Difference between OSPF ABR and ASBR

I wanted to confirm my understanding on OSPF ABR and ASBR, below is what I found and understand of the both also below is the LSA types which is always good to know.


  • LSA Type 1 (Router LSA)
    Router Link State Advertisements Type 1 are generated by every router for each link that belongs to an area. They are flooded only inside of area to which they belong. Link ID of this LSA is the Router ID of the router that generated it.
  • LSA Type 2 (Network LSA)
    Generated by Designated Router (DR) for multiaccess networks and describe the routers that are connected to that segment. They are sent inside the area to which the network segment belong. The Link ID is the interface ip address of the Designated Router which describe that particular segment.
  • LSA Type 3 (Summary LSA)
    Type 3 LSAs are generated by Area Border Routers (ABRs). In type 3 LSAs are advertised networks from an area to the rest of the areas in AS. Advertised networks in Type 4 LSA can be summarized or not. The link-state id used by this LSA is the network number advertised.
  • LSA Type 4 (ASBR Summary LSA)
    Generated by ABRs, this type of LSA contain routes to ASBRs. Link id used is router ID of the ASBR described. Are not flooded in stub areas.
  • LSA Type 5 (External LSA)
    Autonomous system external LSAs are generated by ASBRs and contain routes to networks that are external to current AS. Link-state ID is network number advertised in LSA. Type 5 LSAs are not flooded inside any stub areas.
  • LSA Type 6 (Multicast LSA)
    This type of LSA is used for multicast applications.
  • LSA Type 7 (NSSA External LSA)
    Type 7 LSA allow injection of external routes through Not-so-Stubby-Areas (NSSA). Generally external routes are advertised by type 5 LSA but they are not allowed inside any stub area. That’s why Type 7 LSA is used, to trick OSPF. Type 7 LSA is generated by NSSA ASBR and is translated into type 5 LSA as it leaves the area by NSSA ABR, which is then propagated throughout the network as type 5 LSA.
  • LSA Type 8 (External attributes LSA for Border Gateway Protocol (BGP))
    Used to work with BGP.
  • LSA Type 9
    Graceful restart, allows a routing device undergoing a restart to inform its adjacent neighbors and peers of its condition. During a graceful restart, the restarting device and its neighbors continue forwarding packets without disrupting network performance. Because neighboring devices assist in the restart (these neighbors are called helper routers), the restarting device can quickly resume full operation without recalculating algorithms.
  • LSA Type 9,10,11 (Opaque LSAs)

Nice CBT Nuggets : Key OSPF Areas and LSA Types


An area border router (ABR) is a router that is located near the border between one or more (OSPF) areas. It is used to establish a connection between backbone area 0 and the other OSPF areas. It is a member of both the main backbone area 0 network and the specific areas to which it connects, so it stores and maintains separate routing information or routing tables regarding the backbone and the topologies of the area to which it is connected.

Below are the types of LSA’s you would see from an ABR:

  • LSA Type 3 (Summary LSA)
  • LSA Type 4 (Summary LSA)


An area Autonomous System Border Router (ASBR) is a router that is located near the border between one or more (OSPF) areas. It is primarily the same as the ABR except this ABR is also connected to other routing systems such as BGP or EIGRP as an example which makes it an ASBR if it is advertising those routes into ospf.

Below are the types of LSA’s you would see from an ASBR:

  • LSA Type 3 (Summary LSA)
  • LSA Type 4 (Summary LSA)
  • LSA Type 5 (External LSA)