OSPF CONFIGURATION

[microcisconet]

I expect interface configuration with the following address mask

152.1.10.1/24

152.1.1.1/30

to be configured under OSPF process as

network 152.1.1.0 0.0.0.255 area 0

network 152.1.1.0 0.0.0.252 area 0

BUT i saw it differently in a book as

network 152.1.1.0 0.0.0.3 area 0

network 152.1.10.0 0.0.0.16 area 0

Can someone therefore please help to resolve my confusion on this subject, considering the mask /24 and 30 given in the IP addresses respectfully.

Thanks wishing you all a blissful year

[keeleym]

Hi Rick

Once again I would like to say a big thank you for the clarification. As I read you post I remembered back to my CCNA studies where I was taught that the network statement under an IGP does indeed designate which interfaces will participate in the routing protocol.

My error I suppose comes from the fact that the material I studied always used the same mask as was applied to the interface and never mentioned that you did not have to use the same mask as the interface. I suppose that may have been implied, however it is not something that I picked up on, though I do see now how this is the case.

Again thanks a million for the clarification, it helps enormously.

Best Regards & Best Wishes for 2008 and beyond,

Michael

[Richard Burts]

Michael

First let me say that I did not feel that your first post was coming across as quite arrogant. I had noted most of the issues that you raised in the previous parts of the thread. I feel that you raised some issues and supported them with output that shows that you had researched them in your lab. I feel that this is a very legitimate approach. And your points were not personal - you did not comment on the people or cast aspersion on the people but raised questions about some things that had been said. I believe that this is a very positive way to make contributions. I hope that the others feel the same way that I do.

In their defense I believe that several of the early contributions to this thread were focused on the "big picture" aspect of the question and were a bit careless about some details. I have sometimes done this and been careless about some parts of a contribution because I was focused on some other aspect of it.

I commented (and I do believe) that this point of the useage of the network statement is misunderstood by many people. And much of the course material available, and many of the examples are not helpful in understanding the subtlties of the network statement. They do freequently show a configuration in which the mask in the network statement is the inverse of the subnet mask. At one point I helped to develop a course in the configuration of OSPF. We spent quite a while in the course explaining the various options in using the network statement and mask - for example that the network statements:

network 152.1.1.1 0.0.0.3 area 0

network 152.1.0.0 0.0.255.255 area 0

and

network 0.0.0.0 255.255.255.0

might all produce the same result of including the interfaces into the protocol and not influence the subnets and masks that would be advertised. If you understand the ways in which these 3 statements are similar and the ways in which they are different, then you will have achieved a good understanding of the network statement and its mask.

If we are going to effectively use OSPF then I believe that we should understand this usage.

HTH

Rick

[keeleym]

Hi Rick

Thanks for the comments and information. When I read back through my initial post I felt that I had not worded it inappropriately. I wrote statemetns like "This is incorrect" when what I should have wrote was that "I think this is incorrect". I just felt that it could be construed as an attack on a previous post, which is not what was intended. I will make an effort to be more careful with my future postings.

Can I just ask if my understanding of the 3 network statemetns you provided is correct please?

1. network 152.1.1.0 0.0.0.3 area 0 -- Place the interface on which this subnet is configured into OSPF area 0.

2. network 152.1.0.0 0.0.255.255 area 0 -- Place any interface which is configured with an address 152.1.x.x into OSPF area 0

3. network 0.0.0.0 255.255.255.0 -- Place all interfaces into OSPF

The third one is the one I am least sure about. Should there not be an "area x" at the end and should the last octet not be "255" also as in my lab this gives an "Incomplete command" error without the "area 0" and an "Invalid address/mask combination" error with the "area 0".

===== From a Lab Router ========

R_ACCESS_1(config)#router ospf 1

R_ACCESS_1(config-router)#network 0.0.0.0 255.255.255.0

% Incomplete command.

R_ACCESS_1(config-router)#network 0.0.0.0 255.255.255.0 area 0

OSPF: Invalid address/mask combination

R_ACCESS_1(config-router)#

=============================================

I also tried "network 0.0.0.0 0.255.255.255.255 area 0" and received the same Invalid address/mask combination message

====== From a Lab router ========

R_ACCESS_1(config-router)#network 0.0.0.0 0.255.255.255 area 0

OSPF: Invalid address/mask combination

=========================================

But I could enter "network 0.0.0.0 255.255.255.255 area 0" with no issues.

======== From a Lab router =======

R_ACCESS_1(config-router)#network 0.0.0.0 255.255.255.255 area 0

R_ACCESS_1(config-router)#

=============================================

Best Regards,

Michael

[Richard Burts]

Michael

Your comments about phrasing are very perceptive and appropriate.

Your understanding of the functions of the three network statements is correct. And your questioning of my third network statement is correct on both aspects: yes the mask should have been 255.255.255.255 and the area 0 should have been in the statement. Having just engaged in comments about inaccurate things in posts I am disappointed that I did not proof read my post better. Thanks for catching it. You deserve the rating that I gave it. I look forward to seeing more of your activity in the forum.

HTH

Rick