tricky OSPF question

Unanswered Question
Nov 28th, 2007
User Badges:

ok this question is not very hard, just tricky and i don't agree with the answer. so here it is:


Which configuration commands will enable Router A to advertise all local interfaces over ospf


a)router ospf 1

default-information originate


b)router ospf 1

network 255.255.255.255 255.255.255.255


c)router ospf 1

network 0.0.0.0 0.0.0.0


d)router ospf 1

redistribute connected


Now i chose C. the network 0.0.0.0 0.0.0.0 command advertises all interfaces into OSPF.


the correct answer given was D (redistribute connected)


what is the correct answer, i have configured this in a lab environment and the redistribute connected command does not advertise all interfaces into OSPF.


especially if they are not classfull, since the redistribute connected subnets command would have to be used for that.


any help would be appreciated. thanks

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
Kevin Dorrell Wed, 11/28/2007 - 13:00
User Badges:
  • Green, 3000 points or more

Luca, you are absolutely right: they would need to include the subnets keyword for it to advertise all the local interfaces. It is so lucky IOS reminds you, 'cos I forget almost every time!


As for answer (c), that would advertise all the local interfaces. It would also activate the search for neighbors on all the local interfaces ... something the redistribute connected would not do.


Kevin Dorrell

Luxembourg

crem_cisco Wed, 11/28/2007 - 13:07
User Badges:

The "network" sub-router command does not distribute subnets like eigrp or rip. Common misconception.


//CREM

LucaSalvatore_2 Wed, 11/28/2007 - 13:17
User Badges:

thanks for the fast reaponses guys!


so the correct answer is redistribute connected then?


assuming that the network is classfull

srue Wed, 11/28/2007 - 16:47
User Badges:
  • Blue, 1500 points or more

I think it's a badly worded question - and answers.


If D were the answer, they must have assumed that the network commands have already been configured.

mmolina10855 Wed, 11/28/2007 - 23:12
User Badges:

Not to beat a dead horse here but consider the following:


It is true that if you paste the syntax from answer C into a router, OSPF will be lit up on every interface with an IP. However, the syntax in answer C is technically wrong. The proper syntax for a "catch all" OSPF network statement is "network 0.0.0.0 255.255.255.255 area xxx". When you paste the answer C syntax into a router, IOS is smart enough to adjust the inverse mask to 255.255.255.255 for you. Long story short, answer C is technically a wrong answer even though it works when pasted. I would have chosen answer D because it is closest to being correct. I agree that this is a poorly worded question/answer.

Kevin Dorrell Thu, 11/29/2007 - 01:03
User Badges:
  • Green, 3000 points or more

Crem,


Could you explain that a bit further please? You seem to be implying that if I do something like


interface Lo20

 ip address 192.168.20.65 255.255.255.240

!

router ospf 100

 network 192.168.20.0 0.0.0.255 area 0


then OSPF will not get LSAs for 192.168.20.65/32 (or 192.168.20.64/28 if declared as p2p). But I don't think that is what you are saying. So could you explain please.


Kevin Dorrell

Luxembourg


Actions

This Discussion