OSPF type 4 in stub areas

Hi,

Had one more OSPF snag

Documentation states that in a standard stub area, only type 5 LSA's are being sorted out, but when i create a stub area, i dont get any "Summary ASB Link States" in my OSPF database either, which would lead me to believe that type 4 LSA's get sorted out at the ABR as well. I cant find anything regarding why its not showing up then.

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

nice343

a stub area does not allow type 5 LSA's It generates a default route to the ABR

you are doing something wrong if you don't see this result

Plazma

Stub areas block type 5 LSA's only

Totally stubby areas block type 3,4,5 LSA's

Not so stubby areas block type 3,4,5 LSA's and use a Type 7 LSA inside the NSSA for routes from an ASBR, The ABR for the NSSA then converts it back to type 5 and forwards it out to the area

Not so Totally stubby area's do the same thing as an NSSA except they generate a default route for the external path from the ASBR in the NSSA and inject it into OSPF.

In short, in any stub area you should not see any Type 5 LSA's, just a summary to the ABR that was injected into OSPF

gojericho0

Whats your topology look like? Did you make sure you configured the stub command on all routers within the area?

There is also something called a totally stubby area. This will block inter-area lsa's as well at the ABR.

singh8281

kpjungle wrote:

Documentation states that in a standard stub area, only type 5 LSA's are being sorted out, but when i create a stub area, i dont get any "Summary ASB Link States" in my OSPF database either, which would lead me to believe that type 4 LSA's get sorted out at the ABR as well. I cant find anything regarding why its not showing up then.

Do you have an ASBR router in the topology, if not, then there won't be any type 4 LSA, since they describe the route to the ASBR.

kpjungle

Plazma wrote:

Stub areas block type 5 LSA's only

Totally stubby areas block type 3,4,5 LSA's

Not so stubby areas block type 3,4,5 LSA's and use a Type 7 LSA inside the NSSA for routes from an ASBR, The ABR for the NSSA then converts it back to type 5 and forwards it out to the area

Not so Totally stubby area's do the same thing as an NSSA except they generate a default route for the external path from the ASBR in the NSSA and inject it into OSPF.

In short, in any stub area you should not see any Type 5 LSA's, just a summary to the ABR that was injected into OSPF

yeah. however, the the only difference between NSSA's and regular stubby areas are the same with the exception of NSSA allowing an ASBR.

stubby area= block type 5
totally stubby = block type 2,4 and 5

nssa = block type 5, allow ASBR
totally nssa = block type 3,4 and 5, allow ASBR

(reference is page 258/259 in the BSCI self study)

kpjungle

nice343 wrote:

a stub area does not allow type 5 LSA's It generates a default route to the ABR

you are doing something wrong if you don't see this result

I am seeing the result of LSA type 5's being rejected. What im wondering about, is why do i also see LSA type 4's being rejected (not shown in the OSPF database).

gojericho0

Do be honest, i don't know why a stub area would need type 4 LSAs to tell it the summary of an ABSR since it will be receiving a default route from the ABR anyway.

kpjungle

gojericho0 wrote:

Do be honest, i don't know why a stub area would need type 4 LSAs to tell it the summary of an ABSR since it will be receiving a default route from the ABR anyway.

Exactly, which is what kinda makes me believe that since it wont ever use the information, then why would it list it in the OSPF database to begin with, even though it might receive it. I cant see the purpose either

gojericho0

I have a bonus question for you as well. This one took me a little while to figure out:

Why do all areas need to be adjacent to area 0?

telco1

gojericho0 wrote:

I have a bonus question for you as well. This one took me a little while to figure out:

Why do all areas need to be adjacent to area 0?

they dont have to be physically adjacent. all areas must physically or logically connect back to area 0 since it is the backbone area, and all traffic traverses through the backbone. if an area is not physically connected to the backbone or cannot, then that is when you would configure a virtual link, which would be temporary of coarse until you can get that area physically connected to the backbone.

nicklauscombs

gojericho0 wrote:

I have a bonus question for you as well. This one took me a little while to figure out:

Why do all areas need to be adjacent to area 0?

interarea ospf is distance vector like since there are different link state databases on the abrs so each area hears what it can reach rather then know which means its vulnerable to loops

saw this on a jeff doyle blog, great information to know

kpjungle

gojericho0 wrote:

I have a bonus question for you as well. This one took me a little while to figure out:

Why do all areas need to be adjacent to area 0?

Hehe, I got that question in the lab portfolio. My best answer to the question, was that it could have the potential of ruining a pretty clean design, by attaching areas to areas, back and forth. Also, according to the design specs, you should have the most powerful routers be in the area 0, to provide a high speed backbone. If you all of a sudden grow a regular area into a "2nd backbone" you might have higher convergence times.

