OSPF Stub Area

Christopher Dobkowski

Hello to everyone!

I have one big dilemma about OSPF, the last thing I need to understand about OSPF for CCNA

Namely, Stub Area

When is it a Stub Area and when not?

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Comments

networker050184

It's a stub area when you configure it as such. Do you understand what a stub area is and it's implications on the LSA types advertised within that area?

Christopher Dobkowski

networker050184 wrote: »

It's a stub area when you configure it as such. Do you understand what a stub area is and it's implications on the LSA types advertised within that area?

Not exactly, well I know that stub means that a router uses one link for up/down and there are 11 LSAs i guess... But what's the deal here? I mean ok, I know how to set up a stub area:

Router(config-router)# area 51 stub

what does stub area actually mean, and why and when should I use stub area? and what does it have to do with LSA types?

I'm asking just because knowing to set up some features does not make you CCNA, But i want to really know what each thing means... and the only thing about OSPF that I am supposed to know for CCNA and I don't fully catch it is the Stub Area

veritas_libertas

Maybe this will help you: OSPF area types - Packet Life

networker050184

Sounds like you need to do some research. A quick run down though is that a stub area blocks type 5 LSAs (and therefore type 4) and use a type 3 LSA for a default route to external destinations.

Zartanasaurus

You aren't going to understand stub areas until you understand areas and the 6 major LSA types (1-5,7). Do some review on that and come back to stubs.

atorven

I thought for CCNA you only had to know single area OSPF?

Reck_

Yes sir, but our friend here has chosen to break the objective coverage ,which I see is a good sign of interest in learning.in my CCNA prep, a lot of question about OSPF and find my self reading a CCNP OSPF related document .In turn, some question were answered and cleared out the clouds.However, reaching beyond the objective will add overhead in your time study that you might consider as well.

Christopher Dobkowski

Zartanasaurus wrote: »

You aren't going to understand stub areas until you understand areas and the 6 major LSA types (1-5,7). Do some review on that and come back to stubs.

Thanks a lot !!!!!

It won't hurt to know some CCNP stuff as well :P

veritas_libertas

Christopher Dobkowski wrote: »

Thanks a lot !!!!! It won't hurt to know some CCNP stuff as well :P

You've already moved out of the CCNA map, so you might as well start on CCNP material

Christopher Dobkowski

veritas_libertas wrote: »

You've already moved out of the CCNA map, so you might as well start on CCNP material

Oh stop it, you

haha

Thanks to every one champs

MrBrian

If you're aiming to pass your CCNA soon I'd definitely recommend reading CCNP level stuff for the topics such as OSPF. But like the another poster said, it may add overhead to your current studies, so while it's always great to read and learn more, it might end up adding more to the fire for now. And there's quite a bit to OSPF at the NP level..

Just know that OSPF passes around lsa's, which are little network descriptors.. If a router has 3 interfaces involved in the OSPF process, it will create its own Type 1 lsa describing the details of these links. In fact, each OSPF router will create a Type 1 lsa to describe itself. Then, when OSPF routers form adjacencies, they will give each other their lsa's, kind of like they're profile card. And remember, one of the things that must match for two OSPF routers to become neighbors, is the stub flag. This is in the Hello packet that OSPF sends.. So if one router's a stub, but the other isn't, they won't form a neighborship because that parameter isn't the same..

So what's with all the other lsa types? Well, I'll give you a high level overview because explaining these could go on forever lol. So with OSPF we can put routers into separate areas right? Well inside each area Type 1 lsa's and Type 2 lsa's will be present for the routers in that area only. These describe in detail the networks of the routers only in that area! These Type 1 and 2 lsa's will not be sent by an ABR (Area Border Router= a router attached to two different areas, hence bordering them) into another area.

Instead, the ABR will create Type 3 lsa's for all the networks it has, and then send those into the other area. Now those routers can reach the networks in the other area. But why doesn't the ABR just send the same Type 1 and 2 lsa's from the original area? Well, this is because the OSPF SPF (algorithm it uses to determine best path) is only run on Type 1 and Type 2 lsa's. So the more routers you have in one single area, the more Type 1 and 2 lsa's you'll have, and a larger burden the processor to calculate best paths each time something changes. This is a reason to break up an OSPF area into multiple areas. The other areas will have the routes now, because of the Type 3 "summary" lsa's that are created by the ABR.

The ABR is like the representative for all the networks in one area. Say there's an ABR between area 0 and area 25.. and that there's 10 networks in area 0.. in area 0 there will be Type 1 and Type 2 lsa's to describe these in detail. And then the ABR will represent these networks for the routers in area 25. It will send in Type 3 summary lsa's which are much smaller. They basically just say, to get to x.x.x.x network, come to me, the ABR. And since the ABR has Type 1 and 2 lsa's, it will know exactly how to reach them the best.

