OSPF route choice

[AllezLom63194]

Hi the Community,

I still have the same problem and would need your help.

As you can see on the attached chart. I would like to force the route choice from the blue network to the green network to use "Site main router".

Sometime on my production network this is the "Site Bkp router" that is used to join the green network and I Don't know why.

Regards,

[Joseph W. Doherty]

Your question only concerns the traffic flow from Blue to Green, not the converse too?

How does traffic get from the blue network to DC main or DC Backup?

What area is the DC Main <> DC Backup link in?

[AllezLom63194]

Hello Joseph,

Yes my question concerns flow from Blue to Green.

The blue network use the DC Main router to go away.

The area 0.0.0.2 is a standard OSPF area.

The DC main router has "area 0.0.0.2 default-cost 100" and the DC Backup has "area 0.0.0.2 default-cost 200"

Regards,

[Joseph W. Doherty]

You've answered one of my three questions, the first one.

If my second and third questions are unclear, please let me know.

[AllezLom63194]

Joseph,

I may did'nt uderstood your questions.

The blue network use the DC main as a Gateway. Is it what you need for your second question ?

The link between DC Main and DC Back (Fa0) is on the area 0 as the Blus network. Is it what you need for your third question ?

Regards.

[Joseph W. Doherty]

Yes, and yes - thanks.

Re: DC Main as gateway; i.e. all hosts are hard coded or DHCP provided gateway IP? DC Main's gateway IP is physical IP of interface, not a virtual IP provided by HSRP, GLBP or VRRP?

Forgot to ask, and OSPF cost of interfaces connecting Site Main and Bkp to Green? Also, no summarization of Area Two on DC Main and/or Backup?

[AllezLom63194]

Hello Joseph,

Here is the chart with more configurations informations.

In fact the blue network is my area 0 network. My servers are connected to the red network. They have hard coded ip. There gateway are VRRP IP address configured on virtual interface on R1 and R2 routers. 

I have added the OSPF cost of the green network and there is no summarization one the area 0.0.0.2.

[AllezLom63194]

Hello the community,

Any idees ?

Regards

[Joseph W. Doherty]

I'm sorry, I forgot to follow-up.

So far, I haven't seen an obvious cause for what you describe.

My thinking was to continue to ask more questions and/or request device configurations, as I was thinking of emulating your environment in something like GNS, but I no longer have an user account with the necessary IOS download provisions, and GNS doesn't support current devices, so unsure it would be of benefit.

I also looked into something like "Cisco Modeling Labs - Personal" but don't have that currently, unsure it's device support would be of benefit, and there's also the expense.

I also looked into trying even Cisco's Packet Tracer, which isn't too great for complex simulations, but forgot it doesn't even simulate OSPF.

Perhaps with enough time, and with enough information, maybe I could reason out your issue, but alas, that's more time I can really devote to this.

So, sorry, I don't think I'll be able to help further, and also, again, sorry I forgot to get back on this thread sooner.

What I can do is mention some of Hall of Fame members ( @Richard Burts, @Jon Marshall, @Peter Paluch ), strong in routing issues, who might take a quick peak and see if they can help, or perhaps, reference another expert who might.  NB: FYI, Rick, from this reference, will probably at least say hello, but Jon and Peter haven't been nearly as active as they've been in the past - I presume they are otherwise pressed for time.

[Richard Burts]

I appreciate the mention from @Joseph W. Doherty I am happy to take a look at this and hope that Jon and/or Peter might also join in.

Am I understanding the post correctly that sometimes the path chosen is the desired one but sometimes the path chosen is the backup path? If this is the case it suggests that some network change has taken place causing the different path selection rather than a configuration issue. 

But while we think about that possibility I would ask that the original poster give us the output of show ip ospf interface from each of the 4 routers so that we can get a better understanding of their relationship.

[Joseph W. Doherty]

Rick, thank you for so promptly joining in.

[AllezLom63194]

Hello Rick,

I attached the output of the Show IP ospf interfaces of the site "ETB" main and backup and "DC" main and backup.

I attached the bandwidth usage charts for the main and backup "ETB" routers WAN

As you can see, on the backup router WAN there is only input trafic (black line).

Is it possible that this trafic is the OSPF packets exchanges ?

I'm trying to configure netflow on theses routers to have a look on this flow.

Regards.

[AllezLom63194]

I think that I have found why it doesn't work as I want.

I have "ip ospf cost 100" on the int gi 0/0/1 interface and not on the sub interfaces on the main router. Same with cost 200 on the backup router. It seems that the "ip ospf cost xxx" command is not applied automatically on the sub -interfaces as I think.

I put the "ip ospf cost xxx" comand on each sub-interfaces and it seems that the flow use oly the main router WAN link as I wanted.

Thank for your help and I'll close the case when I'll be sure that the problem is solved.

Regards.

[Joseph W. Doherty]

You're correct, in that a main interface cost is not applied automatically to subinterfaces.

[Richard Burts · Accepted solution]

Thanks for the update. I believe that you have correctly identified the issue being that cost from the main interface is not automatically applied to subinterfaces. Congratulations on finding the solution to your own issue.