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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.
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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).
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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.
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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!
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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)
January 8, 2009 at 4:09 a.m. UTC
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)
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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)
November 24, 2009 at 4:18 a.m. UTC
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 ??
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
7 of 17 9/10/2014 3:41 PM
can any1 help me understand this ?
thx
hanu_blr (guest)
November 25, 2009 at 7:33 a.m. UTC
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)
January 4, 2010 at 3:38 a.m. UTC
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)
October 22, 2010 at 9:19 a.m. UTC
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/)
January 9, 2011 at 9:42 a.m. UTC
Good diagram illustrating area types: https://learningnetwork.cisco.com/docs/DOC-7995
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
8 of 17 9/10/2014 3:41 PM
(https://learningnetwork.cisco.com/docs/DOC-7995)
rain (/users/rain/)
January 11, 2011 at 5:51 a.m. UTC
Dear Stretch,
Thanks a Lot
Your article help me a lot.
Regards,
Rain
Sandesh (guest)
February 8, 2011 at 5:01 a.m. UTC
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
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
9 of 17 9/10/2014 3:41 PM
cool stuff
Justintime52 (/users/Justintime52/)
October 4, 2011 at 1:51 a.m. UTC
Great write up! Thanks for sharing, the role of the NSSA regarding an ASBR has escaped me until now.
Ankit (guest)
October 24, 2011 at 5:05 p.m. UTC
YOU ARE A LIFE SAVER.....NEVER THOUGHT THAT THIS COULD BE EXPLAINED IN SO EASY
TERMS.............KUDOS...
DiMarco (guest)
October 25, 2011 at 7:49 a.m. UTC
Very good page!
Easy to learn, ease to teach.
Good Job.
kwame (guest)
October 25, 2011 at 11:49 a.m. UTC
Best description I have seen on the subject so far. Kudos
Jigs (guest)
January 6, 2012 at 6:05 p.m. UTC
Excellent..!!Good Work
Sadha (guest)
January 16, 2012 at 4:05 p.m. UTC
Great Job.Thanks for sharing!!!!
Alex (guest)
January 17, 2012 at 9:51 p.m. UTC
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)
February 3, 2012 at 1:04 a.m. UTC
Thanks for a wonderful post. As Andy mentioned, in that last diagram, NSSA ABR wouldn't send Type 4 into
the backbone.
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
10 of 17 9/10/2014 3:41 PM
Das Malhi (guest)
February 29, 2012 at 4:28 a.m. UTC
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
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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)
August 12, 2012 at 5:08 p.m. UTC
thanks a lot
Maulin (guest)
October 30, 2012 at 7:35 p.m. UTC
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)
November 24, 2012 at 9:41 a.m. UTC
thank you very much Mr. Jeremy .Itwas really useful for me.
Romania (guest)
December 31, 2012 at 7:56 a.m. UTC
Very well explain. Good job man !
adam (guest)
January 28, 2013 at 7:21 p.m. UTC
very nice work done man
riz (guest)
February 22, 2013 at 9:31 a.m. UTC
SPOT ON!!!!
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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)
October 17, 2013 at 1:24 a.m. UTC
Awesome!!!!
Alof0l (guest)
November 21, 2013 at 2:29 a.m. UTC
Still up-to-date and one of the best resources, congrats.
Pawan (guest)
December 6, 2013 at 2:41 a.m. UTC
Stretch,
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
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 !!!!
OSPF area types - PacketLife.net http://packetlife.net/blog/2008/jun/24/ospf-area-types/
14 of 17 9/10/2014 3:41 PM
Leave a Comment
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)
August 22, 2014 at 7:43 p.m. UTC
nice and easy
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