OSPF LSA Types

OSPF LSA Types

Certiology’s OSPF LSA Types tutorial explains everything about OSPF LSA types for CCNA, CCNP level students.

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

OSPF being an Open / standard Protocol works for each vendor router.

Cisco Router only supports LSA 1, 2, 3,4,5,7 and does not support other LSAs

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

See the below table for a brief glance about LSA Types.

LSA Type Description

1

 Router LSA

2

 Network LSA

3 or 4

 Summary LSA

5

 Autonomous System External LSA

6

 Multicast OSPF LSA

7

 Defined for Not-So-Stubby-Areas

8

 External Attributes LSA for BGP

9, 10, 11

 Opaque LSA

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 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 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 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 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.

To learn OSPF in details, please look at our complete step by step OSPF configuration tutorial

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