Introduction to OSPF Stub Areas

Ahaa, so thats the thing here. People are refering to non-backbone area, that is not a stub or nssa as Standard area, good to know. I was learning OSPF from materials that did not used “standard” in terminology at all. Thanks for pointing this out.

1 Like

Thanks Laz. this question is asked in interview.
and can u please provide details on ospf multicast and broadcast address.

Regards,
Chandra

Hello Chandra

OSPF uses two multicast addresses, specifically, 224.0.0.5 for all OSPF routers, and 224.0.0.6 for all Designated routers. These addresses are used to exchange hello packets and LSAs. Which address is used for which purpose depends on the network topology and the implementation of OSPF. Take a look at this post for more info.

I hope this has been helpful!

Laz

Hello

During cisco test I’ve had a question below.
" Which OSPF areas prevent LSA type 4, LSA type 5? ( choose two)"
Possible answers:

  • NSSA
  • Total Stubby area
  • Stubby area
  • NSSA Totally Stub

I think the answers are Total Stubby and Stubby because in these areas no ASBR allowed.
But according the Cisco guide the 2nd answer should be NSSA Totally Stub.
I’m confusing a bit. Anybody clatify me?

Thanks

Hello Boris,
your mentor is trying to be very sneaky with this question. All the answers are blocking LSA Type 4+5, but the area types with “Totally” in name are blocking LSA Type 3 on top of LSA Type 4+5. Thus right answer from my point of view is:

  • NSSA area
  • Stubby area
2 Likes

Hello Boris

The question is not completely clear, as it does not clarify what it means by “prevent type 4 and type 5”. Does it mean that those LSA types should not exist within the stub area? If so, then all of them should be correct. Take a look at the following table:

image

However, if we take the question to mean “prevent ONLY type 4 and type 5 LSAs and not Types 1, 2 and 3”, then the answer would be NSSA area and Stubby area as @fugazz suggested. The important thing here is to understand the functionality of the LSAs and apply that to any question that may arise in the exam.

I hope this has been helpful!

Laz

2 Likes

Hello Michal,
Thank you very much!

Hello Laz,
Thanks a lot for clear explanation. The table is very helpful for me.

hi there,
I am learning routing at the moment but I have not gone through the dynamic routing protocols sections yet.
could someone tell me what is a stub router?
why default routes cant be configured at a stub router.

Hello Vigneshwarv

The term “stub” is most often used with the OSPF routing protocol. It is used not to refer to a router, but to a non-backbone OSPF area that has only one ABR that connects it to the backbone area. For more info about this, take a look at the following lesson:

When referring to the EIGRP routing protocol, the term stub is applied to a single router. In this case, a stub router is one that has only a single route to “the rest of the network”. You can find out more about that at the following lesson:

Default routes can be configured on a stub router, and it is best practice to do so. In both the cases of a stub area and a stub router, a single default route should be used in order to simplify the routing table. In order to more fully understand this concept, I believe it would be best if you went through the lessons for these routing protocols so that you can more clearly understand the implications of the term stub and how it is applied in each case.

I hope this has been helpful!

Laz

How to filter the external routes inside a default OSPF area “not a stub area”? I have a default area (area 225) and I configure (filter list) in the ABR to filter other areas routes. but, the external routes from other areas still exist in area 225. I can configure (distribute list) in all the routers inside area 225 to filter the external routes, but is it possible to configure something in area 225’s ABR to filter the external routes? for example, the route-map in the ABR’s interface that facing area 225 can filter the external routes?

Thanks in advance,

Hello Mohanad

There are a few things to consider when filtering routes. There’s a difference between filtering LSAs, and filtering routes. Filtering LSAs will block the actual transmission of the LSAs, which means the prefixes they contain will not be found in the OSPF database. However, this can only take place at an ABR, and LSAs can be filtered only between areas. Alternatively, you can prevent specific prefixes from entering the routing table by using distribute lists. The prefixes remain in the OSPF database, and are advertised, but are not placed in the routing table of the specific router.

If you want to filter external routes (Type 5 LSAs) from one area to another, use LSA Type 5 filtering, more about which you can learn at the following lesson:


If you simply want to prevent a particular prefix from entering the routing table, you can use distribute lists as described below:

I hope this has been helpful!

Laz

What’s the purpose of stub areas, what are they trying solve. I understand the fact that stub areas blocks type 3 LSA and/or type 5 LSA, but why do we block these LSA’s.

Hello Alpha

In most lab topologies, the scale of the network is quite small, and there is little or no traffic, so you can’t readily appreciate the benefits of features such as OSPF stubs. This feature is used to minimize the number of useless entries within a router that is found within a stub area. Take a look at the following topology:

Imagine that behind R1, instead of only a single prefix of 1.1.1.0/24, there are many more networks with hundreds of prefixes coming from other areas as well as from autonomous systems outside of OSPF. Without the configuration of any stub networks, R3 would have, in its routing table, hundreds of prefixes for all those networks. This means that each packet that R3 sends anywhere will need to be checked against all of these hundreds of entries, resulting in a lot of wasted CPU and memory.

If you were to take a look at R3’s routing table in such a case, you would see that the next hop IP for all of the routes would be 192.168.23.2, which is R2. This is the case because area 1 is a stub area. By definition, a stub area has only one exit from the area.

When you configure a stub area, and block various types of LSAs, what you are doing is eliminating all of these prefixes in R3, with a single default route for all traffic that is directed to 192.168.23.2. You can understand that reducing the size of a routing table from hundreds of prefixes to one single prefix is quite efficient.

This is not evident in a small lab topology, but in an enterprise network, it is quite evident.

I hope this has been helpful!

Laz

It was quite insightful, Thanks.

1 Like

What about totally stub? This area type will block type 5 external LSAs and type 3 summary LSAs. It’s impossible to have an ASBR in the totally stub area since type 5 external LSAs are blocked.

can we see lsa type 3 from totally stub area in backbone area ?

Thanks

Hello Sims

Yes, the backbone area will receive a type 3 LSA from the ABR connected to a totally stub area.

Remember that the various types of stub areas described in this lesson will block some types of LSAs only in the direction from backbone to stub. In the opposite direction, that is towards the backbone area, the LSAs are not blocked in any way, and should not be, otherwise, prefixes in non-backbone areas will never reach the backbone area to be correctly shared and advertised with the rest of the network.

I hope this has been helpful!

Laz

Hi,

Does an OSPF router in a stub/totally stub area will reach loopback address configured on an ASBR router in another area? If yes, how will it reach because ABRs are not relaying type 4 LSA in stub area which is needed for stub router to reach ASBR.

Thanks,
Nihar

Hello Nihar

An OSPF router in any type of stub area will not receive any Type 4 LSAs. Thus, it will never learn of the location of any ASBRs found in any other areas of the OSFP topology. However, an OSPF router in a stub area will have a default route that points to the ABR of the area as the next-hop IP.

This is the point of having a stub area, to avoid having multiple routing entries for various destinations in an OSPF topology (including ASBRs) and having all of these entries consolidated into a single route to the local ABR. For all routes outside of the stub area, including ASBRs, you will have to go through the ABR to get there anyway, so a single default route is created for this.

I hope this has been helpful!

Laz

There are two questions related to this diagram. R1 is in Area0. R2 and R3 and acting as the ASBR and ABR for Area 0 and Area 1. R4 is acting as the ISP provider and its’ routes are being redistributed into R2 and R3 respectively. Area 1 is an NSSA. 2 Questions come from this

  1. How would R1 Reach R4(ISP) network if both R2 and R3 are acting as the ABR and ASBR?

  2. How could you force R1 to prefer the path of R3?