OSPF LSDB and LSA

OSPF Link-State Database (LSDB)

• Each router stores it’s LSA in it’s LSDB.
• Each router within the same area should have the same LSDB information.
• SPF algortithm for each area it is connected to.
• The router thinks it’s the root of the tree and draws branches via SPF
• LSAs in OSPF LSDB are pieces that the SPF process uses.
• OSPF poisons LSA by setting max-age (3600).

OSPF types of Link State Advertisement (LSA)

• Type 1: Router LSA
• Type 2: Network LSA
• Type 3: Summary LSA
• Type 4: Summary ASBR LSA
• Type 5: Autonomous system external LSA
• Type 6: Multicast OSPF LSA
• Type 7: Not-so-stubby area LSA
• Type 8: External attribute LSA for BGP

Type 1: Router LSA

show ip ospf database router (Router Link states)
show ip ospf database router 1.1.1.1

• Every router creates one Type 1 LSA and floods it throughout the same OSPF Area.
• Router creates on Type 1 LSA for all links in each area.
• LSA is stored in it’s own database and floods it out (224.0.0.5/224.0.0.6 DR).
• Can’t be filtered, can’t be summerized.
• Can’t go between areas, they stop at ABR.
• All Type 1 LSAs are listed under Router Link States (Area x). Show ip ospf database (router)
• OSPF identifies a Type 1 LSA using a 32bit link-state identified (LSID).

Type 1 LSA information about the attached links:

• Interface with no neighbors:
  • subnet/mask is advertised
  • link connected to a stub network
• Interface connected to DR:
  • the IP adress of the DR is send
    
  • Link connected to a transit network
• Interface with no DR:
  • it lists the neighbor’s RID
  • Link connected to another router (point to point)
  • stub link so the subnet+mask is send

Type 2: Network LSA

show ip ospf database (Network Link states)
show ip ospf database network

• Type 2 LSA is generated for multi-access Networks (Needs a DR).
• Type 2 LSA is generated by DR.
• Contains the subnet/mask,
• Contains the neighbors,
• Once a DR is elected it says as a DR (so the SPF tree doesn’t have to be rebuild).
• If the DR fails, the BDR takes over.

Type 3: Summary LSA

show ip ospf database (Summary Net Link States)
show ip ospf database summary

• Type 3 LSA are used when there are multiple areas.
• Type 3 LSA sends the subnet(s)/mask(s) from one area to another area.
• Areas are used to reduce memory and CPU resource.
• ABRs are used to connect different areas.
• Type 1 LSA and Type 2 LSA are not adverstived from one area to another via ABRs.

Type 4: Summary ASBR LSA

• Only seen with an ASBR(0) in your network
• Send from the ABR(0/1) (can reach the ASBR(0) through me) to his area
• Can’t be flooded in the next area
  • ABR(1/2) makes a NEW Type 4 LSA
  • Can reach ASBR(0) through me ABR(1/2)

Type 5: AS Summary external LSA

• Created with redistribution
• Only send from ASBR

Type 7: Not-so-stubby area LSA

• Created with redistribution within an NSSA
• Only send from ASBR