Wednesday, March 08, 2006

 

Understanding OSPF Default Route Generation.

Here are some facts about default route generation with OSPF -
  1. Default routes are not generated by default into normal areas. When configured to do so, the ABR generates type-5 LSAs for these default routes.
  2. Default routes are generated by default into stub and totally stubby areas. The ABR generates type-3 LSAs in this case.
  3. Default routes not generated by default into NSSA and totally stubby NSSA areas. For this case, the default routes are propagated as type-7 LSAs.
For case (1), one needs to type in some flavor of the default-information originate command under the ospf router process. Case (2) needs no configuration. Case (3) needs a configuration command but unlike case (1), you need to say area n nssa default-information originate.

At first glance, all these minor differences in the details of how the default route generation happens with OSPF is maddening. The method behind this seeming madness is not apparent. Why all these differences? When I first encountered these facts, I realized that there was no way I was going to be able to memorize these without reasoning out the story behind these particular choices.

Why were these choices made? Why couldn't the OSPF standard simply use the same type of LSAs for propagation of the default routes regardless of the area into which the route was being propagated? That the default route generation happens by default for stubby and
totally stubby areas makes perfect sense since routing information is withheld from routers in such areas precisely because we want them to treat the ABR as a default gateway. So, then why couldn't the same be done for NSSA and totally stubby NSSA areas which are similar in spirit to the stubby and totally stubby areas? Why did Cisco need to have two different commands for generating the default routes for cases (1) and (3)?

Let's try and reason through these facts to understand why things are as they are. Default routes are not generated by default into normal areas. That makes sense because normal areas have complete routing information and so default routes are likely not even necessary. When configured to do so, the default route is propagated as a type-5 LSA. That seems to be reasonable too because the default route is probably coming from a static default route which is after all considered as a routing process itself. So, generating a type-5 external is fine.

In stubby and totally stubby areas, the default routes are generated by default by the ABR and no configuration is necessary. Routing information is withheld from the routers in stubby and totally stubby areas. Also, there are no exits from a stubby and totally stubby areas other than via an ABR since ASBRs are not allowed in these areas. So, the stub area routers will always definitely need a default route to get out of the area. Since such is the case, it makes sense for the ABR to automatically generate a default route with no configuration being needed. Also, the default route is propagated as a type-3 LSA because type-5 LSAs are not allowed in stub areas. It was a bit surprising to me that even the totally stub receives a type-3 LSA for the default route because by definition totally stubby areas do not receive type-3 LSAs. However, since totally stubby areas are really just a Cisco extension to the stub area, the prohibition of type-3 LSAs from totally stubby areas is not a RFC dictat. So, it looks like ABRs for totally stubby areas filter out all type-3 LSAs but do generate a type-3 LSA for the default route.

Now with this line of reasoning, why type-7 LSAs are used for case(3) with NSSA and totally stubby NSSA areas becomes clear. That is obviously because type-5 LSAs are not allowed in these areas but type-7 external LSAs are. But why don't the NSSA ABRs generate the
default route automatically like they do for stubby and totally stubby areas? Well, the only difference between stubby and NSSA areas is that the NSSA areas can have ASBRs in them. And therein lies the cause for the lack of automatic default route generation. Traffic from NSSA area routers can exit the area either through an ABR or through an ASBR. Unlike the stubby area case, there is more than one exit path from the area! The ABR cannot simply assume that he is the default route of the area and so it would not be right to do automatic default route generation. The user would have to specifically specify if the default exit from a NSSA is via an ABR or an ASBR and so some configuration is needed.

