7.4 Redistribution Example
7.4.3 Phase 3: adding OSPF areas
Each RIP domain can be converted into an OSPF area independently of the other RIP domains. This allows you to migrate one section of your internetwork at a time, if desired.

When all three of our RIP domains have become OSPF areas, the three core routers will serve as ABRs Figure . Recall that ABRs control the exchange of routing information between OSPF areas and the OSPF backbone. Each ABR keeps a detailed record of its respective area's topology and summarizes this information in its updates to other backbone routers.

Note that the figure also presents a new addressing scheme in the core. A 29-bit mask (255.255.255.248) is used to address WAN links and conserve address space. Meanwhile, a 24-bit mask remains on the LAN interfaces, resulting in variable-length subnet masks. OSPF fully supports VLSM, while RIPv1 does not. With OSPF as the sole routing protocol, the network can now take advantage of VLSM's advantages. The example below shows the commands necessary to configure RTA for OSPF operation on all interfaces, with the appropriate masks.

RTA(config)#router ospf 109
RTA(config-router)#network 130.10.62.0 0.0.0.7 area 0
RTA(config-router)#network 130.10.63.0 0.0.0.7 area 0
RTA(config-router)#network 130.10.8.0 0.0.7.255 area 1

Because OSPF is classless, we can configure each ABR to use route summarization. For example, RTA connects to eight networks, which occupy a contiguous address space (see Figure ). Thus, you can configure this ABR to send a single supernet route, which will advertise all eight of the networks:

RTA(config)#router ospf 109
RTA(config-router)#area 1 range 130.10.8.0 255.255.248.0

RTA will advertise one route, 130.10.8.0 255.255.248.0, which covers all subnets in Area 1, into Area 0. Without the range keyword in the area command, RTA 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.

The migration of the network from RIP to OSPF is now complete, and redistribution is no longer necessary.