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A common first step in migrating a RIP
network to an OSPF network is to configure backbone routers that run
both RIP and OSPF, while the remaining network devices run RIP.
These backbone routers act as OSPF ASBRs. Each ASBR controls the
flow of routing information between OSPF and RIP. In the figure, RTA,
RTB, and RTC now act as ASBRs.
Because RIP does not need to run
between the backbone routers, updates can be suppressed using the passive-interface
command. Although the example
below specifies RTA, the same commands could be entered on the other
routers.
RTA(config)# router rip
RTA(config-router)# passive-interface serial 0
RTA(config-router)# passive-interface serial 1
Instead of RIP updates, OSPF updates
will carry the redistributed information across the WAN links. The
necessary OSPF routing and redistribution commands are shown here.
The same configuration is used for all three routers except for the
network numbers.
RTA(config)# router ospf 109
RTA(config-router)# redistribute rip subnets
RTA(config-router)# network 130.10.62.0 0.0.0.255 area 0
RTA(config-router)# network 130.10.63.0 0.0.0.255 area 0
The subnets
keyword tells OSPF to redistribute all subnet routes. Without the subnets
keyword, only networks that
are not subnetted are redistributed by OSPF.
The redistributed RIP routes appear
as external Type 2 routes in OSPF, as discussed in Chapter 5,
Multiarea OSPF.
Mutual redistribution must be
configured for other routers in the RIP domain (not shown in the
Figure) to receive information from OSPF. This example lists the
necessary commands, which are again the same for each router.
RTA(config)# router rip
RTA(config-router)# redistribute ospf 109 match internal external
1 external 2
RTA(config-router)# default-metric 10
Note that the redistribute
command includes the OSPF process ID, 109. The other keywords, match
internal external 1 and
external 2, instruct RIP to
redistribute internal OSPF routes, as well as external Type 1 and
Type 2 routes. This is the default for OSPF redistribution. These
keywords are required only if you want to modify its behavior.
As illustrated in the figure, there
are no paths directly connecting the RIP domains outside the core.
In real-world networks, this is not always the case. If one RIP
domain can communicate directly with another, there is a chance that
they will exchange routes, resulting in a routing feedback loop. You
can use route filters to prevent these potentially disastrous loops.
The configuration below allows RTA's
OSPF process to redistribute RIP information, only for networks
130.10.8.0 through 130.10.15.0:
RTA(config)# router ospf 109
RTA(config-router)# redistribute rip subnets
RTA(config-router)# distribute-list 11 out rip
RTA(config)# access-list 11
permit 130.10.8.0 0.0.7.255
RTA(config)# access-list 11 deny 0.0.0.0 255.255.255.255
These commands prevent RTA from
advertising networks in other RIP domains onto the OSPF backbone,
thereby preventing other boundary routers from using false
information and forming a loop.
When an OSPF backbone area is in place, the RIP domains can easily
be converted into OSPF areas.
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