Friday, 9 November 2018

OSPF Link State Advertisement (LSA)



OSPF uses a LSDB (link state database) and fills this with LSAs (link state advertisement) LSA hold the topology information and LSA held in the LSDB. Instead of using 1 LSA packet OSPF has many different verities of LSAs.

Here is the list:
  
   LSA Types
                          Description
Type 1
Router LSAs
Type 2
Network LSAs
Type 3 or 4
Summary LSAs
Type 5
Autonomous system external LSAs
Type 6
Multicast OSPF LSAs
Type 7
Defined for not-so-stubby areas
Type 8
External attribute LSAs for
 Border Gateway Protocol (BGP)
Type 9, 10, and 11
Opaque LSAs

LSA Type 1: Router LSA
Type 1 router LSA is generated by internal routers. You can identify it “O” routes in the routing table, Type 1 LSA flood within its area only; does not cross ABR. In the type 1 LSA you will find a list with all the directly connected links of this router.


LSA Type 2: Network LSA
The network LSA or Type 2 creates for each multi-access network. LSA Type 2 is advertised by DR of the transit network and flood within its area only, network LSA does not cross ABR. You can identify  “0” routes in the routing table. In Type 2 LSA you will find all the routers that are connected to the multi-access network, the DR and the prefix and subnet mask.


LSA Type 3: summary LSA
 The summary LSA is created by the ABR to sending update from one area to another area. You can identify is “OIA” routes in the routing table. Summary LSA are used to flood network information subnet in the Origin area and cost but does not topology data.


LSA Type 4: ASBR summary LSA
Type 4 LSA is used to advertise as ASBR to all other area in the autonomous system. ASBR summary LSA include the router ID of the ASBR in the link-state ID field.


LSA Type 5: External LSA
Type 5 LSA External LSA also known as autonomous system external LSA, AS external LSA created by ASBR for external routes redistributed into OSPF (LSA 5), external LSAs are advertised and owned by the originating ASBR. External LSAs flood through the entire autonomous system, it advertising router ID (ASBR) is unchanged throughout the autonomous system. Type 4 LSA is needed to find the ASBR.


LSA-6 Multicast OSPF LSA
Type 6 LSA are used in multicast routing (MOSPF routing protocol) multicast LSA (Cisco routers do not support)

LSA Type 7: NSSA external
Type 7 LSA is created by ASBR inside an NSSA, instead of a type 5 LSA. Type 7 LSA flooded only within its area of origin converted to Type 5 LSA on an ABR toward other area.
·         N1- metric increase as it is passed through the network
·         N2- metric doesn’t  increase  (default)


LSA Type 8: external attribute LSA
External attribute LSA is created by ASBR during BGP to OSPF redistribution to preserve BGP attributes of redistribution network.

LSA Type 9, 10, 11
·         Used in OSPF and BGP inter-networking




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