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OSPF default-information originate [always]

CriscoSystems
Level 5
Level 5

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.

1 Accepted Solution

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slmansfield
Level 4
Level 4

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.

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5 Replies 5

slmansfield
Level 4
Level 4

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.

Hello,

Susan is right. When you advertise a default route in a particular routing protocol, you usually want to advertise it to all your neighbors. Actually you expect that the default route advertised by you will spread troughout the network so any router in your network knows about it and eventually routes the traffic towards you. That's why the default-information originate is a command that goes for the entire routing process - it is not just a per-interface setting.

The default-information originate command is present in all routing protocols except EIGRP - what a shame. The EIGRP is the only protocol in which the default route must be redistributed into (or created as a summary route which is basically a not-so-nice trick).

Susan is also absolutely right on the always keyword. By default, OSPF makes sure that your router advertises a default route only if it already knows about it. It does not matter where your did your router get the default route from - it may be static or advertised by some other routing protocol. The point is that it must first be present in your routing table, only then it can be advertised in OSPF. However, with the always keyword, the OSPF skips this sanity check and advertises the default route blindly, whether you have it or not.

Also, as Susan has correctly pointed out, if a route-map is used in the default-information originate command, then the always keyword will be ignored.

Best regards,

Peter

Sorry Peter. I was referring to the EIGRP default-originate command. It may not work exactly the same as in the other routing protocols, but it serves a similar purpose.

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_eig1.html#wp1011454

Hello Susan,

Thank you for your reply. However, I have different experiences with the "default-information allowed" command in EIGRP.

You are saying that this command serves a similar purpose as the "default-information originate" in RIP, OSPF or IS-IS. I am afraid this is not the case. This EIGRP command only allows you to either send or accept a default route (or a candidate default route marked using the command "ip default-network") over EIGRP once it has been imported into EIGRP in the first place. You cannot use the "default-information allowed" to force the EIGRP to send default route. You can merely prevent it from sending it further or from accepting it but you will still have to redistribute a default route into EIGRP.

Frankly, I do not understand what keeps the IOS developers from implementing the simple "default-information originate" to the EIGRP, considering that the code base already exists in other routing protocols and just needs to be adapted for EIGRP.

Best regards,

Peter

Hi Susan,

I just gave it a try and figured out yet another limitation: the "default-information allowed" does not prevent the route 0.0.0.0/0 from being advertised or sent. It just prevents the "candidate default network" flag from being assigned or accepted for a network that is imported in EIGRP by whatever means and is declared as default network using the "ip default-network" command. As the "ip default-network" is quirky at best, it is best unused, which renders the "default-information always" command practically needless.

Best regards,

Peter

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