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A common first
step in converting a RIP network to an OSPF network is to add
backbone routers that run both RIP and OSPF, while the remaining
network devices run RIP. These backbone routers are OSPF ASBRs. Each
ASBR controls the flow of routing information between OSPF and RIP.
In the main figure, router A is configured
as the ASBR.
RIP does not need to run between the backbone routers; therefore,
RIP is suppressed on Router A with the following commands:
router rip
passive-interface serial 0
passive-interface serial 1
The RIP routes are redistributed into OSPF by all three routers
with the following commands:
router ospf 109
redistribute rip subnets
network 130.10.0.0
The subnets keyword tells OSPF to redistribute all subnet routes.
Without the subnets keyword, only networks that are not subnetted
will be redistributed by OSPF. Redistributed routes appear as
external type 2 routes in OSPF. For more detail on route types,
refer to the text earlier in this chapter or see the Cisco
Internetwork Operating System (Cisco IOS) command references.
Each RIP domain receives information about networks in other RIP
domains and in the OSPF backbone area from the following commands
that redistribute OSPF routes into RIP:
router rip
redistribute ospf 109 match internal external 1 external 2
default-metric 10
The redistribute command uses the ospf keyword to specify that
OSPF routes are redistributed into RIP. The keyword internal
indicates the OSPF intra-area and inter-area routes: external 1 is
the external route type 1, and external 2 is the external route type
2. Because the command in the example uses the
default behavior, these keywords might not appear when you use the
write terminal or show configuration commands.
Because metrics for different protocols cannot be directly
compared, you must specify the default metric in order to designate
the cost of the redistributed route used in RIP updates. All routes
that are redistributed will use the default metric.
As illustrated in the main figure, there are no paths directly
connecting the RIP clouds. However, in typical networks, these
paths, or "back doors," frequently exist, allowing the
potential for feedback loops. You can use access lists to determine
the routes that are advertised and accepted by each router.
For example, access list 11 in the configuration file for Router
A allows OSPF to redistribute information learned from RIP only for
networks 130.10.8.0 through 130.10.15.0:
router ospf 109
redistribute rip subnet
distribute-list 11 out rip
access-list 11 permit 130.10.8.0 0.0.7.255
access-list 11 deny 0.0.0.0 255.255.255.255
These commands prevent Router A from advertising networks in
other RIP domains onto the OSPF backbone, thereby preventing other
boundary routers from using false information and forming a loop.
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