As a CCNA / CCNP candidate you are expected to understand how to set and interpret the different types of OSPF LSA’s.
During your career as a Cisco network engineer you will have to deal with setting and manipulating the many different types of OSPF LSA types.
Within an OSPF Area every router must maintain the same link state data base. The link state data base is constructed by the routers by the LSA (Link State Advertisements) it receives from other routers in the area.
From the Link State Data Base (LSDB) the router will run the shortest path first algorithm to calculate the best path based on cost to any network.
OSPF LSA types can be confusing but they all play a vital role within their particular scope of operation . For example some LSA types remain within an area, or advertise routes between areas and advertise routes from outside of OSPF completely.
For the CCNA exam your are expected to know of LSA type 1 and 2
LSA Type 1: Router LSA
Each and every router operating OSPF will generate it’s own Type 1 LSA to present itself. It will create an individual Type 1 LSA for each area it connects to.
The Type 1 LSA will contain the Router ID along with a list of all the interface IP addresses on the router that are within the area that the LSA is advertising into, example if a router is a member of two Areas 0 and 1 the LSA advertised into area 1 will contain a list of all interface IP addresses that are also members of area 1 but none which are members of Area 0. This type of LSA will flood within the same area, the neighbors receiving the Type 1 LSA will forward it to their neighbors until all routers in the area have a copy the advertisement.
Info which is listed inside the LSA
1. Lists the routers interface ip address/mask and interface cost (If no DR has been elected)
2. List the neighbors RID if there is no DR
3. Lists the IP address of the DR and a notation that the link attaches to a transit network.
4. The advertising router’s RID
LSA type 1 never traverse Area border routers. ABR’s create multiple LSA type 1’s, one per area.
