Hello Sivam,
migrating from EIGRP to OSPF is easier because default AD for different route types make internal EIGRP routes AD 90 preferred over OSPF routes ( default AD 110 for each type).
You can configure OSPF on one part of the network on a contiguous group of routers and links to emulate the EIGRP neighborships without even redistributing EIGRP into OSPF in all routers.
You just need to avoid to have OSPF external routes AD 110 preferred over EIGRP external routes AD 170
So you should use
router ospf 1
distance ospf external 180
!
on all routers.
You can check the contents of OSPF LSDB with
show ip ospf database
and you compare it with
show ip eigrp topology.
You should see the same set of internal and external routes on both protocols. EIGRP routes are still installed on all routers IP routing tables.
The ASBR routers for your migration are the last two routers in the block of routers running both EIGRP and OSPF.
These routers can implement mutual redistribution of OSPF into EIGRP and EIGRP into OSPF in a controlled way using route tags.
Using route-tags you don't need to care about specific IP prefixes and the route-tags will avoid re-injection of routes coming from OSPF again in OSPF and routes coming from EIGRP again in EIGRP.
This configuration is needed because you have two ASBR running both EIGRP and OSPF and performing mutual redistribution
Routes coming from EIGRP into OSPF will get route-tag 88
Routes coming from OSPF into EIGRP will get route-tag 110
The key for performing controlled redistribution is to deny all routes that have a route tag that tell that the origin of the route is in the other protocol.
route-map EIGRP-into-OSPF deny 5
match tag 110
route-map EIGRP-into-OSPF permit 10
set tag 88
route-map OSPF-into-EIGRP deny 5
match tag 88
route-map OSPF-into-EIGRP permit 10
set tag 110
router ospf 1
redistribute eigrp <AS-number> route-map EIGRP-into-OSPF subnets
router eigrp 100
redistribute ospf 1 route-map OSPF-into-EIGRP
default-metric 10000 10 1 1 1500
!
Only routes that are in the IP routing table can be redistributed successfully
On routers on the left of the ASBR you can do the following
router eigrp 100
distance eigrp 120 190
!
This should make OSPF routes preferred on all routers on the left side and should populate the right side with D EX routes.
when redistributing into EIGRP a seed metric is needed and it uses 5 values for the metric components and the MTU.
For migrating from OSPF to EIGRP for the distance vector roots of EIGRP you need to redistribute OSPF into EIGRP in each router, otherwise no route is propagated because EIGRP sends updates only for routes installed in the local routing table.
Again you can add EIGRP with initial AD values higher then OSPF and redistribute OSPF on each device to populate the EIGRP topology table.
router eigrp 100
distance eigrp 120 170
redistribute ospf 1
default-metric 10000 10 1 1 1500
!
Note: you do not perform mutual redistribution in all routers but only on the two ASBR nodes at border.
Hope to help
Giuseppe
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