Figure 14-3 illustrates how each of the RIP clouds can be converted into an OSPF area. All three routers are area border routers. Area border routers 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 area border routers on their respective areas.
Figure 14-3 also illustrates variable-length subnet masks (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. Table 14-2 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.
| Network Number | Subnets | Subnet Masks |
|---|---|---|
|
130.10.0.0 |
Area 0: 62 through 64 |
255.255.255.248 |
|
130.10.0.0 |
Area 1: 8 through 15 |
255.255.255.0 |
|
130.10.0.0 |
Area 2: 16 through 23 |
255.255.255.0 |
|
130.10.0.0 |
Area 3: 24 through 31 |
255.255.255.0 |
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 area border router 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
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 alternative technique to find the routers. Cisco routers support the following alternatives to RIP:
The full configuration for Router A follows:
interface serial 0 ip address 130.10.62.1 255.255.255.248 interface serial 1 ip address 130.10.63.1 255.255.255.248 interface ethernet 0 ip address 130.10.8.1 255.255.255.0 ip irdp interface tokenring 0 ip address 130.10.9.1 255.255.255.0 ip irdp router ospf 109 network 130.10.62.0 0.0.0.255 area 0 network 130.10.63.0 0.0.0.255 area 0 network 130.10.8.0 0.0.7.255 area 1 area 1 range 130.10.8.0 255.255.248.0
The full configuration for Router B follows:
interface serial 0 ip address 130.10.62.2 255.255.255.248 interface serial 1 ip address 130.10.64.2 255.255.255.248 interface ethernet 0 ip address 130.10.17.2 255.255.255.0 ip irdp interface tokenring 0 ip address 130.10.16.2 255.255.255.0 ip irdp router ospf 109 network 130.10.62.0 0.0.0.255 area 0 network 130.10.64.0 0.0.0.255 area 0 network 130.10.16.0 0.0.7.255 area 2 area 2 range 130.10.16.0 255.255.248.0
The full configuration for Router C follows:
interface serial 0 ip address 130.10.63.2 255.255.255.248 interface serial 1 ip address 130.10.64.2 255.255.255.248 interface ethernet 0 ip address 130.10.24.3 255.255.255.0 ip irdp router ospf 109 network 130.10.63.0 0.0.0.255 area 0 network 130.10.64.0 0.0.0.255 area 0 network 130.10.24.0 0.0.0.255 area 3 area 3 range 130.10.24.0 255.255.248.0