OK, so the choices start to make sense now. But why did Cisco need two different kinds of configuration commands for default route generation? Well, let's handle this by assuming that the commands could be the same -- default-information originate -- for both cases (1) and (3). Let's then consider the case of an ABR that is ABR for both an NSSA area and a normal area. The command would generate default routes into both areas. What if you now wanted to generate a default route into the NSSA area but not into the normal area? Oops, it looks like I need to be able to specify the area in the default-information originate command. Well, that is exactly what the area n nssa default-information originate command allows you to do. The alert reader might now say that one could get rid of the default-information originate command and always use a command that specifies the area into which the default route generation needs to be done. That could well be the case, but now practical issues intervene. What must have happened is that the default-information originate command must have been implemented first before there was even NSSA support and the area n nssa default-information originate must have come later when that need was identified. But you can't just go remove old commands willy-nilly because users learn to use them and so that remains.

I think all the original questions that the details of OSPF default route generation threw up have now been answered. What was done was that a narrative of sorts was created such that the seemingly strange behavior now seems to be quite reasonable indeed. Once this reasoning is internalized, remembering this behavior will become much easier. This is the smart memorization technique that I referred to a couple of posts ago.

There is a good article on Cisco's website on this topic that explains the hows of default route generation with OSPF. Hopefully this blog post gave you the whys so that OSPF default route generation becomes clearer.


Comments:
This comment has been removed by a blog administrator.
 
This comment has been removed by a blog administrator.
 
As a correction to the original post, NSSA Totally Stubby areas DO automatically generate a default route. It is advertised as a type 3 (summary) route, not as a type 7. (The type 7 route is used when it's a normal NSSA area, and you configure default-information-originate).
 
This comment has been removed by a blog administrator.
 
This comment has been removed by a blog administrator.
 
This comment has been removed by a blog administrator.
 
This comment has been removed by a blog administrator.
 
Hello


G'night
 
This comment has been removed by a blog administrator.
 
[url=http://idisk.mac.com/mabilnik/Public/camera-cellular-phone.html]camera cellular phone[/url]
[url=http://idisk.mac.com/mabilnik/Public/cellular-one-cell-phone.html]cellular one cell phone[/url] - [url=http://idisk.mac.com/mabilnik/Public/cellular-phone-faceplates.html]cellular phone faceplates[/url] - [ur
l=http://idisk.mac.com/mabilnik/Public/cellular-phone-number-search.html]cellular phone number search[/url]
 
[url=http://ringtones.hostingclub.de/free-boost-ringtones.html]free boost ringtones[/url]
[url=http://ringtones.hostingclub.de/free-funny-voice-ringtones.html]free funny voice ringtones[/url] - [url=http://ringtones.hostingclub.de/free-midi-ringtones.html]free midi ringtones[/url] - [url=http://ringtones.hostingclub.de/free-ring
tones-3gforfree.html]free ringtones 3gforfree[/url]
 
[url=http://www.geocities.com/yadrena/causes-of-high-cholesterol.html]causes of high cholesterol[/url]
[url=http://www.geocities.com/yadrena/foods-high-in-cholesterol.html]foods high in cholesterol[/url] - [url=http://www.geocities.com/yadrena/ways-to-lower-cholesterol.html]ways to lower cholesterol[/url]
 
[url=http://www12.asphost4free.com/ringoton/index24.html]converter free mobile ringtones[/url]
[url=http://www12.asphost4free.com/ringoton/index28.html]creator nextel ringtones[/url] - [url=http://www12.asphost4free.com/ringoton/index3.html]free ringtones for samsung sprint phone[/url]
 
[url=http://idisk.mac.com/drugs1/Public/generic-viagra.html]generic viagra[/url]
[url=http://idisk.mac.com/drugs1/Public/purchase-viagra-online.html]purchase viagra online and[/url] - [url=http://idisk.mac.com/drugs1/Public/viagra-prescription-online.html]viagra prescription online[/url]
 
[url=http://idisk.mac.com/drugs1/Public/buy-viagra-on-line.html]buy viagra on line[/url]
[url=http://idisk.mac.com/drugs1/Public/viagra-canada.html]viagra canada[/url] - [url=http://idisk.mac.com/drugs1/Public/viagra-prescription-online.html]viagra prescription online[/url]
 
