OSPF Advertisement Again

[newnetman]

Hello to All my invisible mentors,

I'm here again with OSPF.

This IP ADD 10.1.1.6/30 was advertised as 10.1.1.4 0.0.0.3 area x,

i have no problem with the wildcard mask but would like to know why/how 6 in the network bit turned to 4 when advertised.

Secondly, these IP ADD

10.1.1.2/30 and 10.1.1.5/30 was advertised as

10.1.1.0 0.0.0.3 area x

10.1.1.4 0.0.0.3 area x

Why can't these two addresses be summed up? i.e 10.1.1.0 0.0.0.7 area x ?

OR EVEN THIS WAY

10.1.1.5 0.0.0.0 area 0

and

10.1.1.2 0.0.0.0 area 0

please help me little understanding

thanks

[Marwan ALshawi]

/30 mean this network include only to IP hosts IP address and the 3 in wildcard is becuase 1 is network address and the other two is asigneable address

in ur example

10.1.1.0/30 hosts will be .1 and .2

this is called VLSM now

10.1.1.0/24 is a super address within this address u cam divide it to more spicific networks and smaller number of hosts

this is the idea

[Richard Burts · Accepted solution]

I believe that the original post has some confusion about the relationship between what is entered in the ospf network statement and what is advertised (and I would observe that many people are confused about that). So let me explain it this way:

The network statement does not tell OSPF what to advertise. But the network statement tells OSPF what interfaces to include. And when OSPF has included the appropriate interfaces then OSPF decides based on the interface information what to advertise.

So what that means in slightly more concrete terms:

the OSPF network statement specifies an address and a mask. OSPF uses the address and the mask and compares every interface on the router. Any interface that falls into the range defined by the address and the mask will be included into OSPF. And the subnets of the included interfaces are what OSPF will advertise.

So if 10.1.1.6 is an interface address and it matches the network statement then it will be included into OSPF. Then OSPF looks at the interface configuration and determines what subnet the interface represents and that is what is advertised. So if 10.1.1.6 is in subnet 10.1.1.4, then 10.1.1.4 is what is advertised.

As to the second part of the question:

Why can't these two addresses be summed up? i.e 10.1.1.0 0.0.0.7 area x ?

OR EVEN THIS WAY

10.1.1.5 0.0.0.0 area 0

and

10.1.1.2 0.0.0.0 area 0

the answer is that they can. Any of the following network statements would have the effect of including the two interfaces and of advertising the two subnets:

network 10.1.1.2 0.0.0.0 area 0

network 10.1.1.5 0.0.0.0 area 0

or

network 10.1.1.0 0.0.0.3 area 0

network 10.1.1.4 0.0.0.3 area 0

or

network 10.1.1.0 0.0.0.7 area 0

or

network 10.1.1.0 0.0.0.255 area 0

or

network 10.0.0.0 0.255.255.255 area 0

HTH

Rick

[Jon Marshall]

Rick

Really glad we both came up with the same answer :-).

This seems to be a common issue at the moment !

Jon

[Richard Burts]

Jon

Yes indeed

HTH

Rick

[newnetman]

Hello Rick,

You have always been a blessing to many on this forum, thank God i am no exception.

Many thanks for such a precious time to resolve concerns.

Rregard,

Olu

[Richard Burts]

Olu

I am glad that my explanation was able to resolve your concern. Thank you for the compliment. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there were answers which did resovle the question.

I am glad that you are a participant in the forum and hope that you will continue your participation.

HTH

Rick

[Jon Marshall]

"i have no problem with the wildcard mask but would like to know why/how 6 in the network bit turned to 4 when advertised."

Because OSPF advertises subnets and the subnet for 10.1.1.6 is 10.1.1.4/30.

"Why can't these two addresses be summed up? i.e 10.1.1.0 0.0.0.7 area x ?"

They can if you want to do this. It is up to you.

"OR EVEN THIS WAY

10.1.1.5 0.0.0.0 area 0

and

10.1.1.2 0.0.0.0 area 0"

And yes you can do it this way as well.

It is all about which interfaces you want OSPF to run on. We recently had a thread on this -

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&topicID=.ee71a06&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cc1e1a6/6#selected_message

Jon