As Giuseppe and I discussed on another thread, my lab routers seem to be buggy when it comes to deploying this command. I'm losing a lot of time obsessing over this, since I suspect it will be on the BSCI exam. But while I'm trying to see if I can get it to work on my routers,
a) why is this command configured under router-config mode, rather than interface (like an EIGRP summary address)? If I'm configuring it on an ABR then it gets propagated into the backbone (at least it DID, in my weird lab) and that's not what I want.
b) what about the "always" keyword? It will advertise the default route via OSPF even if there's a manually configured route in there, is what my books say. But if the default route was manually configured, it's not going to be advertised by a protocol in the first place, so why the command...
More to come; thanks in advance guys.
With respect to your first question, I think the default-originate command is always specified in router configuration mode, whether in EIGRP, BGP, IS-IS, RIP, or OSPF. OSPF is the only routing protocol that supports an "always" parameter.
The "always" parameter tells OSPF to advertise a default route even if there is no defined default route in its routing table, by whatever means it would be in the routing table (e.g., static route or another routing protocol). The exception to this rule is when you have a route-map on your default-information originate always statement. In this case the advertisement of a default route is contingent on the satisfaction of the condition(s) of the route-map.