ospf area types - packetlife

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By stretch (/users/stretch/) | Tuesday, June 24, 2008 at 12:31 a.m. UTC

Advancing from last week's discussion on OSPF network types (/blog/2008/jun/19/ospf-network-types/),

today's topic is a source of considerable confusion for many people new to OSPF: area types. Recall that a

large OSPF domain is typically broken into separate areas to restrict the propagation of routes and reduce

the amount of resources required by each router to maintain its link state database. Each area is connected

to a central backbone, area zero.

OSPF relies on several types of Link State Advertisements (LSAs) to communicate link state information

between neighbors. A brief review of the most applicable LSA types:

Type 1 - Represents a router

Type 2 - Represents the pseudonode (designated router) for a multiaccess link

Type 3 - A network link summary (internal route)

Type 4 - Represents an ASBR

Type 5 - A route external to the OSPF domain

Type 7 - Used in stub areas in place of a type 5 LSA

LSA types 1 and 2 are found in all areas, and are never flooded outside of an area. Whether the other types

of LSAs are advertised within an area depends on the area type, and there are many:

Backbone area (area 0)

Standard area

Stub area

Totally stubby area

Not-so-stubby area (NSSA)

Let's begin by examining a standard area. Note that the backbone area is essentially a standard area which

has been designated as the central point to which all other areas connect, so a discussion of standard area

behavior largely applies to the backbone area as well.

OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/

1 of 17 9/10/2014 3:41 PM

In the example above, router 2 acts as the area border router (ABR) between a standard area and the

backbone. R3 is redistributing routes from an external domain, and is therefore designated as an

autonomous system boundary router (ASBR).

As mentioned, type 1 and 2 LSAs are being flooded between routers sharing a common area. This applies

to all area types, as these LSAs are used to build an area's shortest-path tree, and consequently only

relevant to a single area. Type 3 and 5 LSAs, which describe internal and external IP routes, respectively,

are flooded throughout the backbone and all standard areas. External routes are generated by an ASBR,

while internal routes can be generated by any OSPF router.

Note the peculiar case of type 4 LSAs. These LSAs are injected into the backbone by the ABR of an area

which contains an ASBR. This is to ensure all other routers in the OSPF domain can reach the ASBR.

Standard areas work fine and ensure optimal routing since all routers know about all routes. However, there

are often situations when an area has limited access to the rest of the network, and maintaining a full link

state database is unnecessary. Additionally, an area may contain low-end routers incapable of maintaining a

full database for a large OSPF network. Such areas can be configured to block certain LSA types and

become lightweight stub areas.


2 of 17 9/10/2014 3:41 PM

In this next example, R2 and R3 share a common stub area. Instead of propagating external routes (type 5

LSAs) into the area, the ABR injects a type 3 LSA containing a default route into the stub area. This ensures

that routers in the stub area will be able to route traffic to external destinations without having to maintain all

of the individual external routes. Because external routes are not received by the stub area, ABRs also do

not forward type 4 LSAs from other areas into the stub.

For an area to become a stub, all routers belonging to it must be configured to operate as such. Stub routers

and non-stub routers will not form adjacencies.

Router(config‐router)# area 10 stub

This idea of substituting a single default route for many specific routes can be applied to internal routes as

well, which is the case of totally stubby areas.

Like stub areas, totally stubby areas do not receive type 4 or 5 LSAs from their ABRs. However, they also do

not receive type 3 LSAs; all routing out of the area relies on the single default route injected by the ABR.

A stub area is extended to a totally stubby area by configuring all of its ABRs with the no‐summary

parameter:

Router(config‐router)# area 10 stub no‐summary

Stub and totally stubby areas can certainly be convenient to reduce the resource utilization of routers in

portions of the network not requiring full routing knowledge. However, neither type can contain an ASBR, as

type 4 and 5 LSAs are not permitted inside the area. To solve this problem, and in what is arguably the worst

naming decision ever made, Cisco introduced the concept of a not-so-stubby area (NSSA).


