OSPF Difference inter-area and intra-area routes

[mahesh18]

         Hi All,

Need to know understand difference between inter and intra area routes

Thanks

MAhesh

[John Blakley · Accepted solution]

Intra Area routes are the "O" routes that you see in the routing table. These routes are routes that are in the same area. Inter-area routes are the "O IA" routes that are learned in different areas.

If you have 2 areas, area 0 and 1, the routes that are originated in Area 0 and learned by other routers in Area 0 are intra-area routes. These same routes from Area 0 learned by routers in Area 1 are seen as inter-area routes by routers in Area 1. As well as the routes from Area 1 when learned by Area 0 are seen as inter-area routes in Area 0.

HTH,

John

[Richard Burts · Accepted solution]

Mahesh

Intra area routes are routes that are originated and learned in the same area. OSPF uses LSA type 1 and type 2 to advertise intra area routes. OSPF uses the type 1 and type 2 LSA to build the topology map of the area and to determine the best routes within the area. As John tells us in the route table the intra area area routes are identified as O in the routing table.

Inter area routes are routes that originated in some other OSPF area and are advertised into your area. Inter area routes are advertised with LSA type 3. The type 3 LSA is generated by Border routers. So for example let us consider a Border router that has interfaces in area 0 and in area 5. It will take the intra area routes for area 5 (learned from the type 1 and type 2 LSAs) and will generate type 3 LSAs for area 0 and advertise these type 3 LSAs into area 0. The Border router will also take the intra area routes for area 0 and will generate type 3 LSAs for area 5 and will advertise the type 3 LSAs to other OSPF routers in area 5. As John tells us in the route table the inter area routes are identified as O IA.

In OSPF routing an intra area route will always be preferred over an equivalent inter area route.

HTH

Rick

[mahesh18]

Hi Rick,

Many thanks again for answering the question.

Regards

MAhesh

[usmanmani_501]

well explained. Thanks

[shiras k a]

nice expalanation