Hi Rene,
Can a router be in more than 2 areas at a time ?
I know that an ABR can be in area0 and some other area, but can it have 1 interface in area 0 and the other 2 interfaces in area 1 and 2 ?
Hi Mansi,
William V asked this exact question, so please see my answer two responses above --^ (Short answer: Yes).
Excellent method of teaching! Read then watch the video - really helped to reinforce the topic for me.
Keep up the great work!
Rgds,
Shannon
Thanks Shannon!
Hi Rene,
Is there any specific criteria for grouping routes into an area?. Or it is just user configuring thing.
Hi Shanmugasiva,
It really depends on the network design. One advantage of using different areas is that you can use summarization, which results in smaller routing tables and less updates when there are changes in the network.
Rene
Hi,
Each area there will be DR/BDR
?
Thanks
do I need more than one area if I have only small networks with less than 10 routers and firewall?
Hello Hoan.
There is no clear cut yes and no answer to your question. It depends on your topology, the amount of traffic on the network and the type of links you have. As a general guideline, with 10 routers and a firewall, OSPF should function satisfactorily. However, it is always a good idea to check and see what kind of convergence times you are getting and how much OSPF traffic is being generated. Dividing such a network into two or three areas will always make OSPF function more efficiently (if done correctly) but it should be done only if OSPF functionality is sufficiently impaired. Otherwise the benefit you will have is not worth the effort to change it.
I hope this has been helpful!
Laz
Hello Sims.
Will each area have its own DR/BDR? Well, this depends on the topology. A DR and BDR are elected only if there is a multi-access broadcast network. What is that? Well, if you connect several routers to a switch and the interfaces of the routers on that switch are on the same VLAN/subnet, then you have a multi-access broadcast network. Conversely, if your network is composed of point-to-point links between routers, then DRs and BDRs will not be elected.
To return to the what I believe to be the underlying question that you have, if we assume that your whole network is a multi-access broadcast network and you separate that network into OSPF areas, then yes, each area will have its own DR and BDR.
I hope this has been helpful!
Laz
Hi Rene,
I need your help to understand couple of things about ospf:
1)The specific purpose of area 0, why should it be the backbone area that all other areas should be connected to?
2)Can you explain what is the loop prevention mechanism in ospf and how it works?
Thanks you!
Sravanthi,
These two questions are related, actually. The reason for Area 0 has to do with loop prevention of inter-area routing.
To understand loop prevention, you need to understand that OSPF operates in two distinct “modes.” As you probably know, OSPF is known as a Link-State routing protocol. This is true for intra-area routes–routes within the same Area. The loop prevention here is due to the nature of link state protocols. Each node has completely knowledge of all links within its area, and all nodes have the same knowledge. Therefore, loops will not happen within an area.
Loop prevention between areas is much more complex, and this is why there is the requirement of having Area 0 be the backbone that must be directly adjacent to all other areas. In this “mode” OSPF acts like a Distance-Vector protocol in that a node in one area does NOT have complete knowledge of what is going on within another area. Instead, it relies on information being told to it by a neighbor (in this case, an ABR). In this respect, OSPF exhibits distance-vector like behavior.
Thanks Andy! It’s quite a document to read, good resource though 
Hi Rene,
You have very good way of teaching,
will ask few questions once I finish this reading.
Glod bless you.
Thanks,
Rohitendu
Hi Rene,
very good way of teaching the lesson.
i am also looking for layer 2/ layer 3 orchestration lessons plz.
could you plz forward the link for the same?
also Network activation is one area i am interested as well.
how can this forum help me learn that any pointers plz!
Regards
Prem
Dear Rene/Andrew/Lazaros,
Needed a small clarification, when it comes to forming adjacency I think the Network and Subnet mask also need to match or the adjacency wont form. As an experiment I chose 192.168.1.0 for the network on both routers and on one side I used a mask of 255.255.0.0 instead of /24 and the adjacency did not come up.
Hi Srikanth,
You are correct, the subnet mask is one of many variables that has to match in order for an OSPF adjacency to form. I cam up with a little trick to remember them. Are you familiar with the word “shaman”? It means a kind of medicine man / witch doctor. Anyway, with OSPF, just remember “shaman” for what has to match:
Stub flag
Hello and dead timers
Area
Maximum Transmission Unit (MTU)
Authentication settings
Network Mask
Many thanks Mr Andrew.
Great help!!!