OSPF Default routes

Unanswered Question
Feb 7th, 2007

Is there a recommended practice to control/manage multiple default routes as more and more of our OSPF areas/markets get their own Internet?

Is it just using filters/route maps? (OSPF area filtering?) I thought of adjusting the E1 metrics for each default-orig but this may not be possible today without some testing and maintenance windows.

Bringing up a new Internet for an ospf area that has several ABRs and trying to avoid static 0/0 routes from ABR to ABR to get to the FW and would like to have the FW generate the default route but I do not want that route getting to our core and other non-stub areas.

Thanks

Regards

Joe

I have this problem too.
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m-haddad Wed, 02/07/2007 - 16:19

Hello Joe,

I didn't understand why do you have multiple default routes on spokes. Default routes should be originated just by ABRs. And those spokes who have their own internet you can set one static route pointing to their own ISP.

Can you please explain more so that I can help,

Regards,

jtorjman Wed, 02/07/2007 - 18:21

Whether originating the default rte at the ABR or from a FW/router a couple routers away from the ABR is not the issue. The concern is that other areas will learn this other new default route and if metrics are not properly adjusted could become a better route for another area. Just trying to control the advertisement of the default route from an ABR. (Not all areas are converted to stub today.)

Regards

Joe

John Patrick Lopez Wed, 02/07/2007 - 20:04

try to adjust the administrative distance of default routes received from the spoke routers. When the ABR received a default route from the spoke router then set a high administrative distance for 0.0.0.0/0 network and the AD should be higher than 110. I don't know if this will work. It can also serve as a redundant path just incase the ABR lost its default route to area 0.

Richard Burts Thu, 02/08/2007 - 03:57

John

I do not think that adjusting the administrative distance for the 0.0.0.0 route is a workable solution. Setting the administrative distance is a local activity and administrative distance is not advertised. So you would wind up needing to adjust EVERY router.

Joe

I believe that the best solution to your issue is to configure Not So Stubby Areas which allow external routes generated within the area and control external routes from outside the area.

Adjusting the metrics of E1 routes would probably get close but I think that NSSA would be better.

HTH

Rick

m-haddad Thu, 02/08/2007 - 08:51

Hello John,

The admin distance of the router originated through OSPF will have 110 as admin distance. On the spokes that have internet connection you just need a static default route. THerefore, spokes will always use their own static route for internet traffic and they won't install the default route advertised by OSPF because it is of high distance.

Spokes that do not have internet connection, will not need any default static route and they will use the OSPF advertised default route for internet traffic.

I hope this solves your issues,

Regards,

jtorjman Mon, 02/12/2007 - 14:33

I do agree using NSSAs. All of our new markets/ospf areas are configured as NSSAs.

This is an existing, rather large stub area, getting its own Internet conn. We were trying to get away from severall static default routes between the two ABRs and planned on using the default-orig... We may still do it dynamically but will carefully analyze the metrics for other markets.

(We initially wanted to put filters on the two ABRs to restrict the default route out to the core.)

Thanks

Regards

jt

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