Network declaration statement (25-bit sub) in OSPF

CriscoSystems · ‎10-20-2009

Yes, once again stuey is launching an attack on the textbook quizzes!! Who could ever get tired of this engaging drama...

So everyone dig this question and explain your answer (I think the answer is "none of the above" but it's not a given option; and I have been wrong before)

Question: Which of the following commands is the correct command for placing the subnet 172.16.20.128 in Area 1; and all other subnets within the classful address in area 0?

A:

network 172.16.20.128 0.0.0.0 area 1

network 0.0.0.0 255.255.255.255 area 0

B:

network 172.16.20.128 0.0.0.255 area 1

network 172.16.0.0 0.0.255.255 area 0

C:

network 172.16.20.128 0.0.0.0 area 1

network 172.16.20.0 255.255.255.255 area 0

D:

network 172.16.20.128 0.0.0.7 area 1

network 172.16.20.0 255.255.255.255 area 0

#A doesn't put just the other subnets of the 172.16 into area 0; it puts bloody all the subnets in the world in there, which isn't what the question asks us to do

#B comes closest; except with .255 as the fourth octet of the wildcard mask, then the 25th bit isn't visible; which means both subnets 172.16.20.128/25 and 172.16.20.0/25, if the latter were configured, would get into area 1 as well. Which isn't what the question wants...

With #C we have the same problem as with #A, the wildcard mask lets everyone in the world in

#D wants us to have noticed that there's seven unset bits in the network ID and to mistake .7 for the binary equivalent, .127. I shan't fall for that.

Really, I don't see any option here that does _exactly_ what the question wants. How would you answer this question on a live Cisco exam? (I got it from a book, not an exam, so no NDA trouble here.)

Edison Ortiz · ‎10-20-2009

I agree. B is the closest but you said 'subnet', not 'host' I wonder if the book dropped /24 from the question and it wanted to say:

"placing the subnet 172.16.20.128/24 in Area 1"

CriscoSystems · ‎10-20-2009

I wondered that too, but for a 24-bit subnet, why put a value (especially a high-order-bit-set-equivalent value) in the fourth octet?

Anyway, glad to know I'm not the only one who sees a problem there!

Edison Ortiz · ‎10-20-2009

Once entered in a real router, the IOS will replace .128 to .0

The book is trying to make you think..

Regards

Edison.

CriscoSystems · ‎10-20-2009

'Cause of classful summarization? Because in that case it would also replace the .20 with .0; we wouldn't be dealing with any subnets...

Edison Ortiz · ‎10-21-2009

No classful summarization involved but the subnet mask used on the network statement:

network 172.16.20.128 0.0.0.255 area 1

network 172.16.0.0 0.0.255.255 area 0

On real router:

R3#conf t

Enter configuration commands, one per line. End with CNTL/Z.

R3(config)#router os 1

R3(config-router)#network 172.16.20.128 0.0.0.255 area 1

R3(config-router)#network 172.16.0.0 0.0.255.255 area 0

R3(config-router)#do show run | be router os

router ospf 1

log-adjacency-changes

network 172.16.20.0 0.0.0.255 area 1

network 172.16.0.0 0.0.255.255 area 0

Regards,

Edison.

CriscoSystems · ‎10-21-2009

Well, yes of course the .255 in the wildcard mask hides the .128; but now we're right back where we started, aren't we, since the question doesn't specify a 24-bit subnet? (And DOES include the id of a valid 25-bit subnet...)

I'm not trying to give anyone heartburn; it's just that this is exactly the sort of genuinely faulty question I've come across often on Cisco exams. I dread the thought of a flawed question like this coming up on the BSCI...

(Like there's not enough to dread already on that one...)

Edison Ortiz · ‎10-21-2009

We covered that. There isn't a correct answer if you posted the question verbatim. If the question include /24 as I stated before, the answer B is dead on.

I believe it was an edit problem on the book if /24 was not included - as the wording uses 'subnet' not 'host'.

__

Edison.

CriscoSystems · ‎10-21-2009

You're right, I've gotta let this go.

(The answer key says "B" (but we've both proven _we_ know better...))