3 of 17 9/10/2014 3:41 PM

An NSSA makes use of type 7 LSAs, which are essentially type 5 LSAs in disguise. This allows an ASBR to

advertise external links to an ABR, which converts the type 7 LSAs into type 5 before flooding them to the

rest of the OSPF domain.

An NSSA can function as either a stub or totally stubby area. To designate a normal (stub) NSSA, all routers

in the area must be so configured:

Router(config‐router)# area 10 nssa

Type 3 LSAs will pass into and out of the area. Unlike a normal stub area, the ABR will not inject a default

route into an NSSA unless explicitly configured to do so. As traffic cannot be routed to external destinations

without a default route, you'll probably want to include one by appending default‐information‐originate

(thanks to Adam for pointing this out).

Router(config‐router)# area 10 nssa default‐information‐originate

To expand an NSSA to function as a totally stubby area, eliminating type 3 LSAs, all of its ABRs must be

configured with the no‐summary parameter:

Router(config‐router)# area 10 nssa no‐summary

The ABR of a totally stubby NSSA (or not-so-totally-stubby area, if you prefer) injects a default route without

any further configuration.

Standard areas can contain LSAs of type 1, 2, 3, 4, and 5, and may contain an ASBR. The backbone

is considered a standard area.

Stub areas can contain type 1, 2, and 3 LSAs. A default route is substituted for external routes.

Totally stubby areas can only contain type 1 and 2 LSAs, and a single type 3 LSA. The type 3 LSA

describes a default route, substituted for all external and inter-area routes.

Not-so-stubby areas implement stub or totally stubby functionality yet contain an ASBR. Type 7

LSAs generated by the ASBR are converted to type 5 by ABRs to be flooded to the rest of the OSPF

domain.


4 of 17 9/10/2014 3:41 PM

(/users/stretch/)Jeremy Stretch is a network engineer living in the Raleigh-

Durham, North Carolina area. He is known for his blog and cheat

sheets here at Packet Life. You can reach him by email

(/contact/) or follow him on Twitter (http://twitter.com/packetlife).

Posted in Routing (/blog/category/routing/)

(http://www.amazon.com/gp/prime/signup/videos?tag=packetlnet-20)

Comments

Kevin Gillick (guest)

June 24, 2008 at 12:35 p.m. UTC

Very good job on this documentation!

Ed (guest)

June 24, 2008 at 3:17 p.m. UTC

Best description I've seen.

Matt (guest)

June 25, 2008 at 1:27 a.m. UTC

That was really really good...

Adam (guest)

June 25, 2008 at 2:52 p.m. UTC

Just for clarification in the last diagram.....the NSSA area, (R2) will only inject a default route automatically if

the "no-summary" keyword is added. It will not inject a default route otherwise. Try saying Not-So-Totally-

Stubby-Area 3 times fast

stretch (/users/stretch/)

June 25, 2008 at 4:31 p.m. UTC

Thanks for pointing this out, Adam. While you can have a default route injected into an NSSA without

configuring it as totally stubby, the 'default-information-originate' parameter needs to be explicitly included,

and this was overlooked in my review. I've updated the post to reflect the change. Thanks again!


5 of 17 9/10/2014 3:41 PM

Hesham (guest)

July 7, 2008 at 7:44 a.m. UTC

Well done :)

Michael (guest)

November 14, 2008 at 11:11 a.m. UTC

Thank you very much, that was very helpful. But I am confused, what is the benefit of Standard Areas (apart

from the logical partitioning of the topology)? Standard Area internal routers still have a complete link state

database, right?

