An OSPF Open Shortest Path First network is divided into areas. These are logical groupings of routers whose information may be summarized towards the rest of the network. Areas are identified by a 32-bit ID in dot-decimal notation similar to the notation of IP addresses.
Altogether, there are six types of OSPF areas:
- Backbone: The area with ID 0 (or 0.0.0.0) is reserved for the OSPF network backbone, which forms the core of an OSPF network—all other areas are connected to it.
- Normal: A normal or regular area has a unique ID ranging from 1 (or 0.0.0.1) to 4,294,967,295 (or 255.255.255.255). Normal areas handle external routes by flooding them bi-directionally across the Area Border Router (ABR). Note that external routes are defined as routes which were distributed in OSPF from another routing protocol.
- Stub: Typically, a stub area does not have direct connections to any external networks. Injecting external routes into a stub area is unnecessary because all traffic to external networks must be routed through an Area Border Router (ABR). Therefore, a stub area substitutes a default route for external routes to send traffic to external networks.
- Stub no-summary: A Stub no-summary or Totally stubby area is similar to a stub area, however this area does not allow so-called summary routes, that is, it restricts type 3 summary link state advertisements (LSAs) from flowing into the area.
- NSSA: A not-so-stubby area (NSSA) is a type of stub area that in contrast to stub areas can support external connections. Note that NSSAs do not support virtual links.
- NSSA no-summary: A NSSA no-summary is similar to a NSSA, however this area does not allow so-called summary routes, that is, it restricts type 3 summary link state advertisements (LSAs) from flowing into the area.
To create an OSPF area, proceed as follows:
On the Area tab, click New OSPF Area.
The Add OSPF Area dialog box opens.
Make the following settings:
Name: Enter a descriptive name for the area.
Area ID: Enter the ID of the area in dot-decimal notation (e.g., 0.0.0.1 for a normal area or 0.0.0.0 for the backbone area).
Area type: Select an area type (see description above) to specify the characteristics of the network that will be assigned to the area in question.
Auth-type: Select the authentication type used for all OSPF packets sent and received through the interfaces in the area. The following authentication types are available:
- MD5: Select to enable MD5 authentication. MD5 (Message-Digest algorithm 5) is a widely-used cryptographic hash function with a 128-bit hash value.
- Plain-Text: Select to enable plain-text authentication. The password is transmitted in clear text over the network.
- Off: Select to disable authentication.
Connect via Interface: Select an OSPF-enabled interface. Note that to specify an OSPF-enabled interface here it must have been created on the Interfaces tab first.
Connect Virtual Links: All areas in an OSPF autonomous system (AS) must be physically connected to the backbone area (area 0). In some cases where this physical connection is not possible, you can use a virtual link to connect to the backbone through a non-backbone area. In the Connect Virtual Links box, enter the router ID associated with the virtual link neighbor in decimal dot notation (e.g., 10.0.0.8).
Cost: The cost of sending or receiving a data packet in this area. Valid values for cost are in the range from 1 to 65535. Default is 0. If you choose 0, the system calculates an adequate value based on the formula reference bandwidth divided by interface bandwidth. For example, in the case of Ethernet, it is 100 Mbps /10 Mbps = 10.
Comment (optional): Add a description or other information.
The new area definition appears on the Area tab.
To either edit or delete an OSPF area, click the corresponding buttons.