Link State Advertisement

A Link State Advertisement (LSA ) is a message with information about the local topology of a network. It is generated by each router in the network and contains information about adjacent routers (that is, both have an interface in the same network ) and adjazente router ( that is, they exchange routing information ). After the creation of an LSA it is distributed by flooding the entire routing domain. The totality of all LSAs of all routers and networks forms the link state database (LSD ), an adjacency database, the basis of which the best path can be determined for a data packet through the network. The LSAs can be sent again at regular intervals to keep track of changes in the network in the LSD.


  • Router LSA (Type 1): A router sends for each OSPF area to which he is assigned a LSA. It describes the collected states of the router's interfaces. It is only being flooded into the area in which the respective interface is located.
  • Network LSA (Type 2): A network -LSA is sent in a broadcast network, and describes the routers that are connected to the network. There are multiple routers connected together in a broadcast network (eg Ethernet), so network LSAs are sent only from an individual known as Designated Router in order to avoid unnecessary traffic. The Designated Router and the Backup Designated Router are negotiated at the beginning of the network setup. The designated router is the router with the highest router ID of the backup designated router has the second highest ID. The router ID is globally unique in all areas.
  • Summary LSA (type 3 or 4 ): A Summary -LSA sent by the area border routers ( ABR ), they sit at the boundaries between two OSPF areas. An ABR is thus connected to at least two areas and can the information he has learned from an area spread to another area. This is useful so that an area known routes in networks of other areas, the metrics for these routes to be aggregated, ie the development of other areas need not be known, but only the cost of the routing in a network in another area. Summary LSAs of type 3 are exchanged between OSPF areas, Summary LSAs of type 4 describe routes to AS boundary router, so routers that sit on the border areas, where other routing protocols are used. Thus, the ASBR is announced as the next hop for the route in OSPF external areas.
  • Autonomous system external LSA ( Type 5 ): AS-external LSAs are ASBR in all areas being flooded (except in stub networks) that contain information about destinations outside the autonomous system. The metrics in the external networks are either added to the OSPF metrics, or only the external metrics propagated from the ASBR.

Link State ID

The Link State ID identifies the instance that is being described by the LSA. The Link State ID is the question "What describes the LSA? " characterized. The following table shows the possible assignments for the Link State ID, depending on the type of LSA:

LSA header

  • The LS age field is expressed in seconds and increases per hop to InfTransDelay. InfTransDelay takes aging of link state information prior to the measure of the latency that is expected at the interface.
  • LS type indicates one of the five types. See section types.
  • The Link State ID identifies the instance that is being described by the LSA. See Section Link State ID.
  • Advertising router contains the router ID of the router that sends the LSA.
  • Checksum is the checksum in the calculation of the LS Age field is omitted. The reason for this is that the LS age field is modified at each hop.
  • The Length field indicates the length, also here the LS Age field is omitted.