Basic inter-domain protocol redistribution
Route redistribution allows border routers to distribute routes between adjacent routing domains. Thus, the North router can redistribute routes from the northern RIP domain to the OSPF domain and from the OSPF domain to the northern RIP domain. Similarly, the South router can redistribute routes from the southern RIP domain to the OSPF domain and from the OSPF domain to the southern RIP domain. And if both the North and South routers have redistribution enabled in both directions at the same time, the routes that are redistributed from the RIP domains to the OSPF domain will be further distributed to the opposite RIP domain, and routers and hosts in all domains will be able to communicate with each other. (Some subtle complications are explained below.)
For example, in the North and South routers you
might add a redistribute rip command to the router ospf context and a redistribute ospf command to the router rip context, like this:
. . router ospf area backbone redistribute rip exit router rip redistribute connected redistribute ospf exit . .
This causes extensive redistribution of routes within all three routing domains, adding a large number of routes to the route tables of all the routers. For example, the route table in the East router adds routes to subnets in both RIP domains, and looks like this:
East(config)# show ip route
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
--------------- --------------- ---- --------- ---------- ---------- -----
10.1.11.0/24 10.3.32.1 32 ospf External2 10 110
10.1.12.0/24 10.3.32.1 32 ospf External2 10 110
10.1.13.0/24 10.3.32.1 32 ospf External2 10 110
10.1.14.0/24 10.3.32.1 32 ospf External2 10 110
10.2.22.0/24 10.3.33.2 33 ospf External2 10 110
10.2.23.0/24 10.3.33.2 33 ospf External2 10 110
10.3.31.0/24 10.3.32.1 32 ospf IntraArea 2 110
10.3.31.0/24 10.3.33.2 33 ospf IntraArea 2 110
10.3.32.0/24 VLAN32 32 connected 1 0
10.3.33.0/24 VLAN33 33 connected 1 0
10.3.34.0/24 VLAN34 34 connected 1 0
10.3.37.0/24 10.3.33.2 33 ospf IntraArea 2 110
127.0.0.0/8 reject static 0 0
127.0.0.1/32 lo0 connected 1 0
But this route table does not include all the possible routes in all domains: routes to subnets 10.1.15.x, 10.1.16.x, 10.2.21.x, and 10.2.29.x (VLANs 15, 16, 21, and 29) are missing. Host computer M cannot ping host X because there is no route to it, though it can ping through the "invisible" South router to host Y or host Z.
The problem is that those missing subnets are
directly connected to the North and South border routers, and directly
connected routes must be explicitly redistributed with a redistribute connected command even though they are RIP
routes and RIP routes were redistributed. So by adding redistribute
connected commands to the router ospf contexts of the North and South routers, like this:
. . router ospf area backbone redistribute connected redistribute rip exit . .
All existing routes are redistributed and the route table for the East router is now complete:
East(config)# show ip route
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
--------------- --------------- ---- --------- ---------- ---------- -----
10.1.11.0/24 10.3.32.1 32 ospf External2 10 110
10.1.12.0/24 10.3.32.1 32 ospf External2 10 110
10.1.13.0/24 10.3.32.1 32 ospf External2 10 110
10.1.14.0/24 10.3.32.1 32 ospf External2 10 110
10.1.15.0/24 10.3.32.1 32 ospf External2 10 110
10.1.16.0/24 10.3.32.1 32 ospf External2 10 110
10.2.21.0/24 10.3.33.2 33 ospf External2 10 110
10.2.22.0/24 10.3.33.2 33 ospf External2 10 110
10.2.23.0/24 10.3.33.2 33 ospf External2 10 110
10.2.29.0/24 10.3.33.2 33 ospf External2 10 110
10.3.31.0/24 10.3.32.1 32 ospf IntraArea 2 110
10.3.31.0/24 10.3.33.2 33 ospf IntraArea 2 110
10.3.32.0/24 VLAN32 32 connected 1 0
10.3.33.0/24 VLAN33 33 connected 1 0
10.3.34.0/24 VLAN34 34 connected 1 0
10.3.37.0/24 10.3.33.2 33 ospf IntraArea 2 110
127.0.0.0/8 reject static 0 0
127.0.0.1/32 lo0 connected 1 0
Host L can now ping host X and, indeed, any other host in any of the three routing domains.