OSPF - CBT CCNP VID 22 - LSA types and areas

Question 1

Name the 5 OSPF Area types

Answer 1

Backbone area
Standard area
Stub area
Totally stubby area
Not-so-stubby area

Question 2

Name the 5 OSPF LSA types:

Answer 2

LSA type 1: Router link LSA

LSA type 2: Network link LSA (DR generated)

LSA type 3: Summary LSA (ABR Summary Route)

LSA type 4: Summary LSA (ASBR Location)

LSA type 5: External LSA (ASBR Summary Route)

LSA type 7: NSSA external link (type7)
(Generates by ASBR in NSSA, concerted to type 5 by ABR)

Question 3

what is the function of LSA type 1:

Answer 3

Router LSA :
Standard Link state update(LSA) containing router interface/link state routing information. This goes inside LSU (link state updates) like an envelope.
also known as LSA type Run.

Question 4

what is the function of LSA type 2:

Answer 4

Network LSA (DR generated) :
The DR router sends new triggered update to all other ospf routers on the Multicast address 224.0.0.5

Question 5

what is the function of LSA type 3:

Answer 5

Summary LSA (ABR Summary Route) :
these are routes that come from outside the current area. Routing information between ospf intra area. This is normally a type 1 LSA changed into type 3 at the ABR router.

Question 6

what is the function of LSA type 4:

Answer 6

Summary LSA (ASBR Location) :
this notifies every one where the ASBR Router is located and here is the cost to reach me & this is the way to leave the OSPF system.

Question 7

what is the function of LSA type 5:

Answer 7

External LSA (ASBR Summary Route) : 
These are the External Routes redistributed into ospf by ASBR. Mostly summarised.

Question 8

what is the function of LSA Type 7 :

Answer 8

hides external routes through and across an area until it reaches an ABR Router to reveal the hidden LSA type 3&5.

Question 9

What is Totally stubby area ?
What LSA type does it block ?
What is this LSA type contain ?

Answer 9

Does not receive EXTERNAL routes into the area.

It blocks LSA type 5, as they are external Route update for OSPF.

Question 10

What is Totally stubby area ?
What LSA type does it block ?
What is this LSA type contain ?

Answer 10

This block all EXTERNAL routes AND other OSPF INTRA AREA Routes.

Blocks LSA type 3 intra area routes

Blocks LSA type 4 ASBR location - as it doesnt need to know this location in every update.

Blocks LSA type 5 external routes

Question 11

What is Not So Stubby Area ?
What LSA type does it block ?
What is this LSA type contain ?

Answer 11

This allows external routes to be redistributed through the area and across to other area.

• This breaks the rules of ‘stub areas’ as they are normally designed to prevent external and intra area routes.
• But what happens here is the external routes (type3, & 5) area hidden under a TYPE 7 LSA, which is then revealed as type 3&5 LSA once it reaches the ABR router.

Question 12

Why do we need a ‘Stub Area’ ?

Answer 12

• You only needs a default route to the ABR who will forward to the external destination.
• It does not want to keep updating its DBD with these external route updates & its changes.
• Prevent cpu utilisation.
• Prevent update floods.

Question 13

Why do we need a ‘Totally Stub Area’ Why ?

Answer 13

If it only has 1 route out the network to all external and intra area ospf destination, then it just uses that route and it doesn’t need to be disturbed by other area route updates.

Question 14

Why do we need a ‘Not so Stub Area’ Why ?

Answer 14

This allows external routes to be redistributed through the area and across to other area.

• This breaks the rules of ‘stub areas’ as they are normally designed to prevent external and intra area routes.
• But what happens here is the external routes (type3, & 5) area hidden under a TYPE 7 LSA, which is then revealed as type 3&5 LSA once it reaches the ABR router.