Link State Advertisement

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A Link State Advertisement (LSA) is a message containing information about the local topology of a network. It is generated by every router in the network and contains information about neighboring routers (i.e. both have an interface on the same network) and neighboring routers (i.e. they exchange routing information). After an LSA has been created, it is distributed through the entire routing domain using flooding . The totality of all LSAs of all routers and networks forms the Link State Database (LSDB), an adjacency database that can be used to determine the most favorable path for a data packet through the network. The LSAs can be sent again at regular intervals in order to keep track of changes in the network in the LSDB.

Types

  • Router LSA (type 1) : A router sends an LSA for each OSPF area to which it is assigned. It describes the collected states of the router's interfaces. It is only flooded in the area in which the respective interface is located.
  • Network LSA (Type 2) : A network LSA is sent in broadcast networks and describes the routers that are connected to the network. If several routers are interconnected in a broadcast network (e.g. Ethernet ), network LSAs are only sent by a single 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, 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 is sent by Area Border Routers (ABR), these are located at the borders between two OSPF areas. An ABR is therefore connected to at least 2 areas and can disseminate information that it has learned from one area in another area. This is useful so that an area knows routes in networks of other areas; the metrics for these routes are aggregated, i. H. the structure of other areas does not have to be known, only the costs for routing to 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 routers, i.e. routers that are located on the border to areas in which other routing protocols are used. The ASBR is thus announced as the next hop for the route in areas outside the OSPF.
  • Autonomous-System external LSA (type 5) : AS-external LSAs are flooded by ASBR in all areas (except in stub networks) and contain information about targets outside the autonomous system. The metrics in the external networks are either added to the OSPF metrics or only the external metrics are propagated from the ASBR.

Link State ID

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

LSA type description Link State ID
1 Point-to-point connection to another router Router ID of the sending router
2 Connection to a transit network (broadcast) IP address of the designated router
3 Connection to a stub network IP network address of the target network with subnet mask
4th Connection to an ASBR as a gateway Router ID of the described ASBR
5 AS-external targets IP network address of the target network with subnet mask
6th Group Membership LSA Multicast address

LSA header

Link State Advertisement Header
0 1 2 3 4th 5 6th 7th 8th 9 10 11 12 13 14th 15th 16 17th 18th 19th 20th 21st 22nd 23 24 25th 26th 27 28 29 30th 31
LS Age Options LS type
Link State ID
Advertising router
LS Sequence Number
LS checksum Length
  • The LS Age field is specified in seconds and increased by InfTransDelay per hop. InfTransDelay aging link state information with the amount of latency that is forecast at the interface.
  • LS Type indicates one of the 5 types. See the Types section .
  • The Link State ID names the instance that is 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 for which the LS Age field is left out. The reason for this is that the LS Age field is modified on every hop.
  • The Length field indicates the length; the LS Age field is also omitted here.

See also

literature