BSCI Chapter 5 Q&A wrong?

minds-eye

[FONT=&quot]4.[/FONT][FONT=&quot]The address 192.100.56.10/21 has been allocated to an interface on the router. This interface alone is to be included in the OSPF process. State the command that would start the process on this interface.[/FONT]


4. Answer:
network 192.100.56.10 0.0.0.0 area 0
network 192.100.56.8 0.0.0.7 area 0
network 192.100.56.0.0 0.0.0.255 area 0
network 192.100.56.0 0.0.7.255 area 0

[FONT=&quot]Huhh? I got[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]Router(config-router)#network 192.100.56.0 0.0.7.255 area area[/FONT]


Any ideas?

BennyLava

All 4 of those answers are correct because they all match the IP 192.100.56.10. The wildcard mask in the network statement has nothing to do with the subnet mask configured on an interface and does not have to be the 'inverse' of it.

kryolla

This interface alone is to be included in the OSPF process

convert them all to see what ip addreses they encompass but honestly I would of answered 192.100.56.10 0.0.0.0 as the others can include additional interfaces such as /30 interfaces

the first one is a given it will match any interface that has this exact IP 192.100.56.10

the second will match any interfaces with an IP in the range of 192.100.56.9 to 192.100.56.14

the third will match any ip in the range of 192.100.56.1 to 192.100.56.254

see if you can do the fourth

hope this helps

Forsaken_GA

The important thing to remember about ospf network statements is that it's not just a match for the IP per se. It is, essentially, a mask to identify interfaces.

For example, let's say you have you two interfaces with the IP's

192.168.1.1/24
192.168.2.1/24

If you use the statement,

network 192.168.0.0 0.0.255.255 area 0

That would put both interfaces into area 0. If your instructions were to put only 192.168.1.1 into area 0, then that would be wrong, and you'd have to use, at a minimum , network 192.168.1.0 0.0.0.255 area 0

So while using the inverse mask is not a requirement, it's the best way to make sure that only the interface you want is activated.

accely

kryolla wrote: »

convert them all to see what ip addreses they encompass but honestly I would of answered 192.100.56.10 0.0.0.0 as the others can include additional interfaces such as /30 interfaces

the first one is a given it will match any interface that has this exact IP 192.100.56.10

the second will match any interfaces with an IP in the range of 192.100.56.9 to 192.100.56.14

the third will match any ip in the range of 192.100.56.1 to 192.100.56.254

see if you can do the fourth

hope this helps

I'd agree with kryolla, if I had to pick one answer, it'd be:
network 192.100.56.10 0.0.0.0 area 0
because it's the only answer that encompasses JUST that interface.

jason_lunde

Right...the keyword being "alone". It would have to be the first statement.

BennyLava

accely wrote: »

I'd agree with kryolla, if I had to pick one answer, it'd be:
network 192.100.56.10 0.0.0.0 area 0
because it's the only answer that encompasses JUST that interface.

That's not true, all 4 network statements cover either the entire range of addresses in the subnet or a small part of it. For it to match a 2nd interface, that interface would have to use overlapping address space which IOS won't allow.

jason_lunde

BennyLava,
The question states
"This interface alone is to be included in the OSPF process. State the command that would start the process on this interface."

You could have 172.16.56.10/29 on one interface (172.16.56.9 - .14)
and 172.16.56.17/29 that would both be encompassed by a statement of
network 172.16.56.0.0 0.0.0.255 area 0;
yet they are on different interfaces. So the best answer to add only that interface is the first one: network 172.16.56.10 0.0.0.0 area 0

But you are right in looking at the answers given...they will all represent overlapping addresses to a device. I guess this is a case of "best answer" for the question given.

BennyLava

Your example is not the same as the one in the question since the wildcard in your example includes a much larger range than the interface mask.

networker050184

With the wording of the question I'd go with the first answer also.

Forsaken_GA

It's just a tricky question that's designed to make you overthink it. Since they give you no other information, you just have to select the best answer based on the parameters in the question.

mella060

Yes the answer would have to be the first once since you are required to match only that interface.

All the other answers are possible but they could match more then one interface. If you know your wildcard masks then you would know straight away that 0.0.0.0 will match only that interface and should be the correct answer.

What does the book say ? The first one ?

minds-eye

Cheers for the replies..

The book states exactly what i wrote originally... 4 answers

Not surprising as I have found more than 1 error in the book before this.



Given what most people have a agreed, ill have to go with answer 1



But this answer is also correct because it covers exactly the range applied to the interface + based on the fact IOS will not allow 2 interfaces to exist with the same subnet range, it will not include any other interfaces.

network 192.100.56.0 0.0.7.255 area 0

minds-eye

BennyLava what do you mean by this ??

The wildcard mask in the network statement has nothing to do with the subnet mask configured on an interface and does not have to be the 'inverse' of it.

[FONT=&quot]192.100.56.10/21[/FONT][FONT=&quot] [/FONT]covers range 192.100.56.0 to 192.100.63.255

[FONT=&quot]192.100.56.0 0.0.7.255 [/FONT]also covers range 192.100.56.0 to 192.100.63.255

????

networker050184

minds-eye wrote: »

BennyLava what do you mean by this ??




[FONT=&quot]192.100.56.10/21[/FONT]covers range 192.100.56.0 to 192.100.63.255

[FONT=&quot]192.100.56.0 0.0.7.255 [/FONT]also covers range 192.100.56.0 to 192.100.63.255

????

What he is getting at is that the network statements only purpose is to include interfaces in the routing process. The network statement has no influence over what network actually gets advertised for that interface.

ITdude

networker050184 wrote: »

With the wording of the question I'd go with the first answer also.

Ditto here....