Content

| 8.4 |  | Redistribution Example |
| 8.4.3 |  | Phase 3: adding OSPF areas |

Each RIP domain can be converted into an OSPF area independently of the other RIP domains. This allows the migration of one section of the internetwork at a time, if desired.

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

Notice that 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 RIP v1 does not. With OSPF as the sole routing protocol, the network can now make the most of the advantages of VLSM. The following example shows the commands necessary to configure Router A for OSPF operation on all interfaces, with the appropriate masks:

> Router A(config)#router ospf 109
> Router A(config-router)#network 172.16.62.0 0.0.0.7 area 0
> Router A(config-router)#network 172.16.63.0 0.0.0.7 area 0
> Router A(config-router)#network 172.16.8.0 0.0.0.255 area 1

Because OSPF is classless, each ABR can be configured to use route summarization. For example, Router A connects to eight networks, which occupy a contiguous address space. Therefore, this ABR can be configured as follows to send a single supernet route, which will advertise all eight of the networks:

> Router A(config)#router ospf 109
> Router A(config-router)#area 1 range 130.10172.16.8.0 255.255.248.0

> Router A(config)#router ospf 109
> Router A(config-router)#area 1 range 172.16.8.0 255.255.255.0

Router A will advertise one route, 172.16.8.0 255.255.255.0, which 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 172.16.8.0 255.255.255.0, one route for 172.16.9.0 255.255.255.0, and so forth. The migration of the network from RIP to OSPF is now complete, and redistribution is no longer necessary.

| Lab Activity e-Lab Activity: Phase 3: Adding OSPF Areas This lab is to add OSPF areas.

| Web Links RIP and OSPF Redistribution http://www.cisco.com/univercd/cc/td/doc/ cisintwk/idg4/nd2014.htm