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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.
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