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If there's one CCNP / BSCI topic that looks so easy but can lead to a real headache, it's route redistribution. I'm not even talking about the routing loops and suboptimal routing that can result when route redistribution is done without proper planning - I'm talking about the basic commands themselves. Leaving out one single command option, or forgetting what else needs to be redistributed when redistributing dynamically discovered routes, can leave you with a routing table that looks complete but does not result in full IP connectivity.
In this free CCNP / BSCI tutorial series, we'll take a look at three common errors in route redistribution configurations, and how to fix them. We'll use three routers, R1, R3, and R5. R1 and R5 are in a RIPv2 domain and R1 and R3 are in an OSPF domain. R1 will be performing two-way route redistribution.
R5 is advertising its loopback, 5.5.5.5/24, into the RIPv2 domain. R1 sees this route in its RIP routing table:
R1#show ip route rip
5.0.0.0/24 is subnetted, 1 subnets
R 5.5.5.0 [120/1] via 100.1.1.5, 00:00:01, Ethernet0
For R3 to see this route, route redistribution must be configured on R1. We'll use the redistribute rip command to do so.
R1(config)#router ospf 1
R1(config-router)#redistribute rip
% Only classful networks will be redistributed
The router immediately gives us a message that "only classful networks will be redistributed". What does this mean? Let's go to R3 and see if that router is receiving this route.
R3#show ip route ospf
< no output >
When we get no result from a show command, that means there's nothing to show. The only routes that will be successfully redistributed with the current configuration on R1 are classful networks, and 5.5.5.0/24 is a subnet.
To further illustrate the point, a classful network has been added to R5. This network is 16.0.0.0 /8, and is now being advertised by RIP. R1 sees this network as classful...
R1#show ip route rip
R 16.0.0.0/8 [120/1] via 100.1.1.5, 00:00:00, Ethernet0
5.0.0.0/24 is subnetted, 1 subnets
R 5.5.5.0 [120/1] via 100.1.1.5, 00:00:00, Ethernet0
... and R3 is receiving the route through redistribution.
R3#show ip route ospf
O E2 16.0.0.0/8 [110/20] via 172.12.123.1, 00:00:08, Serial0.31
To redistribute both classful and classless networks, the option "subnets" must be added to the redistribute command on R1.
R1(config)#router ospf 1
R1(config-router)#no redistribute rip
R1(config-router)#redistribute rip subnets
R3 will now see both the classful and classless networks being redistributed into OSPF. (100.1.1.0 is the network connecting R1 and R5.)
R3#show ip route ospf
O E2 16.0.0.0/8 [110/20] via 172.12.123.1, 00:00:20, Serial0.31
100.0.0.0/24 is subnetted, 1 subnets
O E2 100.1.1.0 [110/20] via 172.12.123.1, 00:00:20, Serial0.31
5.0.0.0/24 is subnetted, 1 subnets
O E2 5.5.5.0 [110/20] via 172.12.123.1, 00:00:20, Serial0.31
This is one of the most common errors made during route redistribution, but now you know what to look out for! In the next part of this free CCNP / BSCI tutorial, we'll take a look at another such error.
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of over 100 free certification exam tutorials, including Cisco CCNA certification test prep articles. His exclusive Cisco CCNA study guide and Cisco CCNA exam training is also available!
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