Jairo (guest)

December 11, 2008 at 3:40 a.m. UTC

i Am studing for CCNP, this explicattion is the best yhat i have read. thank a lot

M.Farhan Araj (guest)

January 5, 2009 at 7:22 a.m. UTC

This is the best article i have seen regarding OSPF Area & LSA types

Sumit Mittal (guest)


So lucid and descriptive..

marek (guest)

January 13, 2009 at 3:56 p.m. UTC

Hello Stretch

i just wanted to ask about one thing, which i think is a little bit confusing. i am going through all your articles

and i believe this is either unclear or wrong. your sentence in this article "ospf area types" >> quote: Note the

peculiar case of type 4 LSAs. These LSAs are injected into the backbone by the ABR of an area which

contains an ASBR. This is to ensure all other routers in the OSPF domain can reach the ASBR. << end of

quote. if you compare this with your previous article about ospf LSA type 4, it is there where you write, that

the LSA type 4 is being advertised by other ABRs, not the one which contains the ASBR. can you tell me

whether i did not understand it correctly or maybe you did not write it the same way? anyway, thanks very

much for your work. me and my colleagues who read your articles wonder, whether you have any free time

at all...sports, girlfriend, etc. have a good day

Ibrahim (guest)

April 20, 2009 at 8:29 a.m. UTC

Very simple, very clear!

Really best document I've seen.

Many thanks..

Pras555 (guest)


6 of 17 9/10/2014 3:41 PM

June 26, 2009 at 12:41 a.m. UTC

Is type#4 LSA allowed in Stub areas?

In the Summary only types#1, 2 and 3 are mentioned under 'Stub Area'.

Sarabjeet (guest)

July 3, 2009 at 6:44 p.m. UTC

Very simple and clear description.

Thanks a lot.

wum (guest)

July 21, 2009 at 9:17 a.m. UTC

what if we an ASBR in area O, in that case what LSAs will be propogated inside Area 0.

lucky (guest)

August 28, 2009 at 5:54 p.m. UTC

PERFECT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! GOOD EFFORT.... can any one tell me what is the

similarity between area 0 and non area 0.. ? in just one line.

Andy (guest)

August 30, 2009 at 10:24 a.m. UTC

Hi, Good article, but I've found a mistake. R2 on the last diagram is not sending LSA type 4, because NSSA

ABR never sends LSA type 4. As per RFC 3101, page 5: "Also an NSSA's border routers never originate

Type-4 summary-LSAs for the NSSA's AS boundary routers, since Type-7 AS-external-LSAs are never

flooded beyond the NSSA's border." As Type-4 describes how to reach the sender of Type-5, the ABRs

towards other areas will flood Type-4 LSAs

Good luck !

Mahen (guest)

September 11, 2009 at 5:40 a.m. UTC

Really helpful article.....

Thomboy (guest)

October 3, 2009 at 8:31 p.m. UTC

As CCIE running for my Recertification I can honestly say: This is clearly the best explanation of this stuff

I've ever read. Thanks and congrats

hanu_blr (guest)


I have one basic question about type3 and type4 lsa. Why do we need type4 when type3 can do the same to

reach external routes?. Anyways type3/4 are generated by ABR, then y need type4 ??


7 of 17 9/10/2014 3:41 PM

can any1 help me understand this ?

thx

hanu_blr (guest)


In continuation to my earlier post, Take for eg: stub area, where there is a type 3 lsa (Default route) to reach

any inter area or external routes.

B.Ramesh (guest)


thank full it is very good to understand . it is like spoon feeding

rajkrssna (/users/rajkrssna/)

February 2, 2010 at 4:31 a.m. UTC

good work ...amazing

Ismail (guest)

March 31, 2010 at 9:56 a.m. UTC

Fantastic job buddy, Thank you so much

ajay (guest)

October 13, 2010 at 10:45 a.m. UTC

HI I think this is one among the best document regarding the ospf and the LSA types

Fuzz (guest)


Thank you for the post Jeremy.

A guest

December 6, 2010 at 12:21 p.m. UTC

Great work thanks!!

BlueRose (/users/BlueRose/)

December 30, 2010 at 5:11 p.m. UTC

To solve this problem, and in what is arguably the worst naming decision ever made, Cisco introduced the

concept of a not-so-stubby area (NSSA). Love that !! They made it even worse with their 'totally NSSA'

area!!!

