OSPF area types

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

Advancing from last week's discussion on 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.

Standard Areas

ospf_standard_area.jpg

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.

Stub Areas

ospf_stub_area.jpg

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.

Totally Stubby Areas

ospf_total_stub_area.jpg

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).

Not-so-stubby Areas

ospf_nssa.jpg

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.

Summary

  • 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.

About the Author

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 or follow him on Twitter.

Posted in Routing

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
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!


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)
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 ??

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
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
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
January 9, 2011 at 9:42 a.m. UTC

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


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

@Sandesh: Try doing some research yourself: 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

cool stuff


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.


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)
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
thx a lot


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!!!!
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,

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
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 !!!!


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


Raja (guest)
September 28, 2014 at 10:00 a.m. UTC

Very simple and clear explanation,Thanks for sharing.


Eran (guest)
October 1, 2014 at 8:09 p.m. UTC

Thanks for this!


Abigail (guest)
October 3, 2014 at 8:13 p.m. UTC

I read about this on the certification guide book from Cisco but it was very confusing. I was pulling my hair until I found your post. Thank you so much! Now I have a better understanding of ospf area types. Although I still think nssa defeats the purpose of using stub in the first place :/


Mike (guest)
October 22, 2014 at 10:10 a.m. UTC

Thanks Jeremy, That was a next good piece of information which I found here. Anyway as all posted here already.

I wish you all the best buddy!


Nithis (guest)
November 3, 2014 at 7:12 a.m. UTC

Nice document!


Vijay (guest)
November 7, 2014 at 11:54 a.m. UTC

Hi Thanks for the great explanation jeremy.. I have a samll doubt... What is requirement of NSSA area... Let us assume that we have Area0 & Area1 and a RIP/EIGRP External network is directly connected to Area 1.. So what happens.. Does the LSA type 5 flow to Area 0 ?


Mamad (guest)
November 10, 2014 at 6:26 a.m. UTC

Hi,

For the statement

The illustration for NSSA shows a default route towards the NSSA Area, that is actually optional if you explicitly declare default-information originate, isn't it?

Thanks.


steve eyler (guest)
January 7, 2015 at 9:21 p.m. UTC

Well done!


Kevin (guest)
January 15, 2015 at 6:59 a.m. UTC

Well explained. Precise and to the point.


Chris (guest)
January 16, 2015 at 6:16 a.m. UTC

Great article: visually and logically clear and concise. I did notice however, that in the NSSA diagram you did not show the type 3 LSA/s which would be propagated into the NSSA. It may also assist understanding, if the 'default' arrows additionally stated: Single Type 3 LSA.


Ajit Patel (guest)
January 28, 2015 at 2:05 p.m. UTC

Cool explanation


tammy (guest)
February 8, 2015 at 5:38 p.m. UTC

Have not touched OSPF in 3y 11m and 29d and have to recert. Thank goodness for good notes and your diagrams to keep it simple and knock the cob webs out!


Christopher Lemish (guest)
March 10, 2015 at 10:49 a.m. UTC

Jeremy,

Great article man! The NSSA illustration should reflect LSA Type 3 allowed into the NSSA area and the default should reflect being optional. Thanks, Chris


Mapfumo
April 20, 2015 at 5:50 a.m. UTC

Thanks. Well explained and easy to digest.


F-khan (guest)
May 18, 2015 at 6:06 p.m. UTC

Bravo ! superb article which is really well explained Well done


Romy (guest)
June 12, 2015 at 2:41 a.m. UTC

Jeremy you did an excellent and lucid presentation of the most important element of OSPF. Thank You!!!

Romesh


Mohamed (guest)
June 21, 2015 at 6:04 p.m. UTC

Amazing :-)


SINGH (guest)
July 19, 2015 at 6:05 a.m. UTC

Hi Stretch,

You have best notes on the NET, to make it more perfect, for “Totally Stubby Areas” as you said that ABR advertises summary routes into the “Totally Stubby Area” but other areas connected to the ABR receive all route updates of routes that exist in “Totally Stubby Areas”.

I just tested this in the lab. Probably if you put a note that would be BEST.

Thanks


Cypher (guest)
July 21, 2015 at 3:33 a.m. UTC

You explain the topic on stub areas very clear, for us newbies like me. Thank you very much. Btw, hope you can include a diagram on Totally Stubby NSSA. :)


Roy Grubb (guest)
November 4, 2015 at 8:43 p.m. UTC

Described in a way that I can finally put this to rest


Mahosoft (guest)
November 11, 2015 at 5:56 p.m. UTC

thank you for your effort


Naval (guest)
January 12, 2016 at 12:44 p.m. UTC

Very nice article, kudos for the good work. I find little difficult to understand NSSA. I think the diagram must is using area 0 to show as NSSA because the arrows are showing converting the type 7 to type 5 LSA from right to left. But NSSA area is shown in the Right.Seems little confusing to me.


Ye Min Oo (guest)
January 21, 2016 at 9:34 a.m. UTC

Great article with clear explanation!


Randika (guest)
January 22, 2016 at 3:07 a.m. UTC

Thank you very much for the nice explanation man!


Dieter (guest)
February 3, 2016 at 3:32 p.m. UTC

I can only attest that to this day, this system is probably one of the most confusing naming conventions that has ever been made up by a technology vendor.

It's not that difficult, but the naming just doesn't help to make it clear.


Felix (guest)
June 16, 2016 at 1:59 a.m. UTC

Nice explaination, very straightforward.


Jey10 (guest)
June 23, 2016 at 2:12 p.m. UTC

Great, I didn't understand the nssa, now it's ok!


Abdullah Ahmed (guest)
July 25, 2016 at 2:41 a.m. UTC

That is the most accurate interpretation of OSPF any one ever did


Neo.X (guest)
July 28, 2016 at 1:05 p.m. UTC

Very good writing-up, as always. I learned a lot form your articles. Thanks a lot for your hard work.


Dragan (guest)
August 15, 2016 at 7:15 p.m. UTC

Great explanation Jeremy,i would add explanation for one more area: The Totally NSSA blocks external routes (LSA Type 5) + inter-area routes (LSA Type 3) but allows an ASBR within the area


Patrick (guest)
September 20, 2016 at 7:45 p.m. UTC

Clear description but if you need more details please visit my blog :) Cheers!


Tam Quan (guest)
October 8, 2016 at 3:09 a.m. UTC

Thanks a lot, guy

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