Default Route Redistribution into OSPF (from other Routing Protocols)

[john.dejesus]

Hi All,

Is it possible to redistribute a default route 0.0.0.0/0 into OSPF i.e. eBGP or Static Route without using the "default-information originate"? I was just trying to simulate this scenario specifically for redistribution, but failed to redistribute a 0.0.0.0/0 whether its from eBGP or even a static default route. Btw, I'm using GNS3. So even on the ospf debug output there was no type5 or type7 lsa generated for 0.0.0.0/0. Is this a loop prevention in OSPF? Is there any explanation on this?

Thanks.

[Peter Paluch · Accepted solution]

Hi John,

On Cisco IOS-based devices, this is a well-known behavior. You cannot redistribute a default route into OSPF. You can only have it injected using the default-information originate command. In some cases, the default route will be injected automatically, without configuring any extra commands (specifically, when using multiple areas and some of the areas are stubby, totally stubby, or NSSA totally stubby areas). However, even in these cases, you cannot force a default route into OSPF by redistribution.

I suppose that this is a built-in sanity check in Cisco IOS to prevent inadvertent "hijacking" of traffic going to internet by a router that is not intended to be a gateway to the outside world. You can redistribute any other route into OSPF, but if you want to have a default route in OSPF, you have to consciously configure the default-information originate as a means of saying "Yes, I really want this."

Would any of this make sense?

Best regards,
Peter

[john.dejesus]

Hi Peter,

Thank you for responding. Actually, I never tried this simulation before. Because I always use "default-information originate" command in my production network. It's just so happens I'm practicing for my ccie. That's why I posted my query on this forum just to seek clarifications. I tried to look this on the OSPF 2328 RFC but to no avail. One last thing, I assume that Cisco also hide this on ospf debug outputs. I tried debug ip ospf lsa-gen but it only shows the type5 or type7 lsa generation for non-zero prefixes.

[Peter Paluch]

Hi John,

It's just so happens I'm practicing for my ccie.

Perfect - good luck!

I tried to look this on the OSPF 2328 RFC but to no avail.

Yes - this behavior is specific to Cisco IOS implementation. Other vendors and/or operating systems may behave differently. This behavior is not a part of official OSPF specification.

One last thing, I assume that Cisco also hide this on ospf debug outputs. I tried debug ip ospf lsa-gen but it only shows the type5 or type7 lsa generation for non-zero prefixes.

I don't think that the information about ignoring the default route during redistribution is hidden in the debugs. If a router does not redistribute a route into OSPF in the first place, there is no LSA5 or LSA7 for that route whatsoever, so there is nothing to show in the debugs. Try doing another experiment: Create a set of static (or connected) routes, and redistribute only a subset of them into OSPF using a route-map and a prefix-list. I suppose that the debugs will be completely silent about all the routes you have filtered out in the route-map, just like with the default route.

Best regards,
Peter