OSPF: CONCEPTS: STUB AREA INSIGHT


STUB AREA EXPLAINED

Here, you will find the complete insight of Stub Area Type with Twisted scenarios & Questions


 

” REFERENCE LINK “

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/6208-nssa.html

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47867-ospfdb8.html


 

” LSA BACKGROUND “

LSA-1 & 2 : Exist Inside area ,  also called Intra Area

LSA-3:  Exist When exchanging routes between areas, also called Inter Area

LSA-4: Exist When redistributing routes from OSPF to some other protocol ( at ASBR )

LSA-5: Exist When redistributing routes from some other protocol into OSPF ( at ASBR )

LSA-7: Exist When redistributing routes fom some other protocol into OSPF ( at ASBR )

(Exist in Not So Stubby Area Type)


 

” STUB AREA TYPES “

Stub areas are of three types, configured as per requirement in the scenario

1. TOTALLY STUB AREA

LSA’s Allowed: 1,2

we don’t have complete set of LSA’s here, so to reach remaining subnets, Default route is required

Default route: Already present

 

2. STUB AREA

LSA’s Allowed: 1,2,3

we don’t have complete set of  LSA’s here, so to reach remaining subnets, Default route is required

Default route: Already present

 

3. NOT SO  STUBBY AREA (NSSA)

LSA’s Allowed: 1,2,3 & 7

we don’t have complete set of LSA’s here, so to reach remaining subnets, Default route is required

Default route: Need to be configured, using command

Example: Area 1 nssa default-information-originate


 

” POINTS TO NOTE “

By default, LSA-7 gets converted into LSA-5.

 

As in other areas, LSA-7  cannot exist, so LSA-7 gets converted into LSA-5, to advertise routes received by LSA-7 into another areas

 

Area type needs to be configured on every router in the area to make complete area as

Not So Stubby Area (NSSA)


 

” TWISTED CASES “

1. To Convert NSSA   into   NSSA Totally Stub

Means, now LSA-3 will be blocked  or  no Inter-Area routes will be exchanged, as LSA-3 is not allowed Totally Stub area.

NSSA  allowed LSA’s : 1,2,3 & 7

NSSA Totally Stub  allowed LSA’s:  1, 2 & 7

 

Now, LSA-3 is blocked.  It means NSSA will not learn Inter- Area routes. But, communication may be required from NSSA to some other area. So, solution is simply default route

A default route is injected into the NSSA totally stub area as a type 3 summary LSA

 

Command 

Area 1 nssa no-summary

 

2. When LSA-7 is not required in  NSSA

It means Redistribution will not take place.

Allowed LSA: 1,2,3

Command

router ospf 1 Area 1 nssa no-redistribution

 

3. LSA-7 required, but Redistribution needs to be stopped for few subnets

For example, you have a RIP learned route 141.108.10.0/24 that is injected into the OSPF NSSA Area 1.  You do not want this route to be leaked into the rest of the OSPF areas

Command

router ospf 1 summary-address 141.108.10.0 255.255.255.0 not-advertise

 

4. In NSSA, we have one ASBR & two ABR’s. which ABR will do LSA-7 to LSA-5 conversion ?

ABR, which is having lower cost to the ASBR, will convert LSA-7 into LSA-5


 

” ALLOWED LSA’s BRIEFING “


Normal Area= 1, 2, 3, 4, 5

Stub Area= 1, 2, 3 + default route (already present)

Total Stub Area= 1, 2 + default route (already present)

NSSA = 1, 2, 3 & 7

Total Stub NSSA = 1, 2, 7 + default route (Need to be added using command)

 

NOTE

Normal NSSA is the only one that does not automatically get a default route injected. Default route has to be added manually


 

” TWISTED QUESTIONS “

Que. What is the difference between a Stub and NSSA area?

Ans.

Stub Area Allowed LSA’s : 1,2,3

NSSA Allowed LSA’s : 1,2,3 & 7

From above, we can conclude that Stub areas cannot have an ASBR present and a NSSA can have.

 

Que. What is the difference between a Totally Stub and a Totally NSSA?

Ans.

Totally Stub Allowed LSA’s : 1,2

Totally Stub NSSA: 1,2 & 7

From above, we can conclude that Totally-Stub areas cannot have an ASBR present and the Totally Stub-NSSA can have an ASBR.


Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s