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Path Selection problem in OSPF

Hi network gurus,

I am having a path selection problem here, hope someone can help. Please refer to the attached diagram.

Basically, the VPN gateway in HQ needs to communicate with the VPN peer in Partner office. Both A.A.A.0/24 and B.B.B.0/24 subnets are global addresses, and therefore routable across the Internet.

HQ has 4 paths to A.A.A.0, arranged in order of priority

1. Point-to-point between HQ-A and R01

2. Point-to-point between DR-A and R02

3. Via Private ISP cloud

4. Via public Internet

I'm am trying to make Path 3 the preferred path, but unable to do so because of the following reasons:

a> Path 1 & 2 are currently seen by OSPF process as internal routes

b> Router ISP-B is redistributing A.A.A.0/24 from BGP into OSPF. Thus, ISP-B is advertising A.A.A.0/24 as an external route in OSPF to HQ-B, which is then propagated to HQ-A and DR-A.

Thus, Path 3 loses out to Path #1 and #2

Someone recommended me to perform the following actions:

a> Remove network A.A.A.0 from the OSPF process in R01 and R02

b> Add the command 'redistribute static' in the OSPF process of R01 and R02

c> Add the following static routes in R01 and R02

-R01 : ip route A.A.A.0 Gi3/0

-R02 : ip route A.A.A.0 Fa0/1

The objective is to make all the paths advertised as an 'external' type, which would then enable the use of metrics as a deciding factor.

Another reason why this alternative was suggested was because when I run a 'show ip ospf borders', the private ISP's routers (ISP-A and ISP-B) have a better metric than the other routers. Therefore, we assume that if all routes are of the same type (external), the router would make use of this metric.

Would this work?

Thanks a lot!!


Cisco Employee

Re: Path Selection problem in OSPF

I think making all of the routes as external would work. Another way to make path 1 and 2 external routes is to remove the network statement for a.a.a.0 on r1 and r2 and use the "redistribute connected" command instead.

I would also recommend to configure path # 3 as an external type 1 so that it is always prefer over path 1 and 2.

Let me know if I answered your question,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600

Re: Path Selection problem in OSPF

If you are running a MPLS-VPN across the pvt ISP , maybe you could look if your Pvt ISP supports sham-links. But yes making the routes as external would suffice.

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