Best guess however... Couldnt find anything to prove/disprove, explain it in more technical details, did you?

singh8281

Another Quiz

NSSA area would allow external routes from ABR by default under what situation?

gojericho0

singh8281 wrote:

Another Quiz

NSSA area would allow external routes from ABR by default under what situation?

If the ABR was also the ASBR

gojericho0

kpjungle wrote:

gojericho0 wrote:

I have a bonus question for you as well. This one took me a little while to figure out:

Why do all areas need to be adjacent to area 0?

Hehe, I got that question in the lab portfolio. My best answer to the question, was that it could have the potential of ruining a pretty clean design, by attaching areas to areas, back and forth. Also, according to the design specs, you should have the most powerful routers be in the area 0, to provide a high speed backbone. If you all of a sudden grow a regular area into a "2nd backbone" you might have higher convergence times.

Best guess however... Couldnt find anything to prove/disprove, explain it in more technical details, did you?

Yeah, the Jeff Doyle blog explained it great. Basically since the ABR summarizes routes for other areas it is routing by rumor. If area 0 was not adjacent (physically or logically) there is the potential for incorrect routing to take place especially when there are redundant paths.

singh8281

gojericho0 wrote:

singh8281 wrote:

Another Quiz

NSSA area would allow external routes from ABR by default under what situation?

If the ABR was also the ASBR

True, but strange. Why wouldn't it accept from other routers but the NSSA ABR who's also an ASBR

gojericho0

The reason is, if it is attached to another area, that area will still need that type 5 external lsa so the ASBR\ABR will be forced to send it. A plain nssa blocks all type 5 lsas and thats why an original translation from type 5 to 7 is needed to be done by the ASBR

Next question. What is the P-bit used for and give a scenerio when you would want to disable it?

Also...has anyone figured out why the OP might not be seeing type 4s? And why types 4s are even needed if a summary area does not receive external route and all unknown networks are deferred to the ABR?

I'm thinking it maybe the same rationale. The ASBR may also be an ABR and in order to have an accurate topology database the type 4 summaries are needed.

Plazma

kpjungle wrote:

Plazma wrote:

Stub areas block type 5 LSA's only

Totally stubby areas block type 3,4,5 LSA's

Not so stubby areas block type 3,4,5 LSA's and use a Type 7 LSA inside the NSSA for routes from an ASBR, The ABR for the NSSA then converts it back to type 5 and forwards it out to the area

Not so Totally stubby area's do the same thing as an NSSA except they generate a default route for the external path from the ASBR in the NSSA and inject it into OSPF.

In short, in any stub area you should not see any Type 5 LSA's, just a summary to the ABR that was injected into OSPF

yeah. however, the the only difference between NSSA's and regular stubby areas are the same with the exception of NSSA allowing an ASBR.

stubby area= block type 5
totally stubby = block type 2,4 and 5

nssa = block type 5, allow ASBR
totally nssa = block type 3,4 and 5, allow ASBR

(reference is page 258/259 in the BSCI self study)

Totally, NSSA, and Totally NSSA all block type 3,4,5 LSA's, LSA type 1&2 are allowed through.

You are correct in saying NSSA and Totally NSSA allows for an ASBR.

gojericho0

Plazma wrote:

kpjungle wrote:

Plazma wrote:

Stub areas block type 5 LSA's only

Totally stubby areas block type 3,4,5 LSA's

Not so stubby areas block type 3,4,5 LSA's and use a Type 7 LSA inside the NSSA for routes from an ASBR, The ABR for the NSSA then converts it back to type 5 and forwards it out to the area

Not so Totally stubby area's do the same thing as an NSSA except they generate a default route for the external path from the ASBR in the NSSA and inject it into OSPF.

In short, in any stub area you should not see any Type 5 LSA's, just a summary to the ABR that was injected into OSPF

yeah. however, the the only difference between NSSA's and regular stubby areas are the same with the exception of NSSA allowing an ASBR.

stubby area= block type 5
totally stubby = block type 2,4 and 5

nssa = block type 5, allow ASBR
totally nssa = block type 3,4 and 5, allow ASBR

(reference is page 258/259 in the BSCI self study)

Totally, NSSA, and Totally NSSA all block type 3,4,5 LSA's, LSA type 1&2 are allowed through.

You are correct in saying NSSA and Totally NSSA allows for an ASBR.

NSSA's still allow for type 3 and 4, and occasionally type 5. Type 5 is when the P bit is disabled which is the case when the ASBR is also an ABR

Plazma

Apparently you are correct.. and i was misguided.