[url=http://idisk.mac.com/xpills/Public/overnight-tramadol.html]overnight tramadol[/url]
[url=http://idisk.mac.com/xpills/Public/tramadol-and-acetaminophen.html]tramadol and acetaminophen:[/url] - [url=http://idisk.mac.com/xpills/Public/tramadol-hydrochloride.html]tramadol hydrochloride[/url]
 
[url=http://idisk.mac.com/polet1/Public/cheap-england-flights.html]cheap england flights[/url]
[url=http://idisk.mac.com/polet1/Public/cheap-flight-paris.html]cheap flight paris[/url] - [url=http://idisk.mac.com/polet1/Public/cheap-flight-student.html]cheap flight student[/url] - [url=http://idisk.mac.com/polet1/Public/cheap-flights-from-uk.html
]cheap flights from uk[/url]
 
[url=http://idisk.mac.com/polet1/Public/cheap-chicago-flights.html]cheap chicago flights[/url]
[url=http://idisk.mac.com/polet1/Public/cheap-flight-paris.html]cheap flight paris[/url] - [url=http://idisk.mac.com/polet1/Public/cheap-flight-prague.html]cheap flight prague[/url] - [url=http://idisk.mac.com/polet1/Public/cheap-flight-thailand.html]cheap flight thailand[/url]
 
[url=http://idisk.mac.com/mattress2/Public/futon-mattress.html]futon mattress[/url]
[url=http://idisk.mac.com/mattress2/Public/mattress-discounters.html]mattress discounters[/url] - [url=http://idisk.mac.com/mattress2/Public/simmons-mattress.html]simmons mattress[/url]
 
[url=http://blogs.skaffe.com/fioricet]fioricet withdrawl[/url]
[url=http://blogs.skaffe.com/carisoprodol]buy carisoprodol online[/url]
 
[url=http://bama.ua.edu/~tccc/ubb/Forum1/HTML/000125.html]tramadol cod[/url]
[url=http://bama.ua.edu/~tccc/ubb/Forum1/HTML/000134.html]discount fioricet[/url] | [url=http://bama.ua.edu/~tccc/ubb/Forum1/HTML/000135.html]carisoprodol 350mg[/url]
 
[url=http://idisk.mac.com/maluka1/Public/100-free-ringtones.html]100 free ringtones[/url]
[url=http://idisk.mac.com/maluka1/Public/free-real-ringtones.html]free real ringtones[/url] * [url=http://idisk.mac.com/maluka1/Public/free-sprint-ringtones.html]free sprint ringtones[/url]
 
[url=http://idisk.mac.com/dimon2/Public/cialis-generic-viagra.html]cialis generic viagra[/url]
[url=http://idisk.mac.com/dimon2/Public/female-viagra-cream.html]female viagra cream[/url]
 
[url=http://idisk.mac.com/toyo13/Public/cingular-8525-case.html]cingular 8525 case[/url]
[url=http://idisk.mac.com/toyo13/Public/cingular-cellphones.html]cingular cellphones[/url] | [url=http://idisk.mac.com/toyo13/Public/cingular-ring-tones.html]cingular ring tones[/url]
 
[url=http://idisk.mac.com/toyo13/Public/cingular-3125.html]cingular 3125[/url]
[url=http://idisk.mac.com/toyo13/Public/cingular-customer-service.html]cingular customer service[/url] | [url=http://idisk.mac.com/toyo13/Public/cingular-plans.html]cingular plans[/url]
 
[url=http://www.geocities.com/toyoik/free-verizon-wireless-cell-phone.html]free verizon wireless cell phone[/url]
[url=http://www.geocities.com/toyoik/verizon-wireless-broadband.html]verizon wireless broadband[/url] | [url=http://www.geocities.com/toyoik/verizon-wireless-homepage.html]verizon wireless homepage[/url]
 
