Hi Rene,
I found this issue as i passed through the forum but it is not clear to me, that OSPF loop prevention mechanism is based on its distance vector feature while LSA is moving from one area to another. Is there very nice explantion regarding ospf loop prevention
Hello Richard
Take a look at this NetworkLessons note on OSPF loop prevention. If you have any further more specific questions, feel free to let us know!!
I hope this has been helpful!
Laz
Hi Rene,
I am a student. In the OSPF Area discussion, you mentioned area can never communicate with each other other than through the Backbone. What about this command (area 1 virtual-link) wouldnāt that allow you connect to other area without passing through the backbone? Please help me understand this command.
Thank you.
Hello Watson
You are absolutely correct. It is possible to have an OSPF topology such that a non-backbone area is not directly connected to the backbone using a virtual link. However, you must understand what a virtual link does.
In essence, it creates a tunnel between the non-backbone area and Area 0. The result is the non-backbone area acquires a direct link to the backbone area through that tunnel, thus fulfilling the requirement that Rene states in the lesson.
For more information about the virtual link and how it works, take a look at this lesson:
I hope this has been helpful!
Laz
Thatās correct.
Thank you.
thank you so much Rene and by the way happy new year
Hi Rene,
That was a great lesson and well explained. Just a some to clarify something that is not 100% clear to meā¦is this understanding of mine correct?
**
LSAs are generated for each participating Router interface in OSPF, and each LSA from a Router is sent to every other Router in the Area, not just the adjacent Router.
But with DR and BDR, every non-DR and non-BDR Router will only send LSAs to these 2 to form the adjacency/relationship.
**
Thanks.
Hello Rifan
Just one clarification. LSAs are sent either:
More details about LSAs and to which routers they are sent can be found in this lesson:
I hope this has been helpful!
Laz
hi a question i have the follow configuration
router ospf 1
area 1 range 10.2.0.0 255.255.0.0
this command its similar to
router ospf 1
network 10.2.0.0 255.255.0.0
?
Thanks
Hello Ugo
These commands are not the same. They do very different things.
The config that uses the network command simply tells the router to allow all networks configured on local interfaces of the router that fall within the 10.2.0.0/16 range to participate in OSPF. In other words, if an interfaceās IP address is within 10.2.0.0/16, then the subnet of that interface is advertised using OSPF.
The area range command can only be applied at an ABR and it is used to perform route summarization. It takes all learned prefixes from within area 1 that fall within the 10.2.0.0/16 range, and summarizes them before advertising them to other areas. This way you can consolidate multiple OSPF LSAs into a single summarized LSA making OSPF more efficient. More about the area range command can be found at this lesson:
You can also learn more about it at this Cisco command reference:
I hope this has been helpful!
Laz
how do you know when to make another area? so how do you choose when thereās too many routers in one area or if its a good idea to create an extra area?
Hello Peter.
Thatās an excellent question. One reason you would want to create a new area in OSPF is if you have too many routes being exchanged in area 0 (on the order of hundreds or even thousands) and your routing tables and your OSPF databases are getting too large. How large is too large? Well, if they begin slowing down your routers and you see a lot of memory usage and slow reconvergence times that are affecting the performance of OSPF on your network, then you know that itās time to separate the network into multiple areas. Indeed, you should do this well before you perceive any impact on the functionality of your network.
You can determine the use of resources using the following two commands:
show process cpu - This will show how much CPU usage each process is taking. Unfortunately, it might not list OSPF directly, but it can give you some idea.
show process memory- This will give you an overview of the memory usage in the router. Again, it might not break down usage by process.
But there are other reasons to separate your network into areas, and if you follow these, it is unlikely that you will reach the threshold of OSPF resource usage. Typically, the architecture of a network naturally separates it into large sections, and you can typically assign a different area to each section.
For example, a medium-sized enterprise will have a large headquarters, several regional locations, and a series of branch offices. If the networks of those locations are interconnecting using MPLS or some other technology, it makes sense for each physical location to be assigned its own area, and the headquarters can be assigned the backbone, which is also the largest area. This way, you minimize OSPF update sizes being sent over the WAN.
For larger enterprises, it may even be beneficial to separate a particular campus into several areas, where each building is an OSPF area. What you ultimately choose to do depends much more on the hierarchical structure of the sections of a network rather than just the number of routers within an area.
Just for argumentās sake, how many routers are too many? A general rule of thumb for OSPF is 50 routers in an area, but this is not a hard and fast rule. It depends on many more factors such as the number of routes, the CPU and memory power of the routers used, as well as the IP addressing scheme being used. But again, separating your areas according to the natural network design you are using is your best bet to ensure you donāt run into ātoo manyā routers or routes being exchanged in a single area.
I hope this has been helpful!
Laz
Hello!
Could someone please explain the concept of the Forwarding Address to me? Iāve seen some explanations and examples but those were slightly odd and I didnāt fully understand why exactly do we need the forwarding address.
Thank you.
David
Hello David
Take a look at this NetworkLessons note on the topic of OSPF Forwarding Address. It includes some links that may be helpful too. If you have any further questions please let us know!
I hope this has been helpful!
Laz
Great from great
I think if there is post that talking about routing types like IGP and EGP at first of routing section to have great idea about routing because i didnāt see this ???
And i have seen in inter vlan you have talked about routing configuration rip and you are in L2 section !!! , this will make confusing for many people or it best to mention( later in routing section will understand more )
routing topics in switching topics will be big confusing
Hello Barakat
Thanks for pointing that out, Iāll let Rene know to consider it. In the meantime, take a look at the Introduction to Routers and Routing lesson, as well as this brief NetworkLessons note on the difference between EGPs and IGPs.
Hmm, Iām not sure where you found this. Itās not in the āIntroduction to OSPF lessonā forum thread in which you posted. Can you share the lesson and the section where you see this so we can examine it further? Thanks!
I hope this has been helpful!
Laz
Hello Mr Lagapides
Thanks for updating , in fact i have already seen this note , but what i did mean , it was best to be with separate subject at first and top of routing in ccna course as you know for me its ok , but i am talking about perosn & its first time for him in ccna journey it will be hard on him , because first subject is static route directly
About the interVLAN , sorry it was mistake from my side in sections , the post link
you was talking about ospf configuration in interVLAN , and the ospf will be in next section so as i think this lead to big confusion for readers especially if they were with first time
Its only notes to make our zone networklessons.com is best from all 
Hello Barakat
Ah I see, I understand now. Thanks so much for your feedback! Iāll have Rene look into it and consider making any changes that will help simplify the content and make it clearer for newcomers. Thanks again!
Laz