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Exam 200-301 topic 1 question 484 discussion

Actual exam question from Cisco's 200-301

Question #: 484
Topic #: 1

Refer to the exhibit. All interfaces are configured with duplex auto and ip ospf network broadcast. Which configuration allows routers R14 and R86 to form an
OSPFv2 adjacency and act as a central point for exchanging OSPF information between routers?

A. R14# interface FastEthernet0/0 ip address 10.73.65.65 255.255.255.252 ip ospf priority 255 ip mtu 1500 router ospf 10 router-id 10.10.1.14 network 10.10.1.14 0.0.0.0 area 0 network 10.73.65.64 0.0.0.3 area 0 R86# interface FastEthernet0/0 ip address 10.73.65.66 255.255.255.252 ip mtu 1400 router ospf 10 router-id 10.10.1.86 network 10.10.1.86 0.0.0.0 area 0 network 10.73.65.64 0.0.0.3 area 0
B. R14# interface Loopback0 ip ospf 10 area 0 interface FastEthernet0/0 ip address 10.73.65.65 255.255.255.252 ip ospf 10 area 0 ip mtu 1500 router ospf 10 ip ospf priority 255 router-id 10.10.1.14 R86# interface Loopback0 ip ospf 10 area 0 interface FastEthernet0/0 ip address 10.73.65.66 255.255.255.252 ip ospf 10 area 0 ip mtu 1500 router ospf 10 router-id 10.10.1.86
C. R14# interface FastEthernet0/0 ip address 10.73.65.65 255.255.255.252 ip ospf priority 0 ip mtu 1500 router ospf 10 router-id 10.10.1.14 network 10.10.1.14 0.0.0.0 area 0 network 10.73.65.64 0.0.0.3 area 0 R86# interface FastEthernet0/0 ip address 10.73.65.66 255.255.255.252 ip mtu 1500 router ospf 10 router-id 10.10.1.86 network 10.10.1.86 0.0.0.0 area 0 network 10.73.65.64 0.0.0.3 area 0
D. R14# interface Loopback0 ip ospf 10 area 0 interface FastEthernet0/0 ip address 10.73.65.65 255.255.255.252 ip ospf priority 255 ip ospf 10 area 0 ip mtu 1500 router ospf 10 router-id 10.10.1.14 R86# interface Loopback0 ip ospf 10 area 0 interface FastEthernet0/0 ip address 10.73.65.66 255.255.255.252 ip ospf 10 area 0 ip mtu 1500 router ospf 10 router-id 10.10.1.86

Show Suggested Answer

Suggested Answer: D 🗳️

by g_mindset at Sept. 3, 2022, 1:05 p.m.

Comments

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joondale

Highly Voted 2 years, 4 months ago

Selected Answer: D

Going with D A is wrong because ip mtu of R14 and R86 are different B is wrong because because ip ospf priority is configured inside router-config, it should be on the interface C is wrong because ip ospf priority is 0 on R14 and it makes R14 not participate on ospf dr/bdr election, the network type is broadcast so i assume dr/bdr should be elected otherwise the network type should be point-to-point D is correct answer - because mtu are same for both routers, participates in dr/bdr election

upvoted 47 times

AbdullahMohammad251

1 year, 2 months ago

C is wrong because the question says that one of the routers should act as a central point for OSPF information exchange. When setting the priority to 255 we guarantee that R14 or R86 will be the DR, Setting it to zero means excluding the router from the election, and having a priority set to 1 by default doesn't guarantee that the other router will be a DR or BDR.

upvoted 2 times

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LeonardoMeCabrio

1 year, 6 months ago

Shouldnt there be Network commands in D. like there are in C.? Cause I dont see any.

upvoted 2 times

spazzix

1 year, 5 months ago

You can enable OSPF on a per-interface basis with: ip ospf <PROCESS ID> area <AREA #> command in an interface config That can do the exact same thing as network <IP> <WILDCARD MASK> area <AREA #> The benefit of the network command is it allows you activate multiple interfaces at once The downside of the network command is that it may unintentionally add a network into OSPF that you didn't want to. Especially if you configure another subnet or interface down the road.

upvoted 7 times

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zFlyingLotusz

Highly Voted 1 year, 6 months ago

I'm literally skipping these questions.

upvoted 13 times

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ZK23

Most Recent 10 months, 1 week ago

Awful question and Cisco shouldn't try to confuse you like this but here's how I'd answer it quickly. Eliminate the mtu mismatch answers. So it's between B and D now. D is correct because it gives r14 a 255 priority. Step 1: find mtu match (B & D) Step 2: find priority 255 for R14. Only D has 255. A- incorrect mtu mismatch B- incorrect priority for R14 C- incorrect priority for R14 D- Correct mtu and priority. Correct me if I'm wrong or have a faster way.