[url=http://idisk.mac.com/mytoyo/Public/free-metro-pcs-ringtones.html]free metro pcs ringtones[/url]
[url=http://idisk.mac.com/mytoyo/Public/free-mp3-ringtones.html]free mp3 ringtones[/url] > [url=http://idisk.mac.com/mytoyo/Public/index.html]wwe ringtones[/url]
 
This comment has been removed by a blog administrator.
 
mcse labs ebook http://audiobookscollection.co.uk/TypePad-For-Dummies/p197926/ ebook on raising from the dead [url=http://audiobookscollection.co.uk/Robbie-Allen/m107780/]ebook cover creator freeware[/url] free ebook 1914-1918
 
ebook spokane arbitron http://audiobookscollection.co.uk/de/The-Milkshake-Moment-Overcoming-Stupid-Systems-Pointless-Policies-and-Muddled-Management-to-Realize-Real-Growth/p219376/ steven erikson ebook [url=http://audiobookscollection.co.uk/it/Anatomia/c1456/?page=3]best ebook reader for mp3 player[/url] ebook sniper
 
opengl ebook http://audiobookscollection.co.uk/SystemC-From-the-Ground-Up/p104472/ soldering circut board repair ebook [url=http://audiobookscollection.co.uk/Carole-Maggio-Facercise-The-Dynamic-Muscle-Toning-Program-for-Renewed-Vitality-and-a-More-Youthful-Appearance/p163339/]freakanomics ebook[/url] ebook nederlands
 
seize the night ebook http://audiobooksplanet.co.uk/it/To-the-Rescue-of-Art-Twenty-Six-Essays/p208617/ best ebook program [url=http://audiobooksplanet.co.uk/Cerberus-Wolf-in-Fold/p208167/]free ebook the emotion machine[/url] novotni everybody else ebook
 
free last anatomy ebook download http://audiobooksworld.co.uk/es/Ghosts-I-Have-Been/p28961/ ebook reader for ubuntu [url=http://audiobooksworld.co.uk/Demetrio-Labate/m28995/]ebook by weintraub trading[/url] ebook download deutsch
 
aerospace the journey of flight ebook http://audiobooksworld.co.uk/authors/?letter=R&page=38 nag hammadi ebook [url=http://audiobooksworld.co.uk/Amabili-M/m5370/]ebook raus aus schulden[/url] free roulette strategy ebook
[url=http://audiobooksworld.co.uk/F-A-Hayek/m36908/][img]http://audiobooksworld.co.uk/image/5.gif[/img][/url]
 
memoir t-mobil samsung software windows xp http://buyoem.co.uk/category-13/Music-Software?page=23 skills portfolio module software [url=http://buyoem.co.uk/fr/category-100-110/Internet?page=48]word games software[/url] windows age enhancement software
[url=http://buyoem.co.uk/product-34596/Adobe-Creative-Suite-5-5-Design-Premium-MAC]Adobe Creative Suite 5.5 Design Premium MAC - Download OEM, Software Sale, OEM Software[/url] website and forum hosting software
[url=http://buyoem.co.uk/fr/product-36504/MONyog-Ultimate-4-5][img]http://buyoem.co.uk/image/2.gif[/img][/url]
 
password software mac http://buyoem.co.uk/fr/category-100-107/Programmation-et-laboration?page=4 what is software reusability [url=http://buyoem.co.uk/es/product-37094/Mediachance-Dynamic-Auto-Painter-2-5]yacht log software[/url] ornithology free birdsong software view
[url=http://buyoem.co.uk/category-8/Office-Tools?page=3]Office Tools - Cheap Legal OEM Software, Software Sale, Download OEM[/url] where to purchase bicycle brand software
[url=http://buyoem.co.uk/fr/news-38/Oracle-unveils-Solaris-11-hardware-package][img]http://buyoem.co.uk/image/1.gif[/img][/url]
 
