RIP and OSPF Redistribution

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In the topology above R3 is running OSPF and R4 is running RIP. R1 and R2 are running both OSPF and RIP and redistributing OSPF into RIP and RIP into OSPF

R3 and R4 have their loopback addresses with the same IP address 3.3.3.3/24


R1#show run | sec router
router ospf 1
 redistribute rip subnets
 network 192.168.13.0 0.0.0.255 area 0
router rip
 version 2
 redistribute ospf 1 metric 5
 network 192.168.14.0
 no auto-summary
R1#

R2#sh run | sec router
router ospf 1
 redistribute rip subnets
 network 192.168.23.0 0.0.0.255 area 0
router rip
 version 2
 redistribute ospf 1 metric 5
 network 192.168.24.0
 no auto-summary
R2#

R3#sh run | sec router
router ospf 1
 network 0.0.0.0 255.255.255.255 area 0
R3#

R4#sh run | sec router
router rip
 version 2
 network 0.0.0.0
 no auto-summary
R4#




R4#sh ip rout  
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        3.3.3.0/24 is directly connected, Loopback0
L        3.3.3.3/32 is directly connected, Loopback0
R     192.168.13.0/24 [120/5] via 192.168.24.2, 00:00:28, Ethernet0/1
                      [120/5] via 192.168.14.1, 00:00:08, Ethernet0/0
      192.168.14.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.14.0/24 is directly connected, Ethernet0/0
L        192.168.14.4/32 is directly connected, Ethernet0/0
R     192.168.23.0/24 [120/5] via 192.168.24.2, 00:00:28, Ethernet0/1
                      [120/5] via 192.168.14.1, 00:00:08, Ethernet0/0
      192.168.24.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.24.0/24 is directly connected, Ethernet0/1
L        192.168.24.4/32 is directly connected, Ethernet0/1
R4#

On R1 and R2 we can see the route 3.3.3.3 learned by OSPF and RIP. I wonder if this can create a problem or it is acceptable .

Thank you

R1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O E2     3.3.3.0/24 [110/20] via 192.168.13.3, 00:00:25, Ethernet0/0
O        3.3.3.3/32 [110/11] via 192.168.13.3, 00:44:59, Ethernet0/0
      192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.13.0/24 is directly connected, Ethernet0/0
L        192.168.13.1/32 is directly connected, Ethernet0/0
      192.168.14.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.14.0/24 is directly connected, Ethernet0/1
L        192.168.14.1/32 is directly connected, Ethernet0/1
O     192.168.23.0/24 [110/20] via 192.168.13.3, 00:47:12, Ethernet0/0
O E2  192.168.24.0/24 [110/20] via 192.168.13.3, 00:47:12, Ethernet0/0
R1#


R2#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

      3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
R        3.3.3.0/24 [120/1] via 192.168.24.4, 00:00:03, Ethernet0/1
O        3.3.3.3/32 [110/11] via 192.168.23.3, 00:47:50, Ethernet0/0
O     192.168.13.0/24 [110/20] via 192.168.23.3, 00:50:02, Ethernet0/0
O E2  192.168.14.0/24 [110/20] via 192.168.23.3, 00:50:02, Ethernet0/0
      192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.23.0/24 is directly connected, Ethernet0/0
L        192.168.23.2/32 is directly connected, Ethernet0/0
      192.168.24.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.24.0/24 is directly connected, Ethernet0/1
L        192.168.24.2/32 is directly connected, Ethernet0/1
R2#
Screen-Shot-2018-02-18-at-5.07.21-PM.png
jskfanAsked:
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Don JohnstonInstructorCommented:
R3 and R4 have their loopback addresses with the same IP address 3.3.3.3/24

On R1 and R2 we can see the route 3.3.3.3 learned by OSPF and RIP. I wonder if this can create a problem or it is acceptable .

The same IP address on two different devices?  Yeah, that's a problem (maybe not at this time in this very limited situation).  And no, it's not acceptable.
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Hello ThereSystem AdministratorCommented:
R3 and R4 have their loopback addresses with the same IP address 3.3.3.3/24
You shouldn't do this. Maybe it's working now but it's definitely not recommended and it will cause troubles.

