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OSPF LSA Types

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Deep understanding of OSPF Link State Advertisments (LSAs) is crucial to understand OSPF behavior. LSAs build up the OSPF link state database (LSDB), and they are exchanged between OSPF enabled nodes. Here you have the list of OSPF LSAs and a short description.

LSA Type 1 (Router LSA)

Router Link State Advertisments 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 (Summary LSA)

Generated by ABRs, this type os 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 throug 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 9,10,11 (Opaque LSAs)

For future use.

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