dvd software in tacoma http://buyoemsoftware.co.uk/it/category-100-102/Affari?page=4 dvd building software [url=http://buyoemsoftware.co.uk/product-37390/Easy-MP3-to-WAV-Maker-3-0]originpro 7.5 software tutorial[/url] computer software publishers
[url=http://buyoemsoftware.co.uk/es/product-30642/Adobe-Photoshop-CS5-Extended]Adobe Photoshop CS5 Extended - Cheap Legal OEM Software, Software Sale, Download OEM[/url] supporting knowledge management in university software
[url=http://buyoemsoftware.co.uk/fr/product-37177/VeryPDF-PDF2TXT-3-2-Portable][img]http://buyoem.co.uk/image/1.gif[/img][/url]
 
tagged software http://buysoftwareonline.co.uk/es/product-24957/Adobe-Master-Collection-CS4-Mac best ocr software ever [url=http://buysoftwareonline.co.uk/product-37176/VeryPDF-PDF2TXT-3-2]best cd dvd playing software review[/url] memorex cd dvd label printing software
[url=http://buysoftwareonline.co.uk/es/product-34602/Adobe-InDesign-CS5-5]Adobe InDesign CS5.5 - Cheap Legal OEM Software, Software Sale, Download OEM[/url] blackberry 7290 handheld software t-mobile
[url=http://buysoftwareonline.co.uk/es/product-37225/Notepad-Pro-3-2][img]http://buyoem.co.uk/image/1.gif[/img][/url]
 
sieve analysis software http://buysoftwareonline.co.uk/es/product-37102/Alabe-Reports-Complete-Relationship-1-1 best hd dvd player software [url=http://buysoftwareonline.co.uk/it/product-15987/Goldfish-2-1-Mac]server 2008 backup software[/url] adobe photoshop software leak
[url=http://buysoftwareonline.co.uk/product-35363/3herosoft-FLV-Converter-3-5-MacOSX]3herosoft FLV Converter 3.5 MacOSX - Download OEM, Software Sale, OEM Software[/url] pattern design software for macs
[url=http://buysoftwareonline.co.uk/es/product-37360/VariCAD-2007-2-0][img]http://buyoem.co.uk/image/3.gif[/img][/url]
 
Cellulite is an unsightly dimpled appearance, especially around the
thighs. For a faster and reliable result, skin care professionals may recommend these therapies that are less-invasive
to the body. The device utilizes a cutting-edge laser fiber that delivers laser energy
to melt the fat deposits, while also thermally subscising the tissue bands.


Here is my web site velasmooth cellulite treatment miami
 
In the final stages, the dimpling is obvious both when lying down and standing up.
For a faster and reliable result, skin care professionals may recommend
these therapies that are less-invasive to the body.
Many articles report eating healthy foods low in fat,
but high in fruits, vegetables and fiber.

Visit my web-site: How to get rid of cellulite naturally
 
The infection has become too strong for the medications.
They are cures for bacterial infections and many people
do not realize that they are using drugs with
this side effect. There is disagreement about what foods can be
added back into the diet and when, but common sense will help the reader to decide if
they are ready to risk adding a controversial item to their diet.


Look into my weblog ... Yeast infection treatment
 
Finding a mentor or professional who is living the experience you want to acquire for
yourself is a smart business move. Here are some suggestions on how to get started and keep going
in this often overlooked photographic field. In 2013, Real Estate Pune market appears to be strong, with some locations notably poised for growth.



Stop by my web page vuokra-asunto lahti
 
' Finally, Hartman offered, 'These days, it
is so rare to have a group of hundreds of people with basically no naysayers, cynics or haters.
Check out Brandon and Savannah's official website for a tour schedule and the latest news. For a video to rake in this many views,ever, much less in a few days,is historic.

Here is my homepage Buy Youtube Views And Comments
 
The same reason that people buy Youtube views is why they buy Facebook fans.
Google Calendar gives you an ability to customize your view as you like.
Also, economic growth is unlimited in the world and should not be restricted by worry about natural resources.


Here is my website ... buy youtube views and likes
 
Even if you can’t find the coupons, check for the promotional offers
instead

my page - http://sixflagsticketscoupons.com/six-flags-fiesta-texas/
 
In the final stages, the dimpling is obvious both when lying down and standing up.