Stubby/NSSA's have LSA type 3 & 4 but not Type 5

Totally Stubby/ Totally NSSA have 1 LSA type 3 and type 7 LSA's (which get converted back to LSA type 5) - The 1 type 3 LSA would be the default route .. i suppose this is arguable depending who you ask

Not sure much about the P bit.. I would imagine it is also flagged as a type 7 LSA in the NSSA and then converted at the ABR back to type 5.

kpjungle

Heres a thing though.

My NSSA ABR router, connecting to Area 0 and to Area 2, which is the NSSA still list this when doing an sh ip ospf:

R3#sh ip ospf
Routing Process "ospf 1" with ID 3.3.3.3
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
It is an area border and autonomous system boundary router
Redistributing External Routes from,
Initial SPF schedule delay 5000 msecs
...

Does an NSSA ABR not list itself as an NSSA ABR, but instead as an ASBR even though its not one? further down one can see the Translation:

Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:04:53.624 ago
SPF algorithm executed 12 times
Area ranges are
Number of LSA 6. Checksum Sum 0x065683
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 2
Number of interfaces in this area is 1
It is a NSSA area
Perform type-7/type-5 LSA translation
Area has no authentication
SPF algorithm last executed 00:17:29.652 ago
SPF algorithm executed 13 times
Area ranges are
Number of LSA 6. Checksum Sum 0x020237
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0

So are you guys seeing this on your NSSA ABRs too?

gojericho0

If you do not have another router in the NSSA (area 2) that is connecting to a different autonomous system the single router you have designated in the NSSA will assume the ASBR as default. In your case it is also the ABR.

Do a show ip ospf border-routers to confirm

kpjungle

gojericho0 wrote:

If you do not have another router in the NSSA (area 2) that is connecting to a different autonomous system the single router you have designated in the NSSA will assume the ASBR as default. In your case it is also the ABR.

Do a show ip ospf border-routers to confirm

Yep, you are completely right. sh ip ospf border-routers confirms the actual role. No matter what, when an NSSA is in effect, it will say "It is an area border and autonomous system boundary router" on the ABR.

output from different router listing it as an ASBR even though its just an ABR:

i 1.1.1.1 [128] via 192.168.1.5, Serial0/0, ABR/ASBR, Area 0, SPF 6

kpjungle

gojericho0 wrote:

If you do not have another router in the NSSA (area 2) that is connecting to a different autonomous system the single router you have designated in the NSSA will assume the ASBR as default. In your case it is also the ABR.

Do a show ip ospf border-routers to confirm

Hi gojericho0, it seems an NSSA ABR is always listed as:

i 3.3.3.3 [128] via 192.168.1.1, Serial0/0, ABR/ASBR, Area 0, SPF 6

even though its just an ABR, and there IS another ASBR in its area.

Just some info

gojericho0

yeah, i labbed it up at home last night and saw it as well. Are you understanding OSPF better now?

kpjungle

gojericho0 wrote:

yeah, i labbed it up at home last night and saw it as well. Are you understanding OSPF better now?

Yeah, i think im getting it enough. I definately understand the concepts very well. Done all the labs in the study guide, exam guide and lab portfolio. These are all things that werent directly covered.

Thanks alot for the help!

onto IS-IS...

Have you completed BSCI yet btw?

gojericho0

kpjungle wrote:

gojericho0 wrote:

yeah, i labbed it up at home last night and saw it as well. Are you understanding OSPF better now?

Yeah, i think im getting it enough. I definitely understand the concepts very well. Done all the labs in the study guide, exam guide and lab portfolio. These are all things that werent directly covered.

Thanks alot for the help!

onto IS-IS...

Have you completed BSCI yet btw?

Yeah, completed a couple weeks ago. I failed the first time though because I ran out of time with 7 questions left. I was double checking all my answers and didn't allow myself enough time. Second time I only double checked the ones I wasn't sure of and passed pretty easily. Did you schedule yours yet.

nicklauscombs

I got my BSCI scheduled for August 20th, at this point i'm focused mainly on labbing and trying to keep my brain from leaking

kpjungle

Cool, so its doable at least

Nope, didnt schedule mine yet. I want to get at least IS-IS down to see how much time is reasonable to do the rest.. Cant really fit more than 2-3 hour per day into my mind.

Nick, you doing labs from the lab portfolio, or some other material?

nicklauscombs

kpjungle wrote:

Nick, you doing labs from the lab portfolio, or some other material?

I definitely have the lab portfolio, while it is a great resource I really wish it contained more lab challenges sort of like the CCIE workbooks but more geared towards CCNP candidates (does such a thing exist?), the hand holding labs are great when you're first learning the concepts but those challenges really force you to know what's going on or do independent research on how to complete the steps which aides in the learning process. On a side note how many pages in that book are pings good grief

mainly though I am trying to create scenarios in my head then drawing them out on a whiteboard and going that direction for labs

theres also some good lab challenges on http://www.networking-forum.com/ in the lab challenges forum