In this tutorial, I'd like to go through some elementary concepts of mutual redistribution. Redistribution is process by which the routes learnt by one routing protocol are injected into another routing protocol. These routes could be connected or static as well. An example would an organization having to run various routing protocols. In the topology below, OSPF is running for one half of the network and RIP running on the other as indicated by the dotted lines. R3 and R4 are running both OSPF and RIP.
Topology
Dynagen file
| [[Router R1]] model = 3640 console = 2001 autostart = false slot0 = NM-4E slot1 = NM-4T E0/0 = R2 E0/0 S1/0 = R3 S1/0 [[Router R2]] model = 3640 console = 2002 autostart = false slot0 = NM-4E slot1 = NM-4T S1/0 = R4 S1/0 [[Router R3]] model = 3640 console = 2003 autostart = false slot0 = NM-4E slot1 = NM-4T E0/0 = R5 E0/0 [[Router R4]] model = 3640 console = 2004 autostart = false slot0 = NM-4E slot1 = NM-4T E0/0 = R5 E0/1 [[Router R5]] model = 3640 console = 2005 autostart = false slot0 = NM-4E slot1 = NM-4T E0/2 = LAN 1 |
Initial Configurations
| R1 interface Ethernet0/0 ip address 192.168.4.1 255.255.255.0 half-duplex interface Serial1/0 ip address 192.168.3.1 255.255.255.0 clock rate 128000 router ospf 1 router-id 1.1.1.1 log-adjacency-changes network 192.168.3.1 0.0.0.0 area 0 network 192.168.4.1 0.0.0.0 area 0 |
| R2 interface Ethernet0/0 ip address 192.168.4.2 255.255.255.0 half-duplex interface Serial1/0 ip address 192.168.5.2 255.255.255.0 clock rate 128000 router ospf 1 router-id 2.2.2.2 log-adjacency-changes network 192.168.4.2 0.0.0.0 area 0 network 192.168.5.2 0.0.0.0 area 0 |
| R3 interface Ethernet0/0 ip address 192.168.2.3 255.255.255.0 half-duplex interface Serial1/0 ip address 192.168.3.3 255.255.255.0 router ospf 1 router-id 3.3.3.3 log-adjacency-changes detail network 192.168.3.3 0.0.0.0 area 0 router rip passive-interface Serial1/0 network 192.168.2.0 |
| R4 interface Ethernet0/0 ip address 192.168.6.4 255.255.255.0 half-duplex interface Serial1/0 ip address 192.168.5.4 255.255.255.0 serial restart-delay 0 router ospf 1 router-id 4.4.4.4 log-adjacency-changes detail network 192.168.5.4 0.0.0.0 area 0 router rip passive-interface Serial1/0 network 192.168.6.0 |
| R5 interface Ethernet0/0 ip address 192.168.2.5 255.255.255.0 half-duplex interface Ethernet0/1 ip address 192.168.6.5 255.255.255.0 half-duplex interface Ethernet0/2 ip address 192.168.1.5 255.255.255.0 half-duplex router rip network 192.168.1.0 network 192.168.2.0 network 192.168.6.0 |
On examining the route table of R1, it seen R1 has no idea of RIP routes. Obviously, OSPF is not advertising those routes. Since R3 and R4 are running both RIP and OSPF, mutual redistribution can be configured on those devices.