Wow, sorry to go on and on! I'm not even to Type 5 lsa's yet.. jeez. I don't wanna type much longer right now tho lol. But I'll add, that if we redistribute routes into an area, they'll come in as Type 5 "external" lsa's.. and then we can set areas to be Stubby or Totally Stubby. For a Stub area, the ABR won't flood in all these Type 5 lsa's.. And for a Totally Stubby area, the ABR won't flood in Type 5 OR Type 3 lsa's. This will keep the routing tables smaller for routers in the Stub area. Hope the ramblings helped somewhat! Ask me more if you have any questions!

xXErebuS

This and stub areas is a difficult concept to grasp even as a CCNP to fully understand. For your CCNA studies I would just focus on knowing that they exist and think of them as an area that doesn't know all the routes as a regular area does or if as concised as a regular area. There are sub categories of areas as MRBrian stated which can be confusing at first.

networker050184

Why is grasping LSA and area types difficult for a CCNA much less a CCNP? I'd expect someone at the CCNA level to at least know what type 1,2 and 5 are. Stub areas while not covered on the CCNA are certainly not an overly difficult concept.

xXErebuS

networker050184 wrote: »

Why is grasping LSA and area types difficult for a CCNA much less a CCNP? I'd expect someone at the CCNA level to at least know what type 1,2 and 5 are. Stub areas while not covered on the CCNA are certainly not an overly difficult concept.

I wouldn't call it overly difficult but it is one of those things that I highly doubt most people can spit off what LSA's are found in each stub area and when/why you would find a LSA type 4 in an environment without looking it up unless they work in an OSPF environment everyday. It's not something covered on the CCNA studies and you won't find it in the CCNA; but you will find questions digging into LSA types in the CCNP. It may not have been difficult for you but when I took my CCNP getting the LSA types down pat were a much more difficult task than several other CCNP topics.

IMO I don't expect CCNA / junior engineers to know what LSA types are... If I told them to set it up as a normal NSSA and it was not getting a default route out - that would be my fault, not theirs.... once again IMO.

networker050184

I'd expect anyone that claims to know anything about OSPF to know what type 1,2 and 5 are. Its the basic building blocks of the protocol. They don't need to know every field in the packet but at least know that every router creates a type 1 with its links, type 2s are created by the DR and type 5 are for external routes. Simple enough for even the greenest of networkers to keep track of.

I'd say that falls into "Configure, verify, and troubleshoot OSPF' in the CCNA objectives.

xXErebuS

networker050184 wrote: »

I'd expect anyone that claims to know anything about OSPF to know what type 1,2 and 5 are. Its the basic building blocks of the protocol. They don't need to know every field in the packet but at least know that every router creates a type 1 with its links, type 2s are created by the DR and type 5 are for external routes. Simple enough for even the greenest of networkers to keep track of.

I'd say that falls into "Configure, verify, and troubleshoot OSPF' in the CCNA objectives.

You can understand what 1,2,5 LSA types are without touching a single subject on stub areas, every area has 1&2, and 5 is only found in normal areas....

Zartanasaurus

I wouldn't say they are difficult, per se, but it's more in-depth than what you have to learn for RIP or EIGRP. If you want to understand stub areas, these are the questions you have to research and answer.

What is an OSPF area? How do areas affect what is shown in the routing table?
What are the OSPF router types? What makes a router one type vs the other?
For each LSA type, what router type generates them? What do they point to? What is the scope of the LSA?

Once you understand those three prongs, understanding stub areas is a lot easier.

veritas_libertas

networker050184 wrote: »

I'd expect anyone that claims to know anything about OSPF to know what type 1,2 and 5 are. Its the basic building blocks of the protocol. They don't need to know every field in the packet but at least know that every router creates a type 1 with its links, type 2s are created by the DR and type 5 are for external routes. Simple enough for even the greenest of networkers to keep track of.

I'd say that falls into "Configure, verify, and troubleshoot OSPF' in the CCNA objectives.

I can understand you feeling that way, but the types are not even covered in the Official Cisco CCNA book. Your comment made me go back through my books just to make sure, since I felt confident that I hadn't seen it in my Lammle or Odom CCNA books.

networker050184

xXErebuS wrote: »

You can understand what 1,2,5 LSA types are without touching a single subject on stub areas, every area has 1&2, and 5 is only found in normal areas....

I understand that, my response as to this.

xXErebuS wrote: »

IMO I don't expect CCNA / junior engineers to know what LSA types are...

Reck_

For those who would like to cross the barrier, I highly recommend reading Troubleshooting IP Routing Protocols. Contents from this book may also clear the "cloud" you might also be having.

Good luck.

Christopher Dobkowski

I believe I understood the overall concept with the LSAs and Stub Area

Not even finished with CCNA and already tasted CCNP

I love many of these concepts

Only thing left to start my recap and intense labs is Switching/ACL/Frame Relay..

Yesterday I was passing by my fathers room, I saw one of those 36 CCIE etc. books and I was like... really? One of your CCIE books is in size of my whole CCNA book, where you have about 23 books for CCIE

Where do you store all this information?!