And also some of the underweight people are
also such complaints. Vichy is a very popular company in the European market.


My website ... How to Get rid of cellulite naturally
 
When covering the area, it should be enough to use a regular
bandage. In order to reassure yourself that your moles
are safe from cancer, you would have to reinforces your
observation by a doctor's diagnosis. Chocolate and junk foods are often blamed for the cause of acne.

my web-site - how to get rid of warts around the eye
 
Whether it's to launch your own business or to work in another field, there are a lot of options out there. If approved, it would move through their Zero Stage funding group or to some other VC organization. The job of a telemarketer or telecommuting sales representative requires a strong but friendly approach in helping the business connect with potential customers.

Here is my site - Melbourne Family Lawyers
 
[url=http://englandpharmacy.co.uk/products/myambutol.htm][img]http://onlinemedistore.com/2.jpg[/img][/url]
humana pharmacy list in indiana http://englandpharmacy.co.uk/products/levlen.htm bay area pharmacy sturgeon bay [url=http://englandpharmacy.co.uk/products/rogaine-2-.htm]make a pharmacy[/url]
norvasc online pharmacy http://englandpharmacy.co.uk/products/generic-imitrex.htm united pharmacy [url=http://englandpharmacy.co.uk/products/altace.htm]altace[/url]
pharmacy audit description pbm http://englandpharmacy.co.uk/products/tadacip.htm univeristy holland pharmacy [url=http://englandpharmacy.co.uk/products/celexa.htm]prosperity pharmacy annandale virginia[/url]
natural pet pharmacy http://englandpharmacy.co.uk/products/menosan.htm pharmacy technician scholarships [url=http://englandpharmacy.co.uk/products/coreg.htm]coreg[/url]
 
[url=http://fdaapproved.co.uk/products/kamagra-oral-jelly.htm][img]http://onlinemedistore.com/5.jpg[/img][/url]
abbey pharmacy http://fdaapproved.co.uk/products/capoten.htm online mexico international pharmacy [url=http://fdaapproved.co.uk/products/ed-discount-pack-3.htm]comercial pharmacy[/url]
brewerton pharmacy in ny http://fdaapproved.co.uk/products/benicar.htm red river pharmacy services texarkana texas [url=http://fdaapproved.co.uk/products/viagra-soft-tabs.htm]viagra soft tabs[/url]
online vet pharmacy http://fdaapproved.co.uk/products/symmetrel.htm us pharmacy phetermine [url=http://fdaapproved.co.uk/products/silagra.htm]critical care pharmacy[/url]
retail pharmacy supplies http://fdaapproved.co.uk/products/colchicine.htm online pharmacy usa [url=http://fdaapproved.co.uk/products/periactin.htm]periactin[/url]
 
No more longing to become famed at present. Buy Facebook Followers as a tactic to increase status and recognition online in a shorter duration. buy face book follower

 
Are you inclined to get global recognition swiftly? You will need to Buy Facebook Comments to become well known to our world. buy comments for facebook post
 
Are you currently looking for good subjection to your competencies and professional capabilities on LinkedIn? The very best option is through Buying LinkedIn Followers, Connections and Endorsements. buying linkedin connections
 
To buy legitimate and true LinkedIn Followers, Connections and Endorsements, you may contact us. This may help you to strengthen your professional identification on internet. buy linkedin connection

 
If you are looking for best and also simple method to promote your business then this service is flawlessly appropriate with your requirement. buy usa facebook fans
 
Hence this is beneficial for buying twitter followers as a shortcut to become popular. buy twitter follow
 
They possess total understanding about the bitcoin industry that includes the growth, decline and can also forecast the future of market. ico white paper writers
 
buy facebook followers Are you looking for permanent facebook followers here you can easily purchase permanent facebook followers
 
Post a Comment



<< Home

This page is powered by Blogger. Isn't yours?