Routers have to agree on certain parameters (such as timers and being on a common subnet) before they can become neighbors. After its neighbor routers are discovered, the router determines whether it should exchange routes with those neighbors. (If they decide to do so, they become adjacent.) If they are adjacent, they begin to exchange information about networks (links) it knows about, using messages called link state advertisements (LSA). After exchanging all routes, the routers send out updates only when there is a change, and they send information only for that affected link, not the entire routing table.
LSA 1 — Router LSA — Generated by all routers in an area to describe their directly attached links (Intra-area routes). These do not leave the area.
LSA 2 — Network LSA — Generated by the designated router (DR) of a broadcast or non-broadcast segment to describe the neighbors connected to the segment. These do not leave the area.
LSA 3 — Summary LSA — Generated by the area border router (ABR) to describe a route to neighbors outside the area. (Intra-area routes)
LSA 4 — Summary LSA — Generated by the area border router (ABR) to describe a route to an autonomous system boundary router (ASBR) to neighbors outside the area.
LSA 5 — External LSA — Generated by autonomous system boundary router (ASBR) to describe routes redistributed into the area. These routes appear as E1 or E2 in the routing table. Listed as E1 or E2 in the route table.
LSA 6 — Multicast LSA — Not supported on Cisco routers
LSA 7 — Not-So-Stubby Area (NSSA) External LSA — Generated by an autonomous system boundary router (ASBR) inside a not-so-stubby area to describe routes redistributed into the not-so-stubby area. LSA 7 is translated into LSA 5 by area border router (ABR). Appear as N1 or N2 in the routing table.
- Cisco CCNA Routing and Switching 200-120 Exam Cram, Fourth Edition by Michael Valentine and Keith Barker
- OSPF LSA Types document originally located at http://www.ccietalk.com/2008/07/13/ospf-lsa-types and retrieved on December 6, 2017 from https://learningnetwork.cisco.com/docs/DOC-7994