Loopback interface’s IP Address determines a router’s OSPF Router ID.
Loopback interfaces are treated similar to physical interfaces in a router.

From other sources (Cisco...):
You can specify a software-only interface called a loopback interface to emulate an interface. Loopback interfaces are supported on all platforms. A loopback interface is a virtual interface that is always up and allows Border Gateway Protocol (BGP) and remote source-route bridging (RSRB) sessions to stay up even if the outbound interface is down.

You can use the loopback interface as the termination address for BGP sessions, for RSRB connections, or to establish a Telnet session from the device's console to its auxiliary port when all other interfaces are down. You can also use a loopback interface to configure IPX-PPP on asynchronous interfaces. To do so, you must associate an asynchronous interface with a loopback interface configured to run IPX. In applications in which other routers or access servers attempt to reach this loopback interface, you should configure a routing protocol to distribute the subnet assigned to the loopback address.
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jskfanAuthor Commented:
ok so same iP addresses should not be in 2 or more seprate devices in the LAN network.
What about redistribution, routes are redistributed from OSPF to RIP and from RIP to OSPF, is there any risk that the same routes will loop around from OSPF to RIP then back to OSPF and so on ?

OR we can assume that the protocols are intelligent, if route is redistributed from OSPF to RIP then RIP will not redistribute the same route back to OSPF and Vice-versa ?

Thank you
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Hello ThereSystem AdministratorCommented:
Loopback addresses should be unique no matter what.
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Don JohnstonInstructorCommented:
What about redistribution, routes are redistributed from OSPF to RIP and from RIP to OSPF, is there any risk that the same routes will loop around from OSPF to RIP then back to OSPF and so on ?
Yes, depending on the failure, it is possible to create a "black hole" when redistributing routing protocols in a loop scenario.  It's one of the things we used to create in the old ACRC (Advanced Cisco Router Configuration) class.  But it's one of those things that require a failure of a link in a particular location to create the black hole.  

I also created this situation in a multi-vendor routing class where a route would remain in the routing tables for hours after I unplugged the cable to it.

Good times! ;-)
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jskfanAuthor Commented:
in the topology  above:

R1#sh ip route ospf

      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/11] via 192.168.13.3, 00:40:13, Ethernet0/0
O     192.168.23.0/24 [110/20] via 192.168.13.3, 00:40:13, Ethernet0/0
O E2  192.168.24.0/24 [110/20] via 192.168.13.3, 00:17:46, Ethernet0/0
R1#

R2#sh ip route ospf

      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/11] via 192.168.23.3, 00:19:47, Ethernet0/0
O     192.168.13.0/24 [110/20] via 192.168.23.3, 00:19:47, Ethernet0/0
O E2  192.168.14.0/24 [110/20] via 192.168.23.3, 00:19:47, Ethernet0/0
R2#

R3#sh ip route ospf

O E2  192.168.14.0/24 [110/20] via 192.168.13.1, 00:38:18, Ethernet0/0
O E2  192.168.24.0/24 [110/20] via 192.168.23.2, 00:20:36, Ethernet0/1
R3#

R4#sh ip route rip

      3.0.0.0/32 is subnetted, 1 subnets
R        3.3.3.3 [120/5] via 192.168.24.2, 00:00:05, Ethernet0/1
                 [120/5] via 192.168.14.1, 00:00:18, Ethernet0/0
R     192.168.13.0/24 [120/5] via 192.168.24.2, 00:00:05, Ethernet0/1
                      [120/5] via 192.168.14.1, 00:00:18, Ethernet0/0
R     192.168.23.0/24 [120/5] via 192.168.24.2, 00:00:05, Ethernet0/1
                      [120/5] via 192.168.14.1, 00:00:18, Ethernet0/0
R4#


You can see the routes learned by each router above. what scenario will create a Loop ?
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Don JohnstonInstructorCommented:
This paper discusses how redistribution could create routing loops and how to avoid them.

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8606-redist.html#avoidredist
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jskfanAuthor Commented:
Thank you Guys!
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