SeventhBit

 Re-call that OSPF maintains an identical copy of the link state database on every router within an area by flooding LSA's. The maintenance of this database and flooding can sometimes overburden the CPU resources on routers, particularly when there are many routers in the OSPF area. To overcome this issue OSPF has the concept of areas. An OSPF area is just a logical grouping of OSPF routers, breaking one domain into smaller sub-domains, to reduce the flooding domain. Breaking up the single domain into smaller ones accomplishes a couple of things: Hides topology information for routers outside of the domain Reduces the impact on the routers memory as adding areas reduces the size of the database A smaller link state database also reduces the load on the CPU Areas are identified with a 32-bit ID called the Area ID. The area can be expressed as either dotted decimal or a decimal number. Since there can be multiple areas, what do we call the routes from each area? Intra-Area Routes : T...

 *There are more LSA's than what I will cover but for most of us we will only deal with 6 of the LSA's. The nature of a Link State Routing Protocol is that each router has a full "map" of the network (within an area). In order for each router to have a full map of the network it needs to know about every other routers links and the states of those links and if there are any other OSPF neighbors on those links. This is the purpose of the Type 1 LSA - The Router LSA . The mechanism by which OSPF accomplishes this is by flooding. Type 1 LSA's are flooded to all routers within the area they were originated. Router# show ip ospf database router             OSPF Router with ID (0.0.0.2) (Process ID 100) Router Link States (Area 0)   LS age: 397   Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 0.0.0.1   Advertising Router: 0.0.0.1   LS Seq Number: 80000005   Checksum: 0x85C8   Length: 48 ...