Tuesday, March 18, 2008

OSPF - LSA Types



OSPF - LSA Types:

Type 1 - Sent by routers within the Area, including the list of directly attached links. Does not cross the ABR or ASBR.

Type 2 - Generated for every “transit network” within an area. A transit network has at least two directly attached OSPF routers. Ethernet is an example of a Transit Network. A Type 2 LSA lists each of the attached routers that make up the transit network and is generated by the DR.

Type 3 - The ABR sends Type 3 Summary LSAs. A Type 3 LSA advertises any networks owned by an area to the rest of the areas in the OSPF AS. By default, OSPF advertises Type 3 LSAs for every subnet defined in the originating area, which can cause flooding problems, so it´s a good idea to use a manual summarization at the ABR.

Type 4 - It announces the ASBR address, it shows “where” the ASBR is located, announcing it´s address instead of it´s routing table. It works +/- like a Default-Gateway.

Type 5 - Announces the Routes learned through the ASBR

If we do not have any ASBR, there´s no LSA Types 4 and 5 in the network.

A good (and free) document for OSPF is the Cisco´s OSPF Design Guide, which can be found at:


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