This article will help you understand OSPF, and types of OSPF features you can configure. It will also help you find related troubleshooting, and configuration guides on OSPF.
Before we configure OSPF on EXOS lets make sure we understand what OSPF is. OSPF stands for Open Shortest Path First; It is used to dynamically advertise locations of IP networks in your network (AS). Each OSPF router will run a calculation on the distance (aka cost) to each network and pick the shortest cost path. OSPF also allows parts of a network to be grouped together into areas. The topology within an area is hidden from the rest of the AS. Hiding this information enables a significant reduction in LSA traffic and reduces the computations needed to maintain the LSDB. Routing within the area is determined only by the topology of the area.Different Area Types:Backbone Area (Area 0.0.0.0):
Any OSPF network that contains more than one area is required to have an area configured as area 0.0.0.0, also called the backbone. All areas in an AS must be connected to the backbone. When designing networks, you should start with area 0.0.0.0 and then expand into other areas. The backbone area must be contigious and you should be aware to avoid any split backbone scenario's.Stub Areas:
OSPF allows certain areas to be configured as stub areas. A stub area is connected to only one other area. The area that connects to a stub area can be the backbone area. External route information is not distributed into stub areas. Stub areas are used to reduce memory consumption and computational requirements on OSPF routers.Not-So-Stubby-Areas:
Not-so-stubby-areas (NSSAs) are similar to the existing OSPF stub area configuration option but have the following two additional capabilities:
- External routes originating from an ASBR connected to the NSSA can be advertised within the NSSA.
- External routes originating from the NSSA can be propagated to other areas, including the backbone area.
A normal area is an area that is not Area 0, Stub area, or a NSSA. It can be used to summarize routes between areas, and to divide up a large backbone area.