Christmas Calendar: OSPF LSAs

Day 9️⃣: LSAs are the building blocks of OSPF. Each LSA describes a piece of the OSPF routing domain. LSAs are recorded in the link-state database.

This diagram briefly describes the distinct types of LSAs and illustrates the flooding scope.

Day 1️⃣0️⃣: All LSAs begin with a common 20-byte header. This header contains enough information to uniquely identify the LSA and to determine the most recent instance, if multiple instances of the LSA exist.

Day 1️⃣1️⃣: Each router in an area originates a router LSA. The LSA describes the state and cost of the router’s links. All of the router’s links to the area must be described in a single router LSA. The LSA is flooded throughout that area, and no further.

Day 1️⃣2️⃣: A network LSA is generated for every transit network. This LSA is originated by the Designated Router and it describes all routers attached to the network (including the DR itself). The network LSA is flooded inside the area only.

Day 1️⃣3️⃣: Summary LSAs are created by ABRs and they describe inter-area destinations.

NOTE: Summary LSAs do NOT summarize routes. The term ’summary’ comes from detailed topological information being lost with inter-area routing (becoming a ”summary”).

Day 1️⃣4️⃣: ASBR Summary LSAs advertise the presence of an ASBR and the cost to reach it. Like Summmary LSAs, ASBR Summary LSAs are originated by ABRs and have an identical format. The link state ID field in the ASBR Summary LSA is the router ID of the ASBR.

Day 1️⃣5️⃣: AS External LSAs describe routes to external destinations - one LSA for each network. These LSAs are originaged by ASBRs and flooded throughout the entire OSPF domain (excluding special area types).