Sunday, 5 February 2012

OSPF LSA Types & Default Routes

LSA 1 - Router (sent by all routers) and tells about the connected links. It is used to build the graph of which routers are connected to each other inside an individual area

LSA 2 - Network (sent by DR for transit network)

The results of LSA 1 and 2 are the "O" routes in the routing table. These LSA's aren't allowed to cross an area boundary.

LSA 3 - Net Summary (send by ABR, one for each subnet). The goal of LSA 3 is to hide the topology details of one area from another, so SPF calculations in one area do not necessarily affect other areas.

The result of LSA 3 is the "O IA" routes in the routing table. The ABR takes the LSA1 and LSA2 its heard about from its area and summarises them into LSA 3s and sends them into Area 0.

LSA 4 - ASBR Summary (/32 host address of ASBR and is sent by the ABR). This LSA is used by routers to decide how far away the ASBR is from it.

LSA 5 - AS External (external routes. This has a metric type embedded in it which specifically says E1 or E2)

Type E1 - The cost of E1 routes is the cost of the external metric with the additional of the internal cost within OSPF to reach that network. This route consists of LSA 4 + LSA 5. LSA 5 contains the cost of the external metric and the router adds the cost of getting to LSA 4.

Type E2 (default) - The cost of E2 routes will always be the external metric, the metric will takes no notice of the internal cost to reach that network. As a result this route consists of LSA 5 only.

E1 type is preferred over a E2 type.

LSA 6 is for Multicast OSPF, which is a legacy protocol and not used.

LSA 7 - NSSA External. NSSA External routes are similar to regular external routes, where they are originated by the ASBR, but this only applies to NSSA areas. Type 7 LSAs are converted to type 5 LSAs when the ABR of the NSSA originates them into area 0.  The result of the LSA 7 is the "N1" or "N2" routes in the routing table.

Since there could be multiple ABRs in NSSA area, only one is elected to perform 7-to-5 LSA translation – otherwise the routing information will loop back in the area, unless manual filtering implemented in the ABRs (which is prone to errors). Translating ABR is elected based on the highest Router-ID, and may not be on the optimum path toward the advertising ASBR.


Consider the network below. In area 1 routers R1, R4 and R6 all send out LSA 1s. In addition the DR (lets assume R6) will also send out a LSA2 for the network connecting R1 R4 and R6. R1 and R4 will generate LSA3 for each network in area 1 and also it will generate LSA 1 for each link in Area 0.