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OSPF makes use of a LSDB (hyperlink state database) and fills this with LSAs (hyperlink state commercial). As an alternative of utilizing 1 LSA packet OSPF has many several types of LSAs and on this lesson I’ll present all of them to you. Let’s begin with an outline:

  • LSA Sort 1: Router LSA
  • LSA Sort 2: Community LSA
  • LSA Sort 3: Abstract LSA
  • LSA Sort 4: Abstract ASBR LSA
  • LSA Sort 5: Autonomous system exterior LSA
  • LSA Sort 6: Multicast OSPF LSA
  • LSA Sort 7: Not-so-stubby space LSA
  • LSA Sort 8: Exterior attribute LSA for BGP

jigsaw piece

Studying: what’s lsa in ospf

For many individuals it helps to visualise issues with a view to perceive and keep in mind. I like to visualise OSPF LSAs as jigsaw puzzle items. One jigsaw is nothing however all of them collectively give us the whole image…for OSPF that is the LSDB.

This is the primary LSA Sort:

OSPF LSA Type 1

Every router inside the space will flood a kind 1 router LSA inside the space. On this LSA you can see an inventory with all of the instantly linked hyperlinks of this router. How will we establish a hyperlink?

  • The IP prefix on an interface.
  • The hyperlink kind. There are 4 totally different hyperlink varieties:

Don’t be concerned an excessive amount of concerning the hyperlink varieties for now, we’ll see them later. Needless to say the router LSA all the time stays inside the space.

The second LSA kind (community LSA) is created for multi-access networks:

OSPF LSA Type 2

The community LSA or kind 2 is created for every multi-access community. Bear in mind the OSPF community varieties? The printed and non-broadcast community varieties require a DR/BDR. If so you will note these community LSAs being generated by the DR. On this LSA we’ll discover all of the routers which are linked to the multi-access community, the DR and naturally the prefix and subnet masks.

In my instance above we’ll discover R1, R2 and the DR within the community LSA. We can even see the prefix 192.168.123.0 /24 on this LSA. Last item to say: the community LSA all the time keep inside the space.

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Let us take a look at the third LSA kind:

OSPF LSA Type 3

Sort 1 router LSAs all the time keep inside the space. OSPF nonetheless works with a number of areas and also you in all probability need full connectivity inside the entire areas. R1 is flooding a router LSA inside the space so R2 will retailer this in its LSDB. R3 and R4 additionally have to know concerning the networks in Space 2.

R2 goes to create a Sort 3 abstract LSA and flood it into space 0. This LSA will flood into all the opposite areas of our OSPF community. This fashion all of the routers in different areas will know concerning the prefixes from different areas.

The identify “abstract” LSA could be very deceptive. By default OSPF is not going to summarize something for you. There’s nonetheless a command that allow you to summarize inter-area routes. Check out my OSPF summarization lessonif you have an interest. In case you are wanting on the routing desk of an OSPF router and see some O IA entries you’re looking at LSA kind 3 abstract LSAs. These are your inter-area prefixes!

Time for the fourth LSA kind:

OSPF LSA Type 4On this instance now we have R1 that’s redistributing data from the RIP router into OSPF. This makes R1 an ASBR (Autonomous System Border Router). What occurs is that R1 will flip a bit within the router LSA to establish itself as an ASBR. When R2 who’s an ABR receives this router LSA it’s going to create a kind 4 abstract ASBR LSA and flood it into space 0. This LSA can even be flooded in all different areas and is required so all OSPF routers know the place to search out the ASBR.

What about LSA kind 5? Let’s test it out:

OSPF LSA Type 5

Identical topology however I’ve added a prefix (5.5.5.0 /24) at our RIP router. This prefix shall be redistributed into OSPF. R1 (our ASBR) will care for this and create a kind 5 exterior LSA for this. Do not forget we nonetheless want kind 4 abstract ASBR LSA to find R1. For those who ever tried redistribution with OSPF you may need seen O E1 or E2 entries. These are the exterior prefixes and our kind 5 LSAs.

What about OSPF LSA kind 6? Sort 6 multicast ospf LSA I can skip as a result of it is not getting used. It isn’t even supported by Cisco. We use PIM (Protocol Impartial Multicast) for multicast configurations.

Let us take a look at the final LSA kind, quantity 7:

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OSPF LSA Type 7

Final LSA kind…promised! NSSA areas don’t enable kind 5 exterior LSAs. In my image R1 continues to be our ASBR redistributing data from RIP into OSPF.

