Redistribution between EIGRP and OSPF

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Redistribution between EIGRP 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 EIGRP, R1 has all OSPF routes, all OSPF routes are redistributed into EIGRP and show up as EIGRP EX routes. Metric must be specified.
R2(config)#router eigrp 100
R2(config-router)#redistribute ospf 100 metric 100000 100 255 1 1500

R1#sh ip route eigrp
     4.0.0.0/32 is subnetted, 1 subnets
D EX    4.4.4.4 [170/307200] via 100.100.12.2, 00:00:04, FastEthernet0/0
     200.200.34.0/30 is subnetted, 1 subnets
D EX    200.200.34.0 [170/307200] via 100.100.12.2, 00:00:04, FastEthernet0/0
     55.0.0.0/24 is subnetted, 1 subnets
D EX    55.55.55.0 [170/307200] via 100.100.12.2, 00:00:04, FastEthernet0/0
     5.0.0.0/24 is subnetted, 1 subnets
D EX    5.5.5.0 [170/307200] via 100.100.12.2, 00:00:04, FastEthernet0/0
D EX 200.200.45.0/24 [170/307200] via 100.100.12.2, 00:00:04, FastEthernet0/0
D EX 200.200.24.0/24 [170/307200] via 100.100.12.2, 00:00:04, FastEthernet0/0
R1#
After redistribute EIGRP to OSPF, all EIGRP 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)#router ospf 100
R2(config-router)#redistribute eigrp 100 subnets

R5#sh ip route ospf
     1.0.0.0/24 is subnetted, 1 subnets
O E2    1.1.1.0 [110/20] via 200.200.45.4, 00:00:26, FastEthernet0/0
     100.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O E2    100.100.12.0/24 [110/20] via 200.200.45.4, 00:00:26, FastEthernet0/0
O E2    100.100.13.0/30 [110/20] via 200.200.45.4, 00:00:26, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/11] via 200.200.45.4, 01:31:37, FastEthernet0/0
     200.200.34.0/30 is subnetted, 1 subnets
O IA    200.200.34.0 [110/74] via 200.200.45.4, 01:31:37, FastEthernet0/0
O IA 200.200.24.0/24 [110/20] via 200.200.45.4, 01:31:37, FastEthernet0/0
R5#

With EIGRP to OSPF redistribution, the EIGRP EX route (AD 170) will be change to OPSF E1/E2 (AD 110). Let's take the two route 1.1.1.0/24 and 11.11.11.0/24 which injected into EIGRP from R1,
1.1.1.0/24 is using network command so it is EIGRP internal route, 11.11.11.0/24is using redistribution command so it is EIGRP EX route, we can see them on R2
R2#sh ip route eigrp
     1.0.0.0/24 is subnetted, 1 subnets
D       1.1.1.0 [90/409600] via 100.100.12.1, 00:11:49, FastEthernet0/0
     100.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       100.100.13.0/30
           [90/2195456] via 100.100.12.1, 00:11:49, FastEthernet0/0
     11.0.0.0/24 is subnetted, 1 subnets
D EX    11.11.11.0 [170/409600] via 100.100.12.1, 00:00:34, FastEthernet0/0
R2#
So another border router R3, after the mutual redistribution on R2, R3 reach 11.11.11.0/24 is no longer go to the short path which is in EIGRP domain, instead, it goes to OSPF domain because OSPF has better AD (110); in EIGRP domain, the route has AD 170.
R3#sh ip route | i 11.11.
O E2    11.11.11.0 [110/20] via 200.200.34.2, 11:36:33, Serial1/1
R3#
Note the next hop is R2 which is ASBR.

Then if we redistribute EIGRP to OSPF on the 2nd border router R3, since at this moment, R3's routing table no longer has EIGRP EX route, so only EIGRP internal routes are injected into OSPF domain, EIGRP EX routes will behaviors like RIP2OSPF redistribution. The new EIGRP2OSPF routes change their AD from 90 to 110, when these routes reach R2, R2 already have them in routing table with AD 90, so they will no be injected into routing table, so no loop.

The if we redistribute OSPF to EIGRP on the 2nd border router R3, the OSPF2EIGRP routes will change their AD from 110 to 170, when they reach R2 via EIGRP domain, R2 will discard them since R2 already has them with better AD 110.

So with EIGRP and OSPF mutual redistribution, there is no loop. Only for the EIGRP EX route, 2nd border router will go through OSPF domain to reach it.










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