7.6 RIP and OSPF Redistribution
7.6.5 Adding OSPF areas
Figure illustrates how each of the RIP clouds can be converted into an OSPF area. All three routers are ABRs. ABRs control network information distribution between OSPF areas and the OSPF backbone. Each router keeps a detailed record of the topology of its area and receives summarized information from the other ABRs on their respective areas.

Figure also illustrates VLSMs. VLSMs use different size network masks in different parts of the network for the same network number. VLSM conserves address space by using a longer mask in portions of the network that have fewer hosts.

Figure lists the network address assignments for the network, including the network number, subnet range, and subnet masks. All interfaces indicate network 130.10.0.0.

SPF address assignments

To conserve address space, a mask of 255.255.255.248 is used for all the serial lines in area 0. If an area contains a contiguous range of network numbers, an ABR uses the range keyword with the area command to summarize the routes that are injected into the backbone:

router ospf 109
network 130.10.8.0 0.0.7.255 area 1
area 1 range 130.10.8.0 255.255.248.0

These commands allow Router A to advertise one route, 130.10.8.0 255.255.248.0 that covers all subnets in Area 1, into Area 0. Without the range keyword in the area command, Router A would advertise each subnet individually; for example, one route for 130.10.8.0 255.255.255.0, one route for 130.10.9.0 255.255.255.0, and so forth.

Because Router A no longer needs to redistribute RIP routes, the router rip command can now be removed from the configuration file; however, it is common in some environments for hosts to use RIP to discover routers. When RIP is removed from the routers, the hosts must use an alternate technique to find the routers. Cisco routers support the IRDP and ARP alternatives to RIP.

The ICMP Router Discovery Protocol (IRDP) technique is illustrated in the example at the end of this section. IRDP is the recommended method for discovering routers. The ip irdp command enables IRDP on the router. Hosts must also run IRDP.

With proxy Address Resolution Protocol (ARP), if the router receives an ARP request for a host that is not on the same network as the ARP request sender, and if the router has the best route to that host, then the router sends an ARP reply packet giving the router's own local data link address. The host that sent the ARP request then sends its packets to the router, which forwards these packets to the intended host. Proxy ARP is enabled on routers by default. Proxy ARP is transparent to hosts.

The proxy ARP alternative is not illustrated in this case study. See the Cisco IOS command references for details on this feature.