After the network command has been entered, OSPF identifies which interfaces are participating in OSPF by comparing the interface IP address with the address given in the network command, filtered through the wildcard mask. The wildcard mask states how much of the address to pay attention to. It could just look at the class of address, such as everything in network 10.0.0.0, for example. At the other extreme, it can be more specific and identify an interface address. All interfaces that match the given network number will reside in the area specified in the network command.
WARNING Take great care in the wildcard mask. Remember that it follows the same format as the wildcard mask in an access list. It is extremely easy to make errors in the configuration, and those errors may be difficult to find.
After identifying the interfaces on the router that are participating in the OSPF domain, the following happens:
• Updates will be received on the interface.
• Updates will be sent out of the interfaces.
• The interface will be placed in the defined area.
• If appropriate, the Hello protocol will be propagated. Depending on the interface type, a default hello and dead interval are defined (see Table 5-4).
This network command has many of the same characteristics as an access list. The wildcard mask has the same format and enables you to group interfaces into an area. It follows the same top-down logic of a link list that you saw before in configuring access lists in Chapter 2, "Managing Scalable Network Growth."
NOTE If there are stub networks connected to a router, it is useful to issue the command redistribute connected subnets. This command is issued as part of the router process configuration, and it includes the connected subnets in OSPF advertisements without actually running OSPF on them. This is very useful for real OSPF configurations, particularly those that involve WAN pay-per-packet, low-bandwidth links.
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