OSPF Network Command

The OSPF network command is used to activate OSPF on the specified interfaces. Specifying an interface with the command will include the interface into the OSPF routing process. On broadcast and point-to-point network types, the router will automatically start the neighbor discovery process by sending hello packets to the AllSPFRouters multicast address (224.0.0.5) after an interface has been integrated into the OSPF process.

If there’s another OSPF router on that link and their Hello packets match, they will try to form an adjacency but that’s another story.

The OSPF network command uses the following syntax:

network ip-address wildcard-mask area area-id

The usage of the ip address and the wildcard mask allows us to specify one or multiple interfaces with one statement and assign them to the specified OSPF area. The Cisco IOS software sequentially evaluates the ip-address wildcard-mask pair for each interface as follows:

1. The wildcard-mask argument is logically ORed with the interface IP address.
2. The wildcard-mask argument is logically ORed with the ip-address argument in the network command.
3. The software compares the two resulting values. If they match, OSPF is enabled on the associated interface and this interface is attached to the OSPF area specified.

When enabling OSPF on interfaces you got three different options to do so:

1. The definition of only one interface
Definitions of only one interface are done with a wildcard-mask of 0.0.0.0 and the exact IP address of the given interface. This will match exact on that interface and nothing else.

2. The definition of mutiple but not all interfaces into one area
If you have to specify multiple interfaces and the IP concept has been done nicely, you might be able to summarize them all into one network statement. So if we have an address range like 116.1.16.0/21 which belongs to area N we can use network 116.1.16.0 0.0.7.255 area N to define them in one network statement. Every new interface on that router with an IP in that range will automatically be added into the OSPF process and added to that area.

3. The definition of all interfaces into one area
If you want to specify all interfaces on the router, the network statement network 0.0.0.0 255.255.255 area N will do the trick. Every new interface with an IP address on the router will automatically join OSPF and this area.

In older IOS versions the order in which the OSPF network statements have been configured if the area definitions have been overlapping. Nowadays Cisco IOS still doesnt allow an interface to be in more then one area:

Any individual interface can only be attached to a single area. If the address ranges specified for different areas overlap, the software will adopt the first area in the network command list and ignore the subsequent overlapping portions. In general, we recommend that you configure address ranges that do not overlap in order to avoid inadvertent conflicts.

but its not a problem any more in what order the statements got configured. The Cisco IOS will automatically reorder the network statements and places the most specific statements at the top and the least specific at the bottom. The following example will show this behavior:

R6(config)#do sh ip int brief | e una
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            116.1.100.6     YES manual up                    up
FastEthernet0/1            116.1.99.6      YES manual up                    up
Loopback0                  99.99.6.6       YES manual up                    up

R6(config)#router ospf 1
R6(config-router)#network 0.0.0.0 0.0.0.0 area 20
R6(config-router)#network 116.1.0.0 0.0.255.255 area 10
R6(config-router)#network 116.1.99.6 0.0.0.0 area 0

R6#sh run | sec router ospf
router ospf 1
log-adjacency-changes
network 116.1.99.6 0.0.0.0 area 0
network 116.1.0.0 0.0.255.255 area 10
network 0.0.0.0 255.255.255.255 area 20

R6#sh ip ospf int | i is up|Area
FastEthernet0/1 is up, line protocol is up
Internet Address 116.1.99.6/24, Area 0
FastEthernet0/0 is up, line protocol is up
Internet Address 116.1.100.6/24, Area 10
Loopback0 is up, line protocol is up
Internet Address 99.99.6.6/24, Area 20

So based on the above note from Cisco and the shown behavior it is possible to create overlapping network statements without the need to think about the network statement order, but also as Cisco says it should be avoided if possible.