D is right.
R1 just redistribute RIP in EIGRP. R2 learn 192.168.200.0 route from EIGRP and R2 redistribute EIGRP in OSPF, then R2 advertive 192.168.200.0 to R1. R1 learns 192.168.200.0 from R2 via OSPF. OSPF has AD 90 and RIP 120, so OSPF route become better than RIP route, but, for a route redistribution there is a rule that says the route must be in routing table and the source of redistribution must be the source of this route in route table, in this case, the RIP. When R1 learn this route in OSPF, the OSPF route replace the RIP route in route table, so the rule is broken and the redistribution stop working, then R1 stops redistribuing 192.168.200.0 to EIGRP and R2 stop reciving this route and R2 stops redistribute this route in OSPF so R1 won't recive this route from OSPF anymore, then OSPF route is removed from LSDB and RIB so RIP route go to the route table and the redistribution to EIGRP starts again and the problem starts over and over
If you redistribute RIP in OSPF in R1, R2 is gonna have this route as the best route from OSPF, so it does not matter if R2 learns it from EIGRP or NOT, because OSPF has AD 90 and External EIGRP 170.
Confirmed in Lab. One correction to topology above I had to include network 10.20.0.00 0.0.0.3 in router ospf 100 under R2 as not shown in diagram. I did debug on R1:
192.168.200.0/24 and debug ip routing on R1: Mar 17 09:21:55.996: RT: add 192.168.200.0/24 via 10.40.0.2, rip metric [120/1]
*Mar 17 09:21:56.045: IP: s=10.20.0.2, d=224.0.0.5, pak F1682428 consumed in input feature , packet consumed, MCI Check(82), rtype 0, forus FALSE, sendself FALSE, mtu 0, fwdchk FALSE
*Mar 17 09:21:56.045: RT: updating ospf 192.168.200.0/24 (0x0):
via 10.20.0.2 Et0/0
and the route 192.168.200.0 no exist on R2 or R3 either (as it seems to be flapping in and out of routing table in R1): R2: Show Ip route: 10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.20.0.0/30 is directly connected, Ethernet0/0
L 10.20.0.2/32 is directly connected, Ethernet0/0
D 10.30.0.0/30 [90/307200] via 172.16.23.2, 00:06:28, Ethernet0/3
D 10.40.0.0/30 [90/332800] via 172.16.23.2, 00:06:28, Ethernet0/3
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.23.0/30 is directly connected, Ethernet0/3
L 172.16.23.1/32 is directly connected, Ethernet0/3
and now 192.168.200.0 is in routing table of R1, R2, and R3 after redistribution:
R2: Show ip route:
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.20.0.0/30 is directly connected, Ethernet0/0
L 10.20.0.2/32 is directly connected, Ethernet0/0
D 10.30.0.0/30 [90/307200] via 172.16.23.2, 00:15:31, Ethernet0/3
D 10.40.0.0/30 [90/332800] via 172.16.23.2, 00:15:31, Ethernet0/3
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.23.0/30 is directly connected, Ethernet0/3
L 172.16.23.1/32 is directly connected, Ethernet0/3
O E2 192.168.200.0/24 [110/1] via 10.20.0.1, 00:06:17, Ethernet0/0
I wrote some comments down.
For me is still not clear if the config that we apply on R1 is substituting the redistribution between RIP to EIGRP.
The key problem is that RIP is redistributed in EIGRP and then EIGRP is redistributed in OSPF.
With D the redistribution of RIP will be done via OSPF, but I do not get if the EIGRP redistribution is deconfigured.
If this is the case, then the loop will be solved.
If the config about the redistribution between RIP to EIGRP is not unconfigured then the problem will still there.
There are a lot of explanations but none explain why D is correct. Here my take on why D is correct: EIGRP routes are redistributed on R2 to OSPF (redistribute eigrp 100). This would result in a OSPF type 2 route. If you would have any route "competing" with another identical type 2 route, the more preferred route is a OSPF type 1 route, If you redistribute RIP on R1, and make the routes a type 1 route, they always have preference in the OSPF domain.
The answer fixes the problem but there are better solutions to solve this problem. In order of preference: There should be no redistribution from OSPF to EIGRP on R2 (option 1) OR only routes originating from EIGRP itself should be redistributed on R2 (option 2) OR you can lower the AD on RIP learned routes on R1 (option 3).
More info on OSPF path selection order: https://networklessons.com/ospf/ospf-path-selection-explained
Option "D" correct.
the key point is router 1. because it has to redistribute rip into eigrp and rip into ospf. In this way Router 2 knows the network 192.168.200 with AD 110 and R3 with AD 170. and R1 wirh AD 120. :). Avoid fflapping :)
the problem here is that R2 will send the ospf E2 to R1 that only got the RIP route, so actually the problem should be there, R2 will receive the E1 route and that's good.
The problem is after that you redistribute RIP in EIGRP and then EIGRP is gonna be redistributed back into OSPF from R2 then you will receive the E2 route and R1 I think will prefer that path
I also labbed this scenario, and concluded that the static route of "B" only resolves the issue if we redistribute the static route on R1 (under ospf for instance). Then the prefix 192.168.200.0/24 is received by the members of the ospf and eigrp domain as an external route. Without redistributing the static route, only R1 can reach 192.168.200.0/24. No other router can.
With solution "D": After redistributing rip into ospf, 192.168.200.0/24 became reachable from R2, but not from R3. If R3 also needs to reach 192.168.200.0/24, then "D" is not enough.
Sorry for this post, I tested different configurations, and made a mistake when I readded the rip configuration on R1. I spotted the error, and after applying the correct configuration "D" actually worked.
The point there is that you will receive the E2 redistribute route from EIGRP into OSPF.
Even if you receive the type E1 from R1, R2 will send the network in OSPF as E2 cause it have only the RIP route I think that R1 is gonna prefer that path
B works in this case but R4 needs a route back which isn't shown. So D is best. I would say D would work 50/50 depending on who did redistriution first into OSPF, R1 or R2, this way the route gets into the OSPF LSDB and routing table first, however on R1 we see "metric-type 1" which is preferred over metric-type of R2, so R1 will win no matter what.
In my understanding:
On R1 RIP is redistributed on EIGRP. Therefore it will have on EIGRP
D EX 170 192.168.200.0/24
Best route on R1 would be RIP route 120<170
On R2 EIGRP is redistributed in OSPF so R2 will have route:
O E2 110 192.168.200.0/24
Now R1 adds OSPF route on it because 110<120 and points to R2,
therefore the routing loop.
Based on alternatives B looks as the best option.
We might need to redistribute this static aswell on R1, for other routers to know about this route, otherwise they should have the default pointing to R1
This flapping would actually happen if you redistribute RIP into EIGRP!!!!!!
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