R4 redistributes loopback 0 in EIGRP, which is further redistributed by R2 into OSPF. R3 then redistributes OSPF back to EIGRP. This causes R2 to have 2 paths to reach the loopback of R4 and this may cause routing loops
When there are multiple points of redistribution, and the network which is being redistributed could only be reached via one path, then the initial router which is redistributing must not choose any other path other than the one from which it initially learned.
R4#sh run | sec router eigrprouter eigrp 234 redistribute connected metric 1 1 1 1 1 route-map CONNECTED network 10.0.24.4 0.0.0.0 no auto-summary eigrp router-id 4.4.4.4R4#sh run | sec route-map route-map CONNECTED permit 10 match interface Loopback0R2#sh run | sec routerrouter eigrp 234 network 10.0.23.2 0.0.0.0 network 10.0.24.2 0.0.0.0 no auto-summaryrouter ospf 1 router-id 2.2.2.2 log-adjacency-changes redistribute eigrp 234 subnets network 10.0.12.2 0.0.0.0 area 0R3#sh run | sec routerrouter eigrp 234 redistribute ospf 1 metric 1 1 1 1 1 network 10.0.23.3 0.0.0.0 no auto-summary eigrp router-id 3.3.3.3router ospf 1 router-id 3.3.3.3 log-adjacency-changes network 10.0.13.3 0.0.0.0 area 0R2 receives the route to loopback 0 of R4 as an EIGRP external route. When it redistributes this into OSPF, R3 will prefer the OSPF path because of the better AD. R3 then again redistributes the 4.4.4/24 network into EIGRP from OSPF. In this case R2 will have 2 paths to reach the loopback of R4 because the metric values are the same.
R2#sh ip route 4.4.4.0Routing entry for 4.4.4.0/24 Known via "eigrp 234", distance 170, metric 2560025856, type external Redistributing via ospf 1, eigrp 234 Advertised by ospf 1 subnets Last update from 10.0.23.3 on Ethernet0/1, 04:08:25 ago Routing Descriptor Blocks: * 10.0.24.4, from 10.0.24.4, 04:08:25 ago, via Ethernet0/2 Route metric is 2560025856, traffic share count is 1 Total delay is 1010 microseconds, minimum bandwidth is 1 Kbit Reliability 1/255, minimum MTU 1 bytes Loading 1/255, Hops 1 10.0.23.3, from 10.0.23.3, 04:08:25 ago, via Ethernet0/1 Route metric is 2560025856, traffic share count is 1 Total delay is 1010 microseconds, minimum bandwidth is 1 Kbit Reliability 1/255, minimum MTU 1 bytes Loading 1/255, Hops 1R2#sh ip eigrp topology 4.4.4.0 255.255.255.0 | in Originating|External Composite metric is (2560025856/2560000256), Route is External External data: Originating router is 3.3.3.3 External protocol is OSPF, external metric is 20 Composite metric is (2560025856/2560000256), Route is External External data: Originating router is 4.4.4.4 External protocol is Connected, external metric is 0To resolve this issue R2 must not accept the route to loopback of R4 from any other sources. This could be done at R2 with the following methods:
Method 1. Increasing the metric of the interface connecting to R3
R2(config-router)#int e0/1R2(config-if)#delay 10000RT: delete route to 4.4.4.0 via 10.0.23.3, eigrp metric [170/2560025856]The problem with this method is, even when R4 withdraws the route, R2 will install the path via R3. This could be simulated by shutting down the loopback at R4. The debug ip routing output below shows that the path via R3 is the entered to RIB
RT: delete route to 4.4.4.0 via 10.0.24.4, eigrp metric [170/2560025856]RT: add 4.4.4.0/24 via 10.0.23.3, eigrp metric [170/2562560256]R2#sh ip route 4.4.4.0Routing entry for 4.4.4.0/24 Known via "eigrp 234", distance 170, metric 2562560256, type external Redistributing via ospf 1, eigrp 234 Advertised by ospf 1 subnets Last update from 10.0.23.3 on Ethernet0/1, 00:06:55 ago Routing Descriptor Blocks: * 10.0.23.3, from 10.0.23.3, 00:06:55 ago, via Ethernet0/1 Route metric is 2562560256, traffic share count is 1 Total delay is 100010 microseconds, minimum bandwidth is 1 Kbit Reliability 1/255, minimum MTU 1 bytes Loading 1/255, Hops 1Method 2. Filtering the route inbound, by changing AD to 255
Method 3. Filtering the route by using a distribute list
Method 4. Filtering the route by using an offset list
Method 2 to 4 is effective because it denies the path via R3. The filtering method using distribute list , altering distance does not seem to work for external routes.
R2(config-router)#offset-list R4_LOOP_STA in 2147483647 e0/1ip access-list standard R4_LOOP_STA permit 4.4.4.0RT: delete route to 4.4.4.0 via 10.0.23.3, eigrp metric [170/2560025856]
To conclude, if a route 'X' could only be originally learned from one particular source, the same route 'X' must not be relearned from any other source.