Redistribution between EIGRP and OSPF with 2 ASBRs

[Hamed Karimi]

Hello guys,

On the below topology, I tried to redistribute between OSPF and EIGRP. On CLE1 I redistributed a static route (4.4.4.0) into EIGRP, so DC1 and DC2 see that route as External EIGRP.

But after redistribution between OSPF and EIGRP (bidirectional), I am not sure why DC2 does not inject the 4.4.4.0 as OSPF route (cause of lower metric)

The problem is that DC2 sees it as external EIGRP but DC1 as External OSPF:

. Here is the OSPF database output:

Here is the OSPF and EIGRP configuration on both routers:

DC1#
router eigrp 100
network 10.30.30.0 0.0.0.255
network 172.20.20.0 0.0.0.255
redistribute ospf 1 metric 100 0 0 1 1500

!
DC1#
router ospf 1
redistribute eigrp 100 subnets
network 172.20.20.0 0.0.0.255 area 0

----------------------------

DC2#
router eigrp 100
network 10.50.50.0 0.0.0.255
network 172.20.20.0 0.0.0.255
redistribute ospf 1 metric 100 0 0 1 1500

!
DC1#
router ospf 1
redistribute eigrp 100 subnets
network 172.20.20.0 0.0.0.255 area 0

[Giuseppe Larosa · Accepted solution]

Hello @Hamed Karimi ,

>> I redistributed a static route (4.4.4.0) into EIGRP, so DC1 and DC2 see that route as External EIGRP.

So the route is originally a D EX route in EIGRP in both DC1 and DC2

You have two routers that perform mutual redistribution between EIGRP AS 100 and OSPF process-id 1.

You see the effects of a race condition:

the first router to redistribute the EIGRP D EX route for 4.4.4.0/24 into OSPF  is DC2.

DC1 installs the OSPF LSA type 5 for network 4.4.4.0/24 in the IP routing table as OSPF external routes have lower AD 110 instead of 170.

You can check this using

on DC1:

show ip ospf database external 4.4.4.0

You will see the advertising router as DC2.

As the EIGRP external route is not installed anymore on DC1, DC1 will not inject the route into the OSPF domain .

Also DC2 keeps the original EIGRP D EX route installed in the IP routing table.

If you clear the OSPF process on both routers the opposite can happen.

You understand that when having two ASBR performing mutual redistribution some care is needed. Here EIGRP provides a different AD 170 for external routes so it solves many issues, but in general to avoid route re-inejction the use of route tags in route maps is recommended.

What is learned from a protocol should be never be sent back to the same protocol even at the other border router.

At the end what you see is normal and it is the effect or a race condition that is both DC1 and DC2 are in competition to inject the prefix into OSPF domain , however one of them is faster and the other device installs the OSPF route and this prevents it from injecting the route into OSPF domain . Only routes installed in IP routing table can be redistributed in your case DC1 accepting the route from DC2 removed the D EX route and it cannot redsitribute from EIGRP to OSPF that prefix.

Hope to help

Giuseppe

[Hamed Karimi]

Thank you for the good explanation.