The command requirements for configuring OSPF in a single area is relatively (compared to say other routing protocols) few in number and simple; the implications of the commands are somewhat complicated but need to be understood.
Required Commands for Configuring OSPF WITHIN a Single Area
We are going to configure an OSPF internal router. An Internal router being one with all interfaces that lie within a single area. The sole OSPF function on an internal router is to route within an area.
The Router needs to understand how to participate in the OSPF network:
- OSPF Process – Declare an OSPF process.
- Participating interfaces – Identify the interfaces to be used by OSPF.
- Area – Definitions are done per interface. This discussion assumes that all active interfaces are in the same area.
- Router ID – A unique 32-bit ID, usually drawn from an interface IP Address.
Enabling the OSPF Routing Protocol
Router(config)#router ospf process-number
In the above the process-number is not globally significant. It is possible to have more than one process running on a router (although that would be an unusual configuration, but not unheard of) and two OSPF processes could route for different parts of the network. The process number does not have to be the same on every router in the area.
The OSPF Network Command
Once OSPF is turned on (the above command), you must define the interfaces that are to participate in OSPF and the area that they reside in:
Router(config-router)#network network-number wilcard-mask area area-number
NB.Take note of the above command. Many errors occur in configuration with this command, normally due to misapplication of the wildcard-mask parameter, either including too many or too few interfaces in a particular OSPF area.
Similar to other routing protocols like RIP, the network command identifies the interface on which the OSPF process is to be active. Unlike RIP however this command has a wilcard mask that allows it to be very specific. All interfaces that match the network wildcard mask will be active within the given area.
One can apply the network command in different ways, each method will yield different yet similar results.
- FE 0/0 : 192.168.0.1 / 24
- FE 0/1 : 192.168.1.1 / 24
- FE 0/2 : 192.168.2.1 / 24
- FE 0/3 : 192.168.3.1 / 24
- S o/1 : 10.10.1.1 / 30
- s 1/1 : 10.10.2.1 / 30
We can enable OSPF area 0 (zero) on all interfaces with the following command:
Router(config-router)#network 0.0.0.0 255.255.255.255 area 0
When using this approach you may include interfaces inadvertently that you may not want to include (as this is a sweeping statement config line).
The second method would be to break the network into the 10 network and the 192 network, as follows:
Router(config-router)#network 10.0.0.0 0.255.255.255 area 0
Router(config-router)#network 192.168.0.0 0.0.3.255 area 0
This approach gives a little more control over the two different networks (192.x.x.x and 10.x.x.x) splitting them into two config lines.
Another method would be to separately enable OSPF on each interface;
Router(config-router)#network 192.168.0.1 0.0.0.0 area 0
Router(config-router)#network 192.168.1.1 0.0.0.0 area 0
Router(config-router)#network 192.168.2.1 0.0.0.0 area 0
Router(config-router)#network 192.168.3.1 0.0.0.0 area 0
Router(config-router)#network 10.10.1.1 0.0.0.0 area 0
Router(config-router)#network 10.10.2.1 0.0.0.0 area 0
This option is more time consuming to deploy but gives much more control over what interface specifically is included and not included in area 0 (zero) which will enable much more control over the routing process.
All the above achieve the same thing (six interfaces places in area 0 (zero) begin to process OSPF traffic).
The technique that is used should be functional, effective and efficient given the topology and application on the network while still maintaining the ability to be documented thoroughly (Keep It Simple Stupid KISS or as simple as possible, because you might not be the one to always maintain the network).
NB.Be intimately familiar (CCNA) with wildcard masks and the network command to enable OSPF on router interfaces
The area parameter puts the designated interface into an area. A router can have different interfaces in different areas (as mentioned earlier thus making the router an Area Border Routers (ABR)). The area-number is a 32-bit field and format can either be a simple decimal (0, 1, 2, 3, 4) or dotted decimal( 0.0.0.1, 0.0.0.2, 0.0.03, 0.0.0.4). Some implementations of OSPF might only understand one of the formats (keep in mind that some vendors throw the dotted decimal around 0.0.0.1 will become 1.0.0.0), Cisco understands both formats.
After identifying the interfaces on the router that are participating in the OSPF domain, hellos are exchanged, LSAs are sent, and the router inserts itself into the network.
NB.If there are stub networks connected to a OSPF router, it is useful to issue the command redistribute connected subnets. This command includes the connected subnets in OSPF advertisements without actually running OSPF on these routers. A route-map is often used with this command to exclude interfaces that are explicitly configured with OSPF
Next up Internal Router Config in more detail….
Notes and Notices: This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I cannot lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.