BlueRose (/users/BlueRose/)


Good diagram illustrating area types: https://learningnetwork.cisco.com/docs/DOC-7995


8 of 17 9/10/2014 3:41 PM

(https://learningnetwork.cisco.com/docs/DOC-7995)

rain (/users/rain/)


Dear Stretch,

Thanks a Lot

Your article help me a lot.

Regards,

Rain

Sandesh (guest)


Hi Stretch,

I appreciate your effort on this one. However this is good for a entry level person. Please include bit level

information so that it looks more interesting.

Thank you,

-Sandesh

stretch (/users/stretch/)

February 8, 2011 at 2:55 p.m. UTC

@Sandesh: Try doing some research yourself: http://tools.ietf.org/html/rfc2328 (http://tools.ietf.org

/html/rfc2328)

iniasky (guest)

February 26, 2011 at 5:03 p.m. UTC

Real good article!!! A short clear summary of LSAs and OSPF areas

Thanks

--Iniasky--

Shweta (guest)

September 17, 2011 at 6:34 p.m. UTC

Good explanation, easy to understand .

Many Thanks

Shweta

sat (guest)

September 23, 2011 at 12:46 p.m. UTC


9 of 17 9/10/2014 3:41 PM

cool stuff

Justintime52 (/users/Justintime52/)


Great write up! Thanks for sharing, the role of the NSSA regarding an ASBR has escaped me until now.

Ankit (guest)


YOU ARE A LIFE SAVER.....NEVER THOUGHT THAT THIS COULD BE EXPLAINED IN SO EASY

TERMS.............KUDOS...

DiMarco (guest)


Very good page!

Easy to learn, ease to teach.

Good Job.

kwame (guest)


Best description I have seen on the subject so far. Kudos

Jigs (guest)


Excellent..!!Good Work

Sadha (guest)


Great Job.Thanks for sharing!!!!

Alex (guest)


Not-so-stubby areas implement stub or totally stubby functionality yet contain an ASBR. Type 7 LSAs

generated by the ASBR are converted to type 5 by ABRs to be flooded to the rest of the OSPF domain.

This article is very well done, but this two rows are genial. this is the first time i can summarize the NSSA

confusion cisco implemented!!!

Brendan (guest)


Thanks for a wonderful post. As Andy mentioned, in that last diagram, NSSA ABR wouldn't send Type 4 into

the backbone.


10 of 17 9/10/2014 3:41 PM

Das Malhi (guest)


Great job..

Mathew (guest)

March 1, 2012 at 5:47 p.m. UTC

Best description of OSPF area/LSA types I'v ever seen! Thanks.

Kamil (guest) (http://www.4pronetworks.com/)

March 9, 2012 at 3:33 p.m. UTC

Anybody can shed some light on why in NSSA area, when we redistribute static routes pointing to the

client's PI address space are not being redistributed into OSPF backone 0 area ?

A guest

March 28, 2012 at 10:46 p.m. UTC

Thank this clear explaination is just what i needed

David (guest)

March 29, 2012 at 6:51 a.m. UTC

Very good article!!! This article helped me get a much better handle on OSPF. Thank you Jeremy!

ceekay (guest)

May 29, 2012 at 4:41 p.m. UTC

One just can't leave without thanking you for this explanation :)

Vamsi (guest)

June 2, 2012 at 4:34 p.m. UTC

Wow......finally could decipher the LSA's and area types.....one stop repository....thanks!

Ashwin (guest)

June 5, 2012 at 1:13 p.m. UTC

Isn't the NSSA picture (the 3rd picture) actually depicting a Totally NSSA area as the type 3 messages are

missing?

Suresh (guest)

June 11, 2012 at 2:02 a.m. UTC

clear explanation...

