OSPF LSA TYPES

OSPF uses LSAs or Link state Advertisements to share information of each network and populate the LSDB (Link State Database)

Below table shows OSPF LSA Types –

ospf-lsa-types

 

LSA Type 1: Router LSA

  • This LSA is flooded by every router within an area
  • The LSA consists of information about directly connected links
  • It is identified by the Router ID or the originating router
  • It floods within an area and the LSA Type 1 does not cross the ABR.
  • We see this as “O” routes in the routing table
  • LSA 1 – O, Router LSA contains all Link IDs – network, generated by every router and is local to the area.

 

LSA Type 2: Network LSA

  • Generated by the DR Router on the broadcast network
  • It includes the network ID, subnet mask and also the list of attached routers in the transit
  • In OSPF we can have one Network LSA or Type 2 LSA for each transit broadcast of NBMA Network
  • This LSA also can be flooded only within the area and cannot cross the ABR
  • We see this as “O” routes in the routing table
  • LSA 2 – O, Network LSA contains all routers attached to the segment, generated by DR and is local to the area

LSA Type 3: Summary LSA

  • The Type 3 LSA is used to share the network information with other areas. Inter-area ( IA)
  • These LSAs are advertised by the ABR Router
  • These LSA are represented in the routing table as “OIA” routes
  • To flood the information through the Autonomous System, they are regenerated by the ABRs following subsequently
  • LSA 3 – O IA, Network Summary LSA describes network from another area, generated by ABR and is propagated between areas

LSA Type 4: Summary ASBR LSA

  • This LSA type is also known as summary LSA is used to advertise about the ASBR to other area in the same autonomous System.
  • It is generated by the ABR of the originating area.
  • These type of LSAs are flooded throughout the Autonomous System
  • These LSA will contain the Router ID of the ASBR only
  • LSA 4 – O IA, Summary ASB Link States, generated by the ABR and is propagated between areas

LSA Type 5: External LSA

  • These are advertised by the ASBR only and also owned by the ASBR
  • These LSAs are used to advertise networks from other Autonomous systems
  • These LSAs are flooded throughout the entire Autonomous System
  • The advertising router ID is not changed throughout the AS when it is propagating.
  • An LSA Type  4 is used to find an ASBR
  • Routes are not summarized by default
  • LSA 5 – O E1, O E2, External Link States, generated by ASBR and is propagated

LSA Type 6 – Multicast OSPF LSA

  • These are used when routing using multicast (the MODPF Routing protocol)
  • These LSAs are not supported by Cisco Routers

 

LSA Type 7 – External LSA

  • As LSA Type 5 is not allowed in NSSA Areas, a solution to that was thought as LSA 7.
  • It carries the similar information as a Type 5 LSA but this is not blocked in the NSSA Area.
  • When it reached the ABR it is translated back to Type 5 LSA and flooded into other areas.
  • LSA 7 – O N1, O N2, NSSA External Link States, generated by ASBR into NSSA area and is propagated into area 0 as E1 or E2

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