Redistribution between RIP and OSPF

Redistribution between RIP and OSPF


R2 has all four types OSPF routes:
R2#sh ip route ospf
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/11] via 200.200.24.4, 00:04:42, FastEthernet0/1
     200.200.34.0/30 is subnetted, 1 subnets
O       200.200.34.0 [110/74] via 200.200.24.4, 00:04:42, FastEthernet0/1
     55.0.0.0/24 is subnetted, 1 subnets
O E1    55.55.55.0 [110/40] via 200.200.24.4, 00:04:42, FastEthernet0/1
     5.0.0.0/24 is subnetted, 1 subnets
O E2    5.5.5.0 [110/20] via 200.200.24.4, 00:04:42, FastEthernet0/1
O IA 200.200.45.0/24 [110/20] via 200.200.24.4, 00:04:42, FastEthernet0/1
R2#
After redistribute OSPF to RIP, R1 has all OSPF routes, so all type OSPF routes are redistributed into RIP by default.
R2(config)# router rip
R2(config-router)# redistribute ospf 100 metric 5
R2#

R1#sh ip route rip
     4.0.0.0/32 is subnetted, 1 subnets
R       4.4.4.4 [120/5] via 100.100.12.2, 00:00:22, FastEthernet0/0
     200.200.34.0/30 is subnetted, 1 subnets
R       200.200.34.0 [120/5] via 100.100.12.2, 00:00:22, FastEthernet0/0
     55.0.0.0/24 is subnetted, 1 subnets
R       55.55.55.0 [120/5] via 100.100.12.2, 00:00:22, FastEthernet0/0
     5.0.0.0/24 is subnetted, 1 subnets
R       5.5.5.0 [120/5] via 100.100.12.2, 00:00:22, FastEthernet0/0
R    200.200.45.0/24 [120/5] via 100.100.12.2, 00:00:22, FastEthernet0/0
R    200.200.24.0/24 [120/5] via 100.100.12.2, 00:00:22, FastEthernet0/0
R1#
After redistribute RIP to OSPF, all RIP routes inject into OSPF as E2 route by default, if no metric is specified in the redistribute command, the default metric is 20. subnets keyword is important.
R2(config)# router ospf 100
R2(config-router)# redistribute rip subnets

R4#sh ip route ospf
     1.0.0.0/24 is subnetted, 1 subnets
O E2    1.1.1.0 [110/20] via 200.200.24.2, 00:13:57, FastEthernet0/1
     100.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O E2    100.100.12.0/24 [110/20] via 200.200.24.2, 00:13:57, FastEthernet0/1
O E2    100.100.13.0/30 [110/20] via 200.200.24.2, 00:13:57, FastEthernet0/1
     55.0.0.0/24 is subnetted, 1 subnets
O E1    55.55.55.0 [110/30] via 200.200.45.5, 00:13:57, FastEthernet0/0
     5.0.0.0/24 is subnetted, 1 subnets
O E2    5.5.5.0 [110/20] via 200.200.45.5, 00:13:57, FastEthernet0/0
R4#

After the mutual redistribution, on another border router R3, it will no longer have any RIP routes in its routing table, because the previous RIP routes now are redistributed into OSPF show up as OSPF E2 routes, since OSPF route has better AD, they replace the previous RIP routes.
R3#sh ip route rip
R3#
R3#sh ip route | i 1.1.1.0
O E2    1.1.1.0 [110/20] via 200.200.34.2, 00:43:41, Serial1/1

R3#traceroute 1.1.1.1
Type escape sequence to abort.
Tracing the route to 1.1.1.1
  1 200.200.34.2 40 msec 24 msec 16 msec
  2 200.200.24.2 48 msec 36 msec 20 msec
  3 100.100.12.1 72 msec 68 msec 104 msec
R3#
So if R3 do redistribution from RIP to OSPF, it really does nothing.

Then if R3 do redistribution from OSPF to RIP, will this cause routing loop? let's take a look R5's loopback route 5.5.5.0/24, it exists on both R2 and R3's routing table as OSPF route, then it is redistributed by both routers into RIP show up as RIP route, when this route transit RIP domain reach the another end border router R2 and R3, they are not going to be loaded into routing table as R2/R3 already have it from OSPF which has better AD.

So with multiple mutual redistribution points between RIP and OSPF, without any filters on the border routers, there is no loop at all. The only issue is on the 2nd border router, reach other RIP routes need go through OSPF domain.