.4.0 mask 255.255.255.0 – more specific
network 10.100.4.0 0.0.1.255 area 2 – net 10.100.4.0 and 10.100.5.0 mask 255.255.254.0 – low priority – not change area for network 10.100.4.0
Answer D – is correct.
I would like to add about question No7
from the documenatation it has been deduced that :
On point-to-multipoint, broadcast networks, there is no need to specify neighbors. However, you can specify neighbors with the neighbor command, in which case you should specify a cost to that neighbor.
•On point to multipoint, nonbroadcast networks, you now use the neighbor command to identify neighbors. Assigning a cost to a neighbor is optional.
For point to multipoint,Broadcast
===============
encapsulation frame-relay
ip ospf cost 100
ip ospf network point-to-multipoint
For Point to Multipoint, Nonbroadcost
==========================
ip ospf network point-to-multipoint non-broadcast
encapsulation frame-relay
Note: Dont get confused with encapsulation framerelay thats common in both types, the factor that distinguishes is ip ospf network , which will either broadcast or non braodcast…
comments well come
regards
Hi Guys,
In question 1, i think that option “D” is incorrect, because when a router is configured with Loopback interface, that becomes the Router id for that router to be identified by its neighbors, if i am not wrong. Hence A,B & C should be correct.
Please verify
@bala, no with the neighbor command you should state the IP address of the neighbor in the common subnet, not the Router ID of the neighbor. D is correct.
Q1: Why can’t use loopback address to form neighbor? Can someone help?
I agree with Bala, loopback beats ip address of the neighboring interfaces by definition, “D” should be the right answer, because any loopback configured in the router takes precedence over physical interfaces.
Sorry, in my last comment I meant “C” should be right answer.
Could somebody explain the answer C for the question 5? Thanks
Why can not it be answer B? –> there will be no more area mismatch….
@ frize. The routers A and B E0/1 interfaces are in subnet 10.3.0.0/24 which is in AREA 0.
You can see that Routers A and B are ABR’s connected to AREA 0. So the E0/1 interfaces of those routers must be in the backbone AREA (AREA 0).
@josue @bala
i don’t agree with you, you should always use neighbor IP address you can reach, not the router-id.
with neighbor set to lo, the router will never set an adjencancy, couse he will never find any neighbor.
Think you might be confising the BGP neighbor command where you can use the RID
Correction: I mean loopback
Question number 2 is confusing. When simulated on GNS3 the the 2 ethernet interfaces are not coming under area 2. Why would interface having ip address in 10.100.5.0 will come under area 2 ?
The comment made by DIV are is not true when verified in GNS3.
Q2, I mean, how are we supposed to know how that will turn out? I know that I’d never do it like that and have ever tried it like that in a LAB for obvious reasons (since it’s just plain silly to do it like that). I don’t mind that they’re testing knowledge or something, but these kind of questions are rediculous, really, unbelievale.
Ranjeet Badhe… good one, gonna try this in a LAB as well :P.
Q1: The question is very confusing, it just wants to know if you know how to configure OSPF to form adjancency in Frame-Relay network.
A correct: EITHER you shall use ip ospf network broadcast
B correct: EITHER you shall use ip ospf network point-to-point
D correct: OR you leave everything DEFAULT which is non-broadcast and then you should use NEIGHBOR command, because NEIGHBOR command is used only in non-broadcast mode (non-broadcast or ip ospf network point-to-multipoint non-broadcast mode)
I hope it might help. (and I hope I am right ;))
Q:1
A, B, D are correct…
When configuring neighbor statements you need to use the directly connected routers interface ip address (which needs to be in the same network… which is obvious)
When configuring ***virtual-links*** you use the two connecting routers RID’s. If a loopback is configured, the rid will be the highest loopback and does NOT need to be part of the ospf process
HTH :-)
Question 1 <~~ This is a sketchy question
After using GNS3 to confirm..
Answer A is correct – However the assumption is that the * Router(config-if)#frame-relay map ip broadcast* has been issued on the interface
Answer B is also correct with the same assumption as answer A
Answer C is not correct – It will not work.
Answer D is correct – Read Fatfighter’s response above. Non Broadcast will not elect neighbors. They must be set manually.
@Dacollins But why would they have to be in area 0? 10.3.0.0/24 cab be in different subnet? I’ll try and simulate quickly on GNS3 just to see if I see any problems.
@dacollins @frize Verified, it wouldn’t be a problem if interface E0/1 on router A is area 2.
http://i.imgur.com/A641T.jpg
I believe B is wrong because we don’t refer to the area as area 0.0.0.2 in the exam.
@Q9) Can someone please explain
Why not C??
because stub no summary only run on ABR router
R4 is ABR router
Q9– C is not correct Besause stub no summary cmd only run on ABR router..
@SOLID area 2 and area 0.0.0.2 are the same thing. Both the area ID and router ID are described in octets (###.###.###.###) just like IP. That doesn’t mean they’re IP addresses.
When you see area numbers in the debug ip output, they’re seen in octet form
Q5 could be B, or C.
C is the safest bet. The diagram is poorly done, is as if someone went out of their way to make it look ambiguous. it doesn’t implicitly draw a circle between all routers to say router A and B are inside area 0. You could make the link between A & B be in either area 1 or 2, and as long as they’re in a backbone area while being connected to other ospf areas, they’d still be considered ABRs
question 6
why F is not correct ?
Q3: D is right. Why isn’t A right as well?
Because the only adress that replied to the ping command is the loopback address.
for Q 5
the right answer is 5 , why ,
1- because if you adjust router A & B E0/1 to be in area 1 or area 2
they will form a relationship OK but the catch is there will be 2 DRs for this area , which is wrong
try it and do #sho ip os data command
2- there will be two paths to networks out of area 0
see under summary Net link OK
3- in the routing table you will see only routes coming from area 0 , why , because ospf believes in LSAs coming from backbone than from nonbackbone ,,,, so anything coming from nonbackbone will be discarded
hope it helps
for Q 5
SORRY GUYS I MEANT CCCCCCCCCCC
MY BAD
@ABDO.
F is wrong because OSPF forwards link changes not to all OSPF devices but only to OSPF routers in the local area..and ofcourse the backbone.
For Q 5. If I were to weigh between the two correct answers B and C.. my first answer would be “B” because it requires less amount of configuration. However, if we take the Ethernet LANS connected to routers A and B (assuming they are going to participate in OSPF), OSPF at router C will have less possibility of suboptimal routing..and the network is so small to be breaking it up into multiple areas.
Am I correct in thinking this way –about suboptimal routing in router C?? I personally prefer “B” as a choice, because it works too..Or is “C” the “safest” and best answer just because it simplifies everything? Well-thought input would be greatly appreciated.
SubOp in OSPF??:
Answer B will make router A an “ABR Area 1 AND(!) 2″, while answer C makes it the “ABR Area 1(!)” ONLY.
I think the key here is “router A is ABR AREA _1_”, not “Area 1 and 2″ .
I back up answer C.