Sunday, 18 September 2016

OSPF - Part 4

OSPF Link State Advertisement (LSA)

OSPF uses a LSDB and fills this LSAs. OSPF have many different type of LSA.

LSA Type 1 (Router LSA)

Each router within the area will flood a type 1 router LSA. In this area we will find list with all the directly connected  links of this router. How do we identify a link?

   1.) The IP Prefix on an interface
   2.) The link type. There are four type of link


LSA type 1 always stays within the area. It generated by each router for each area it is located.

LSA Type 2 (Network LSA)

The network LSA is created for each multi-access network. Remember the OSPF network type?
 The broadcast and non-broadcast network types require a DR/BDR. If this is the case we will see these network LSAs being generated by the DR. 
In this LSA we will find all the routers that are connected to the multi-access network, the DR and of course the prefix and subnet mask.

The network LSA is stay within the area.  Network LSAs are generated by the DR.

LSA Type 3 (Summary LSA)

Type 1 router LSA always stay within the area. OSPF however works with multiple areas and we probably want full connectivity within all of the areas. R1 is flooding a router LSA within the area so R2 will store in its LSDB. R3 and R4 need to know about the networ.

 In area 2.
R2 going to create Type 3 summary LSA and flood it into area 0. This LSA will flood into all the other areas of our OSPF network. This way all the routers in other areas will know about the prefixes from the areas.

The name “summary” LSA is very misleading. By default OSPF is not going to summarize anything for you.
LSA Type 4 (ASBR Summary LSA)

In this example we have R1 that is redistributing information from the RIP router into OSPF.
 This makes R1 an ASBR. What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. 
When R2 who is an ASBR receives this router LSA it will create  a type 4 summary  ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so all OSPF routers know where to find the ASBR.

LSA Type 5 (External LSA)


Some topology but I’ve added a prefix ( at our RIP router. This prefix will be redistributed into OSPF. R1 (our ASBR) will take care of this and create a type 5 external LSA for this.

This type of LSA generated by ASBR.

LSA Type 6  (Multicast LSA)

Note:    LSA type 6 is being not used. It’s not even supported by CISCO. We use PIM (Protocol Independent Multicast) for multicast configuration.


We can see area 1 is a NSSA (not-so-stubby-area) which doesn’t allow external LSAs (type 5). To overcome this issue we are generating type 7 LSAs. 

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