OSPF Area Question

Unanswered Question
Dec 24th, 2008

I am working a solution to provide redundancy to a network that is running OSPF via satellite (ethernet via DMVPN tunnel) connections in area 0. The terminals are geographicaly located to take advantage of a fiber connection that is in OSPF area 1. I would like to take advantage of setting up the fiber as the redundant link (I know, fiber is better than satellite ... but there is a need to keep the primary route on satellite). Can I just set them up with a /30 area 1 network at each location and set the ospf cost higher than the primary links? Do I have to use the area default-cost command? How about floating static routes?

I have this problem too.
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Edison Ortiz Wed, 12/24/2008 - 12:08

Can I just set them up with a /30 area 1 network at each location and set the ospf cost higher than the primary links?

Not sure about your topology based on the information provided but keep in mind, intra-area routes will be preferred over inter-area routes regardless of the cost of the link.

In other words, if you set the secondary links as area 1 and the network located behind this router is also under area 1, the secondary link will be the preferred route over the DMVPN area 0.

How about floating static routes?

What about? What's the purpose for having floating routes in your topology?



richard.m.gilbert Wed, 12/24/2008 - 13:14

The topology is a DMVPN mesh of 7 locations. 2 of the locations are set up with a higher priority than the others so that they will always be the DR and BDR. They are also the ones mapped using nhrp. So ... the DR and BDR have ospf adj with all routers ... and obviously the lower priority routers have just 2 adj (DR and BDR). That is all located in area 0.

Each router is situated where they can take advantage of a fiber ring. I thought of doing a PTP with each ... or just doing a mgre tunnel through the area 1. With the nature of this network being one that is taken down and re-established often, I lean more to the opinion that it may be too complicated for the actual network managers out at each site. So I was trying to think of different solutions that would make it easier for them.

The floating static route could be inserted into each of the "spoke" routers so that if the ospf link is lost then it will go in the routing table. By setting the AD of the static route to higher than ospf (110) it will float it until needed.

I think my options are:

tunnel (ptp or mp)


floating static

/30 connection (but I am unsure of what this will cause ... like recursive routing issues).

Take a look at the attached for topology. Lines in blue are the connections I want to make physically ... just not sure how to do it logically. Also ... it has to be ethernet.


Edison Ortiz Wed, 12/24/2008 - 13:51

Thanks for posting the topology. Yes, assigning area 1 to the secondary link(s) will indeed create problems as the intra-area routes between sites will be preferred over the area 0 backbone inter-area routes.

You can use OSPF on the secondary links and use area 0 as well and modify the cost of the links as you mentioned in your original post.

You can also use floating static for the secondary connection, though I'm not sure how many spokes you will have /you also need the statics at the hub/ - thus this can potentially bring some scalability issues.

As for virtual-link, you implement this option when you have to connect partitioned area 0s over non-area 0s devices. I don't see the need for virtual-link per your diagram.



richard.m.gilbert Wed, 12/24/2008 - 18:52

You are right about the virtual-link ... forgot about that. So, if I am getting what you are saying ... the best option is to use tunnels?

Also ... I think I am getting confused with your use of intra and inter. Intra means within the area, and inter means across areas ... right?

Edit - Forgot to mention ... for the redundant link, it isn't necessary for all routers to be able to have adjency with eachother ... they all just need to get back to the one router that has all the services which is typically the DR.

Edison Ortiz Wed, 12/24/2008 - 19:10

the best option is to use tunnels?

I didn't imply to use tunnels. Personally, I'm not a big fan of tunnels. You can place the secondary links on area 0 and change the cost so the primary links remain preferred. I believe this is the simplest solution of all unless I'm missing something.

Also ... I think I am getting confused with your use of intra and inter. Intra means within the area, and inter means across areas ... right

Correct. OSPF routing decision prefers intra over inter.




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richard.m.gilbert Wed, 12/24/2008 - 19:12

Understand. Just do the /30 connection but move the ABR out to the area 1 router making the link an area 0 link. Correct?

richard.m.gilbert Wed, 01/07/2009 - 10:23

I was about to put this into use but then I suddenly realized that there is no way that this can work. With the secondary links being ones that are for redundancy, when the area 0 primary link goes down the router is then effectively cut off from area 0 with its only way back to area 0 through area 1.

I think tunnels are the only way to go ... making sure the ospf cost is higher than the primary link ... using an unadvertised /30 network to connect a spoke router to the area 1 router.


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