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OSPF configuration

In OSPF the backbone area is always defined as area 0. All routers will be in the backbone area with a flat design. In this, all the routers will reculate their best paths when a single link goes down. There can thus be scalability problems with a flat design. The basic OSPF com-mands are:

router ospf {process-id}

and

network {address} {wildcard-mask} area {area-id}

where

Process ID. A numerical value that uniquely identifies the OSPF routing process. It only has local significance and does not have to match the OSPF process ID used on any other router.
Network. This identifies the interfaces that participate in OSPF routing and to assign these interfaces to the proper OSPF area. It can be used with either an absolute address or a sub-net address. An absolute address with a 0.0.0.0 wildcard-mask assigns only one interface to the OSPF area, and, for example, a network or subnet address with a 0.0.0.255 wildcard-mask assigns all interfaces that fall within that range to the OSPF area.
Area ID. This identifies the area associated with the absolute address, network address, or subnet address. Where a single domain is used there will be only area 0, which will typically be referenced as area 0.0.0.0.

For example on Router A:

(config)# int s0
(config-if)# ip address 10.64.0.1 255.255.255.0
(config-if)# exit
(config)# int e0
(config-if)# ip address 10.32.0.1 255.255.255.0
(config-if)# exit
(config-if)# router ospf 1
(config-if)# network 10.0.0.0 area 0

and on the router connected to it (Router B):

(config)# int s1
(config-if)# ip address 10.64.0.2 255.255.255.0
(config-if)# exit
(config)# int s0
(config-if)# ip address 10.16.0.1 255.255.255.0
(config-if)# exit
(config-if)# router ospf 55
(config-if)# network 10.64.0.2 area 0
(config-if)# network 10.16.0.1 area 0

It can be seen in this case that port S0 on router A connects to S1 on router B. On router A, OSPF is defined with a process ID of, and that it is run on all interfaces whose address is part of network 10.0.0.0, on area 0. The OSPF process ID on router B is 55, and uses absolute ad-dresses for the network statements. This requires a separate network statement for every interface that will participate in OSPF routing. For example:

OSPF configuration (multiple areas)

On OSPF the common area is any area other than the backbone area, which may have multi-ple exit and entry points to and from a common area. It can also have a stub area which only has one entry and one exit point to the backbone area, and is used where smaller routers are used to run OSPF. In a hierarchal design there are four types of routers:

Internal router. These are routers which exist within the boundary of a common area, and will know about every subnet within its area. It will also know the identity of all its neighbors within the area, and will send updates to all of the routers within its area. It will also receive routing updates only from routers in its own area, where information about subnets in other areas comes from the area border routers.
Backbone router. These are the core routers in area 0. They thus know how to get to all subnets in all areas, and are typically located at the border of the backbone and a common area. They send updates to other backbone routers and learn about the rest of the network through the area border routers.
Area Border Router (ABR). These provide the foundation of the hierarchical design and route traffic between areas. They thus attach to multiple areas and maintain a separate topo-logical database for each connected area, and can summarize routing information passed between a common area and the backbone area. In a design with additional redundancy, a common area may have more than one area border router.
Autonomous system boundary router (ASBR). This is an OSPF router that connects to an external network, which may or may not be running OSPF, and can import or export in-formation to and from the OSPF autonomous system.

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