Since kind 5 isn’t allowed now we have to think about one thing else. That is why now we have a kind 7 exterior LSA that carries the very same data however isn’t blocked inside the NSSA space. R2 will translate this kind 7 into a sort 5 and flood it into the opposite areas.

Let me summarize the LSA varieties for you:

  • Sort 1 – Router LSA: The Router LSA is generated by every router for every space it’s situated. Within the link-state ID you can see the originating router’s ID.
  • Sort 2 – Community LSA: Community LSAs are generated by the DR. The link-state ID would be the interface IP deal with of the DR.
  • Sort 3 – Abstract LSA: The abstract LSA is created by the ABR and flooded into different areas.
  • Sort 4 – Abstract ASBR LSA: Different routers have to know the place to search out the ASBR. This is the reason the ABR will generate a abstract ASBR LSA which is able to embody the router ID of the ASBR within the link-state ID subject.
  • Sort 5 – Exterior LSA: often known as autonomous system exterior LSA: The exterior LSAs are generated by the ASBR.
  • Sort 6 – Multicast LSA: Not supported and never used.
  • Sort 7 – Exterior LSA: often known as not-so-stubby-area (NSSA) LSA: As you may see space 2 is a NSSA (not-so-stubby-area) which does not enable exterior LSAs (kind 5). To beat this subject we’re producing kind 7 LSAs as an alternative.

The one factor left to do is check out these LSAs in motion…time to configure some routers!

Verification

We will see all of the OSPF varieties within the LSDB, to show this I’ll use the next topology:

OSPF Two Areas 3 routers

It is a easy setup with 3 routers and a pair of areas. I’ve added a few loopbacks so now we have prefixes to take a look at. This is the configuration:

R1(config)#router ospf 1 R1(config-router)#community 192.168.12.0 0334390000 space 0 R1(config-router)#community 1.1.1.0 0334390000 space 0 R2(config)#router ospf 1 R2(config-router)#community 192.168.12.0 0334390000 space 0 R2(config-router)#community 192.168.23.0 0334390000 space 1 R3(config)#router ospf 1 R3(config-router)#community 192.168.23.0 0334390000 space 1 R3(config-router)#community 3.3.3.0 0334390000 space 1

Let’s begin by wanting on the LSDB of R1:

R1#present ip ospf database OSPF Router with ID (1.1.1.1) (Course of ID 1) Router Hyperlink States (Space 0) Hyperlink ID ADV Router Age Seq# Checksum Hyperlink rely 1.1.1.1 1.1.1.1 30 0x80000003 0x004CD9 2 2.2.2.2 2.2.2.2 31 0x80000002 0x0048E9 1 Web Hyperlink States (Space 0) Hyperlink ID ADV Router Age Seq# Checksum 192.168.12.2 2.2.2.2 31 0x80000001 0x008F1F Abstract Web Hyperlink States (Space 0) Hyperlink ID ADV Router Age Seq# Checksum 3.3.3.3 2.2.2.2 17 0x80000001 0x00D650 192.168.23.0 2.2.2.2 66 0x80000001 0x00A70C

Through the use of the present ip ospf database we are able to have a look at the LSDB and we are able to see the kind 1 router LSAs, kind 2 community LSAs and the kind 3 abstract LSAs right here. What else do we discover right here?

  • Hyperlink ID: That is what every LSA.
  • ADV router: the router that’s promoting this LSA.
  • Age: The utmost age counter in seconds. The utmost is 3600 seconds or 1 hour.
  • Seq#: Right here you see the sequence quantity which begins at 0x80000001 and can enhance by 1 for every replace.
  • Checksum: There’s a checksum for every LSA.
  • Hyperlink rely: It will present the whole variety of instantly linked hyperlinks and is simply used for the router LSA.

In order that’s LSA kind 1,2 and three. To indicate you quantity 4 and 5 I’ve to make some modifications:

OSPF Two Areas 3 routers ABR ASBRTo perform this I’ll redistribute one thing on R1 into OSPF.

R1(config)#1 . loopback interface R1(config-if)#ip deal with 11.11.11.11 255.255.255.0 R1(config-if)#exit R1(config)#router ospf 1 R1(config-router)#redistribute linked subnets

I created a further loopback interface and configured an IP deal with. Then I am telling OSPF to redistribute the instantly linked interfaces into OSPF. Let us take a look at the LSDB of R2 and R3:

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