Understanding "area range" in ospf

[Kevin Melton]

I understand that an ABR will use a range of networks with a summary statement. I am unclear at this point how to do the summarization to come out with the right decimal number based on the bit patterns...

Please help...

thanks

[Gilles Dufour]

not sure what you have problem to understand.

You jsut need to configure the summary address you want to be advertise.

So, you decide which decimal you want to put there.

If you want to make sure you're advertising the right thing, give us the network addresses you want to summarize.

[Kevin Melton]

I guess I am not quite sure how to perform summarization. Lets use the example as follows:

Lets say I have networks 192.168.1.0/24 thru 192.168.7.0/24. I want to advertise these routes...

[Gilles Dufour]

That's a problem of bits and masks.

You can only group 2 ** n (1,2,4,8,16,...)addresses

So, x.x.1.0 to x.x.7.0 is 7 addresses. Not enough for 8 and too much for 4.

If you want to match 192.168.0.0/24 to 192.168.7.0/24 the command will be

area 1 range 192.168.0.0 255.255.248.0

TIPS: 248 = 256 - 8 (number of addresses in the pool)

if you want to match 192.168.4.0 to 192.168.7.0 the command is :

area 1 range 192.168.4.0 255.255.252.0

So, for the addresses x.x.1.0 to x.x.7.0 there is no match.

You have to advertise more or less.