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MaxwellSilver
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Joined: Thu Oct 07, 2021 10:11 pm

OSPF and default routes

Sat Oct 09, 2021 1:59 am

Diagram1.png
I'm having an issue with default routes not failing back properly regardless of how the distance is weighted. This problem seems similar to two other posts, but it's not exactly the same issue as far as I can tell.

There are three routers in OSPF area 0 (see pic). Default routes are redistributed via Router1 and Router3. Default routes are only distributed if available. Initially all outbound traffic egresses as planned via Router1 to the ISP (Distance=1). If I simulate a failure of Router1, Router2 and Router3 learn the 'backup' default route attached to Router3. This is as planned and Router2 and Router3 are able to send traffic out the slow and expensive cell modem (Distance=240). If I look at the routing tables on Router2/Router3, it is clear that the Cell route has now been distributed via OSPF and caused the change. However, once the connection via Router1 is restored, the less desirable route persists in the OSPF tables on Router2 and Router3. The only way I have found to dislodge it is to disable the default route on Router3. When that happens, the preferred default route via Router1 propagates through OSPF and traffic flows correctly. I can re-enable the default route on Router3 and traffic still flows correctly since it is won't be propagated via OSPF until the preferred default route is unavailable. This seems to be a "There can only be one" scenario. Ideally, the process of failing back to Router1's default route would happen automatically. I've tried adding the egress networks to the OSPF instances with appropriate weighting applied to the interfaces, but that doesn't make a difference in the behavior I'm seeing.

Any thoughts?

Any help would be much appreciated.
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