upvoted 3 times

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elgato69

11 months, 3 weeks ago

Impossible answer, the MTU must match or OSPF will not work.. come on man, this was the first thing covered in my course material on OSPF

upvoted 2 times

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tubirubs

1 year, 5 months ago

Selected Answer: D

WTFFFFF. A have a diferente size of MTU! 1400 and 1500. its the 1st for exclude. lol this dump is very dump

upvoted 2 times

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TechJ

1 year, 8 months ago

Selected Answer: D

going with D, A is clearly wrong due to MTU mismatch For the people that think D is wrong, you need to look at the entire command line, because I got tricked at the beginning as well. It does provide "ip ospf 10 area 0" on both the loopback and actual interface (not just loopback only)

upvoted 1 times

AbdullahMohammad251

1 year, 2 months ago

DR and BDR serve as the central point for exchanging OSPF routing information. So we need their priority to set the priority to a value that guarantees that R14 or R86 is the DR. That's why we need to pick R14 or R86 to have a priority = 255

upvoted 1 times

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krzysiew

1 year, 8 months ago

Selected Answer: C

An OSPF priority of 0 does not prevent the router from establishing OSPF adjacencies.

upvoted 2 times

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dropspablo

1 year, 8 months ago

Selected Answer: D

Router with "priority 0" and another with "priority default (1)" formed adjacency and exchanged LSAs and LSDBs normally (I tested it in P.Trace and OSPF dynamic routing works normally), the difference is that there will not be a DR Backup in case fail (that's all). One will be DR Other (neighbor Full/DR) and one DR (neighbor Full/DROther), and BDR appears written that it does not exist, because priority 0 cannot be neither DR nor BDR. (Observation: "point-to-point type" is recommended for this type of connection.) However, the exercise asks them to act as a central point for exchanging information, in this case "it gives the impression" that he asked us to select a "DR". Letter "D" would be the most correct because using "ip ospf priority 255" (in the interface) we define R14 as DR.

upvoted 3 times

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MJBM

1 year, 8 months ago

C is for me. A is mtu issue B is network issue D is network issue

upvoted 1 times

PHIL95240

9 months, 1 week ago

The answer C doesn't work because in the command : network 10.73.65.64 0.0.0.3 area 0 you have an ip address which is used for the network. The both valid ip addresses are 10.73.65.65 and 10.73.65.66. The good answer should be B or D. The difference between those questions is for the command "ip ospf priority 255" it isn't placed at the same place.

upvoted 1 times

PHIL95240

9 months, 1 week ago

Finally it should be the answer D because the command : "ip ospf priority 255" comes just after the Global interface configuration, Interface Specific configuration in the Router Specific configuration like this : R14(config)# interface Fa0/0 R14(config-if)# ip address 10.73.65.65 255.255.255.252 R14(config-if)# ip ospf priority 255

upvoted 1 times

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linuxlife

1 year, 9 months ago

as per the standard OSPF specification defined in RFC 2328, “OSPF Version 2”. Specifically the RFC states the following: 10.6 - Database Description Packet. If the Interface MTU field in the Database Description packet indicates an IP datagram size that is larger than the router can accept on the receiving interface without fragmentation, the Database Description packet is rejected. Basically this means that if a router tries to negotiate an adjacency on an interface in which the remote neighbor has a larger MTU, the adjacency will be denied. The idea behind this check is two-fold. The first is to alleviate a problem in the data plane, in which a sending host transmits packets to a receiver that are too large to accept. Typically, Path MTU Discovery (PMTUD) should be implemented on the sender to prevent this case, however this process relies on ICMP messages that could possibly be filtered out in the transit path due to a security policy. The second, and most important issue, is to alleviate a problem in the control plane in which OSPF packets are exchanged.

upvoted 1 times

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daddydagoth

1 year, 11 months ago

Selected Answer: D

It's absolutely D

upvoted 4 times

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gewe

1 year, 11 months ago

option A would be best if MTU match... option D has no routes advertised...

upvoted 4 times

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Netcmd

2 years, 2 months ago

D is wrong because the network cmd is not configured for the networks. How can you form neighbours without it

upvoted 4 times

Ceruzka

1 year, 10 months ago

intf are advertised instead with cmd "ip ospf 10 area 0" so no network cmd needed in router ospf process..

upvoted 1 times

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Ceruzka

1 year, 10 months ago

IMHO correct is "C" I agree with Netcmd. @joondale comment: Priority 0 on R14 just mean that this intf will neither be DR nor BDR, but it will work. So for me "C"

upvoted 2 times

Ceruzka

1 year, 10 months ago

my BAD, looks like D" is correct.

upvoted 1 times

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mzu_sk8

2 years, 2 months ago

ip ospf 10 area 0 subinterface command, does the same , is even better , it has preemption over "network" command

upvoted 7 times

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GigaGremlin

2 years, 3 months ago

Selected Answer: D

D is the correct answer

upvoted 1 times

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nicombe

2 years, 4 months ago

I'm also going with D. While OSPF Routers can create an adjacancy having mismatched MTUs, they won't be able to exchange LSDBs and therefore won't be able to act as a central point for exchanging OSPF information between routers.

upvoted 1 times

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