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# OSPF routing loop?

I have the following setup

cost 100
EC01-A -------OSPF------------ WA01-A --------------------------------- site 2 WA01-A
|                                     |
O |(Cost 100)                 O | (cost 110)
|                                     |
EC01-B---------OSPF------------ WA01-B ------------------EIGRP-------------- Site 2
cost 100    (distance lowered to 80)

I am learning the routes VIA EIGRP on WA01-B  from site 2 and redistributing into OSPF with a metric of 100. WA01-B has a distance of 80 and sending the routes to EC01-B and WA01-A.

Will the routes be learnt by WA01-B from EC01-B and WA01-A?

WA01-B - EC01-B  -------- cost 200
EC01-B - EC01-A ----------cost 300
EC01-A - WA01-A --------- cost 400
WA01-A - WA01-B -------- cost 500

WA01B - WA01-A - cost 210
WA01-A - EC01-A - cost 310
EC01-A - EC01-B - cost 410
EC01-B - WA01-B - cost 510

So WA01-B learns the same route through WA01-A with a lower metric. Will this cause a routing loop since the OSPF distance on WA01-B is 80?

WA01-B learns route from EIGRP with distance 90 and metric 307899 AND it learns OSPF route from WA01-A distance is 80 and cost is 500

Is there something where OSPF does not receive routes on the advertising routers?

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hkdv
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2 Solutions

Commented:
WA01-B will learn the route from all sources, but it will populate the RIB with routes via *its* lowest admin distance.  So if you've changed the admin distance of OSPF only on WA01, it will still view the EIGRP route as most preferable.  If you want WA01-B to prefer OSPF routes over EIGRP routes, you need to change its admin distance for OSPF, or raise the AD of EIGRP above 110.
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Author Commented:
I changed the admin distance on WA01-B to 80.

So WA01-B RIB will be populated with the OSPF route?

I currently am testing this and see that the WA01-B RIB is populated with the EIGRP route.

It made sense to me that it should be populated with OSPF route and not EIGRP. But does not seem to be the case.

Something I am missing?
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Commented:
Hi,

The routes learned from EIGRP will be redistributed into OSPF as OSPF type 2 external routes. This will make the router WA01-B an OSPF ASBR.

In this case the path will be the lowest cost to reach the ASBR.
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Commented:
As per my understanding..router WA01-B will c subnet from site 2 learned via self IP in database of
for e,g if u check Type-5 AS External Link States on WA01-B it will c self IP in database.
So routing loop wont happen.
when WA01 advertises same route to WA-01B will reject it as it already has it in database.

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Author Commented:
Really Sorry, but don't understand this well..

EIGRP is redistributed into OSPF on ROUTER WA01-B as type 2 external routes.

So when a packet comes to WA01-B to go to site B.. it will use the EIGRP route even though the OSPF distance on the router is 90?

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Commented:
Your statements are little confusing ....
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InstructorCommented:
>So when a packet comes to WA01-B to go to site B.. it will use the EIGRP route even though the OSPF distance on the router is 90?

The router will use whatever route has the lowest administrative distance. If the AD is the same, then it will use the route with the lowest metric. If the metrics are the same, then it will use the four routes that have the same lowest metric (by default).
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Author Commented:
I meant with a OSPF distance of 80.

@Sanjeevloke - I do see the route in the OSPF data base as Type 5 external link state on WA01-B

10.20.0.0       10.7.255.5      760         0x8000002B 0x0078E5 20
10.21.0.72     10.7.255.5      760         0x80000051 0x0023D2 20

What is the reason for this? Is it because I re-distributed EIGRP into OSPF on WA01-B?
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InstructorCommented:
>I meant with a OSPF distance of 80.

If you haven't changed the AD of EIGRP to something lower than 80, the router will prefer the OSPF route.
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Author Commented:
It is taking the EIGRP route and I havent changed the AD of EIGRP. It is at 90!
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InstructorCommented:
Please post the output of "show ip route" and also show your topology with the IP addresses.
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Commented:
Yes its because u redistributed EIGRP in OSPF ...
I am sure it wont create any loop ...

10.20.0.0       10.7.255.5      760         0x8000002B 0x0078E5 20
10.21.0.72     10.7.255.5      760         0x80000051 0x0023D2 20

so is this 10.7.255.5  WA01-B  interface IP ? it should be ..
If it is then looping wont happen ..
Also i can see tag of 20 is ur EIGRP AS number 20 ?

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Commented:
Hi,

It won't create a loop.

Since WA01-B is redistributing routes from EIGRP into OSPF as OSPF type 2 externals, WA01-B will become an OSPF ASBR. You're OSPF internal routers will have already calculated a route to the ASBR using SPF. The other OSPF routers will see the type 2 external routes and will know how to get to them via the ASBR.

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