Mo'men Ibrahem (guest)

June 24, 2012 at 2:35 a.m. UTC

i appreciate your efforts

it was wonderful, and it was my only way to understand this subject


11 of 17 9/10/2014 3:41 PM

thx a lot

victorastacio25 (/users/victorastacio25/)

July 28, 2012 at 7:40 a.m. UTC

Really good explanation, one of the best I've seen.

Thanks.

Victor. CCNP

jatinder thakur (guest)


thanks a lot

Maulin (guest)


Hi,

Very good & easy discription...but i have a question....

How many stub areas can connected to Backbone area through ABR? means what is the amount of stub

arae 1,2,3,4,5.....

robin (guest)

November 16, 2012 at 1:26 p.m. UTC

Thanks for that. A great article on area types and also helped clarify LSA types into the bargain, definitely

one of the best documents i have read on the subject. This entire is awesome, although it can get a little

detailed at times, i often wind up here as a point of reference for my CCNP studies.

Dania (guest)


thank you very much Mr. Jeremy .Itwas really useful for me.

Romania (guest)


Very well explain. Good job man !

adam (guest)


very nice work done man

riz (guest)


SPOT ON!!!!


12 of 17 9/10/2014 3:41 PM

dont ask me why but i was getting confused before..

this has cleared it for me...

:-)

A guest

March 21, 2013 at 7:27 p.m. UTC

Excellent

trueffel (guest)

April 14, 2013 at 11:15 a.m. UTC

Great explanation, Cisco can't even top this. Good work!

gunasekaran (guest)

April 24, 2013 at 9:23 a.m. UTC

Very simple and clear explanation.

Raghav (guest)

June 12, 2013 at 6:54 a.m. UTC

Good blog , understood how ospf area and types are propagated.

Me (guest)

July 19, 2013 at 8:05 a.m. UTC

Really good.

Arthur (guest)

September 9, 2013 at 11:00 a.m. UTC

As always Jeremy great job. This is probably one of the best summarization of OSPF areas and their

roles/purpose on the net.

Awsome!!!

Gatekeeper509 (guest)


Awesome!!!!

Alof0l (guest)


Still up-to-date and one of the best resources, congrats.

Pawan (guest)


Stretch,


13 of 17 9/10/2014 3:41 PM

I have a doubt. I think NSSA does not generate a default route (by default) untill

we configure

Area nssa default-information-originate.

Am i wrong in thinking that?

I Must say. Your articles are so helpful.

Thank you for sharing all of your beautiful stuffs with us.

Jonathan (guest)

May 17, 2014 at 5:19 p.m. UTC

Thanks, best explanation on the net!

Ayyaz Ul Haq Qureshi (guest)

May 20, 2014 at 7:08 a.m. UTC

I was always confused on OSPF Area types since i have known them, but thanks to this article i am no more

confused. Also the ambiguities about LSA-types are also cleared. The best Explanation ever. keep it up

jorge galan (guest)

May 20, 2014 at 9:15 p.m. UTC

Good information, thanks from Spain

Priyanga Smith (guest)

June 9, 2014 at 6:54 a.m. UTC

Great job. Thanks a lot mate!

Antimus (/users/Antimus/)

June 12, 2014 at 2:12 a.m. UTC

This is just what I needed. Thank you.

Shailesh Sherekar (guest)

June 23, 2014 at 6:01 a.m. UTC

Great explanation, Thank you for this post.

Patrick (guest)

June 30, 2014 at 7:45 p.m. UTC

Wow, this is still the best explanation of area types in the web. Thanks for the great job, this made

understanding way easier for me.

Gurmeet (guest)

July 1, 2014 at 9:23 a.m. UTC

Nice !!!!


14 of 17 9/10/2014 3:41 PM

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Mircea (guest)

August 10, 2014 at 10:32 p.m. UTC

Very good presentation! Thanks! :)

raj (guest)

August 20, 2014 at 6:59 a.m. UTC

Super stuff :)

omer (guest)


nice and easy

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