10-01-2008 05:17 PM - edited 03-03-2019 11:45 PM
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
Solved! Go to Solution.
10-01-2008 07:23 PM
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
10-01-2008 07:01 PM
/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
10-01-2008 07:23 PM
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
10-01-2008 07:28 PM
Rick
Really glad we both came up with the same answer :-).
This seems to be a common issue at the moment !
Jon
10-01-2008 07:44 PM
Jon
Yes indeed
HTH
Rick
10-02-2008 12:34 AM
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
10-02-2008 07:05 AM
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
10-01-2008 07:27 PM
"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 -
Jon
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