| R3#sh ip route Codes: 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 Gateway of last resort is not set O 192.168.4.0/24 [110/74] via 192.168.3.1, 00:04:49, Serial1/0 O 192.168.5.0/24 [110/138] via 192.168.3.1, 00:04:49, Serial1/0 R 192.168.6.0/24 [120/1] via 192.168.2.5, 00:00:19, Ethernet0/0 R 192.168.1.0/24 [120/1] via 192.168.2.5, 00:00:19, Ethernet0/0 C 192.168.2.0/24 is directly connected, Ethernet0/0 C 192.168.3.0/24 is directly connected, Serial1/0 |
| R3#sh ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 588 0x80000004 0x009D1F 3 2.2.2.2 2.2.2.2 585 0x80000003 0x00DECC 3 3.3.3.3 3.3.3.3 1179 0x80000002 0x00BDED 2 4.4.4.4 4.4.4.4 1163 0x80000003 0x00E3B5 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 192.168.4.2 2.2.2.2 585 0x80000002 0x00E5CF |
| R1#sh ip route Codes: 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 Gateway of last resort is not set C 192.168.4.0/24 is directly connected, Ethernet0/0 O 192.168.5.0/24 [110/74] via 192.168.4.2, 00:03:55, Ethernet0/0 C 192.168.3.0/24 is directly connected, Serial1/0 |
Table 1 : Routing Tables and OSPF database
On R3 and R4, redistribution is configured. When redistributing a routing procotol, for instance, RIP into OSPF (receiving protocol), specify the protocol that must redistributed, in this case RIP and the metric associated with routes of the protocol being redistributed, unless a default-metric parameter is configured. Optional parameters can be specified [route-maps], which is outside the scope of this tutorial
R3(config-router)#redistribute rip metric 100
R4(config-router)#redistribute rip metric 100
Similarly we need redistribute OSPF into RIP
R3(config-router)#redistribute ospf 1 metric 2
R4(config-router)#redistribute ospf 1 metric 2
Lets take a look at R1 and R3 routing table now.
| R3#sh ip route Codes: 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 Gateway of last resort is not set O 192.168.4.0/24 [110/74] via 192.168.3.1, 00:04:31, Serial1/0 O 192.168.5.0/24 [110/138] via 192.168.3.1, 00:04:31, Serial1/0 O E2 192.168.6.0/24 [110/100] via 192.168.3.1, 00:02:20, Serial1/0 R 192.168.1.0/24 [120/1] via 192.168.2.5, 00:00:23, Ethernet0/0 C 192.168.2.0/24 is directly connected, Ethernet0/0 C 192.168.3.0/24 is directly connected, Serial1/0 |
| R4#sh ip route Codes: 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 Gateway of last resort is not set O 192.168.4.0/24 [110/74] via 192.168.5.2, 00:11:11, Serial1/0 C 192.168.5.0/24 is directly connected, Serial1/0 C 192.168.6.0/24 is directly connected, Ethernet0/0 O E2 192.168.1.0/24 [110/100] via 192.168.5.2, 00:07:41, Serial1/0 O E2 192.168.2.0/24 [110/100] via 192.168.5.2, 00:07:41, Serial1/0 O 192.168.3.0/24 [110/138] via 192.168.5.2, 00:11:11, Serial1/0 |
| R1#sh ip route Codes: 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 Gateway of last resort is not set C 192.168.4.0/24 is directly connected, Ethernet0/0 O 192.168.5.0/24 [110/74] via 192.168.4.2, 00:06:36, Ethernet0/0 O E2 192.168.6.0/24 [110/100] via 192.168.4.2, 00:04:25, Ethernet0/0 O E2 192.168.1.0/24 [110/100] via 192.168.3.3, 00:03:06, Serial1/0 O E2 192.168.2.0/24 [110/100] via 192.168.3.3, 00:03:06, Serial1/0 C 192.168.3.0/24 is directly connected, Serial1/0 |
We see R1 is learning RIP routes now as OSPF External Type 2, sweet. However, take a closer look at R3's routing table, it is now learning the route 192.168.0.6/24 via OSPF even when the same route via RIP is the optimal path. Similarly, R4 is learning routes to 192.168.1.0/24 and 192.168.2.0/24 via OSPF instead of RIP. When routes are advertised by two routing protocols, the router will always select the routes from the routing protocol which has a better administrative distance, in this case OSPF. We need to somehow tell the router to choose specific routes from a particular protocol and this is were distribute-lists comes in play.
By using a distribute list and applying it under the OSPF process of R3 and R4, specific routes learned via OSPF/RIP can be inserted into the routing table.
distribute-list [access-list no] in
applied under a routing process, specifies which routes learnt via the routing protocol should be filtered in the routing table.
