Introduction

Link State Advertisements are used to propagate routing information within the OSPF routing domain.

LSA Types

• Each LSA has a different Link ID and Link Data. See below:

Type 1 (Router Link LSA)

• Describes the state of each connected link.
• Only flooded within the local area.

R1#sh ip ospf database router

            OSPF Router with ID (141.34.200.1) (Process ID 1)

                Router Link States (Area 0)

  LS age: 774
  Options: (No TOS-capability, DC)
  LS Type: Router Links                <- LSA Type 1
  Link State ID: 141.34.200.1            <- Link ID, RID of this router
  Advertising Router: 141.34.200.1        <- Link ID, RID of this router
  LS Seq Number: 80000003
  Checksum: 0x874B
  Length: 36
  Number of Links: 1

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 192.168.1.0    <- Link ID
     (Link Data) Network Mask: 255.255.255.0        <- Link ID
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

Type 2 (Net Link LSA)

• Only generated by the DR.
• Represents DR's (pseudo node) attached routers.
• Only flooded within the local area.

Type 3 (Summary Net Link LSA)

• Only generated by the ABR.
• Describes destination of an area it's representing.
• In stub type areas, default routes are represented as network LSA replacing individual routes.
• Flooded to other areas that don't originate these advertised LSAs.
• See ospf-loop-avoidance-summary for more information on loop avoidance.

Type 4 (ASB Summary Link LSA)

• Generated by the Area Border Router not the ASBR.
• Describe the route to the ASBR.
• In the same area, the ASBR routers are type 1 then converted by ABR to type 4.
• This route is not inserted in the router's routing table, only database, used for calculation of external routes.
• This type of routes are host routes.

Type 5 (Type-5 AS External LSA)

• Originated by ASBR, advertise a destination outside of the OSPF domain.
• These LSAs are not associated with any areas.
• Flooded throughout the OSPF domain (with exception of stub type areas and virtual links).
• If the forwarding address is 0.0.0.0, it will use the advertising router as the destination.
• If the forwarding address is not full of 0s, that means the ASBR is advertising an external route that's should be forwarded to another host not running OSPF which is on the same subnet. This is similar to ICMP redirect or BGP next-hop on the same subnet.
• There are two types of External LSA: Type 1 (E1) and Type 2 (E2).
  • Type 1 (E1) - consider the total cost to the external destination.
  • Type 2 (E2) - consider the injected cost of the advertised route. If there is a tie, consider the forwarding metric which is the cost to the ASBR. If the forwarding metric is the same use both routes. One exception is if the LSA has a forwarding address. In that case pick the router with highest ID to advertise and win the route.
• LSA Type 4 has to be advertised to reach external types. This could be a problem when Summary loop prevention, stops type 4s.

R2#sh ip ospf database external 

            OSPF Router with ID (10.24.0.2) (Process ID 1)

        Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 648
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 34.34.34.34 (External Network Number )
  Advertising Router: 10.23.0.3
  LS Seq Number: 80000002
  Checksum: 0x9E54
  Length: 36
  Network Mask: /32
    Metric Type: 2 (Larger than any link state path)
    MTID: 0 
    Metric: 20 
    Forward Address: 0.0.0.0
    External Route Tag: 0

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 439
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 34.34.34.34 (External Network Number )
  Advertising Router: 10.24.0.4
  LS Seq Number: 80000002
  Checksum: 0x9060
  Length: 36
  Network Mask: /32
    Metric Type: 2 (Larger than any link state path)
    MTID: 0 
    Metric: 20 
    Forward Address: 0.0.0.0
    External Route Tag: 0

R2#sh ip route
O E2     34.34.34.34 [110/20] via 10.24.0.4, 00:36:29, GigabitEthernet1/0
                     [110/20] via 10.23.0.3, 00:36:29, GigabitEthernet0/0

Type 6 (Multicast)

• Not implemented in IOS.

Type 7 (NSSA LSA)

• LSA originated by ASBR in NSSA areas.
• Can be translated by the ABR to LSA type 5 to cross the NSSA area boundary.
• Flooded only in the NSSA area.
• There are two types of NSSA LSA: Type 1 (N1) and Type 2 (N2), which are similar to external types of LSA 5.

Type 8 (External Attributes )

• Not implemented in IOS.
• It was proposed as an replacement to iBGP for OSPF to carry attribute info over the OSPF domain.

Type 9 (Opaque Link Local Scope)

• Opaque LSA with a link local scope.

Type 10(Opaque Area Local Scope)

• Opaque LSA with an area-local scope. Can only be advertised within the same area.
• MPLS traffic engineering

Type 11(Opaque AS Local Scope)

• Opaque LSA with an AS range. Can be advertised throughout the OSPF domain, with the exceptions of stub and NSSA areas. This is similar to LSA type 5s.

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