Here, our objective is to permit 192.168.4.0/24 and 192.168.5.0/24 to be learnt via OSPF by R3 and 192.168.3.0/24 and 192.168.4.0/24 to be learnt via OSPF by R4. The similar logic is applied to the RIP routing process as well.
| R3 configuration access-list 1 permit 192.168.4.0 0.0.0.255 access-list 1 permit 192.168.5.0 0.0.0.255 access-list 2 permit 192.168.6.0 0.0.0.255 access-list 2 permit 192.168.1.0 0.0.0.255 router ospf 1 router-id 3.3.3.3 log-adjacency-changes detail redistribute rip metric 100 network 192.168.3.3 0.0.0.0 area 0 distribute-list 1 in router rip passive-interface Serial1/0 network 192.168.2.0 distribute-list 2 in |
| access-list 1 permit 192.168.4.0 0.0.0.255 access-list 1 permit 192.168.3.0 0.0.0.255 access-list 2 permit 192.168.2.0 0.0.0.255 access-list 2 permit 192.168.1.0 0.0.0.255 router ospf 1 router-id 4.4.4.4 log-adjacency-changes detail redistribute rip metric 100 network 192.168.5.4 0.0.0.0 area 0 distribute-list 1 in router rip redistribute ospf 1 metric 2 passive-interface Serial1/0 network 192.168.6.0 distribute-list 2 in |
Please remember that applying a distribute-list [access-list no] in only filters routes from the routing table of R3 and R4 . R3 and R4 will continue to advertise LSAs for the following routes 192.168.0.2/24, 192.168.6.0/24, 192.168.1.0/24. By examining R1's routing table and R3's OSPF database, this can be verified.
| R1#sh ip route Codes: 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 Gateway of last resort is not set C 192.168.4.0/24 is directly connected, Ethernet0/0 O 192.168.5.0/24 [110/74] via 192.168.4.2, 00:36:26, Ethernet0/0 O E2 192.168.6.0/24 [110/100] via 192.168.3.3, 00:09:31, Serial1/0 O E2 192.168.1.0/24 [110/100] via 192.168.3.3, 00:07:27, Serial1/0 O E2 192.168.2.0/24 [110/100] via 192.168.3.3, 00:07:27, Serial1/0 C 192.168.3.0/24 is directly connected, Serial1/0 |
| R3#sh ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 1391 0x80000005 0x009B20 3 2.2.2.2 2.2.2.2 1187 0x80000004 0x00DCCD 3 3.3.3.3 3.3.3.3 459 0x80000004 0x00BFE7 2 4.4.4.4 4.4.4.4 416 0x80000005 0x00E5AF 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 192.168.4.2 2.2.2.2 1187 0x80000003 0x00E3D0 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 192.168.1.0 3.3.3.3 211 0x80000003 0x00EFE7 0 192.168.1.0 4.4.4.4 548 0x80000001 0x00D5FF 0 192.168.2.0 3.3.3.3 211 0x80000003 0x00E4F1 0 192.168.2.0 4.4.4.4 548 0x80000001 0x00CA0A 0 192.168.6.0 3.3.3.3 669 0x80000001 0x00BC18 0 192.168.6.0 4.4.4.4 161 0x80000003 0x009A34 0 |
Finally, examining R3 and R4s routing table , it can be verified that the routes mentioned in Table 2 is now learned via RIP.
| R3#sh ip route Codes: 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 Gateway of last resort is not set O 192.168.4.0/24 [110/74] via 192.168.3.1, 00:14:44, Serial1/0 O 192.168.5.0/24 [110/138] via 192.168.3.1, 00:14:44, Serial1/0 R 192.168.6.0/24 [120/1] via 192.168.2.5, 00:00:04, Ethernet0/0 R 192.168.1.0/24 [120/1] via 192.168.2.5, 00:00:04, Ethernet0/0 C 192.168.2.0/24 is directly connected, Ethernet0/0 C 192.168.3.0/24 is directly connected, Serial1/0 |
| R4#sh ip route Codes: 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 Gateway of last resort is not set O 192.168.4.0/24 [110/74] via 192.168.5.2, 00:13:03, Serial1/0 C 192.168.5.0/24 is directly connected, Serial1/0 C 192.168.6.0/24 is directly connected, Ethernet0/0 R 192.168.1.0/24 [120/1] via 192.168.6.5, 00:00:05, Ethernet0/0 R 192.168.2.0/24 [120/1] via 192.168.6.5, 00:00:05, Ethernet0/0 O 192.168.3.0/24 [110/138] via 192.168.5.2, 00:13:03, Serial1/0 |
I have tried to keep this tutorial simple. When I started, I thought it would a piece of cake to write one. Well the answer my friends, it's not. However, I'd encourage more people to write tutorials because it benefits others and familiarises oneself with topics that are not their strong areas.
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