Default information originate in ospf

[ncnaveen_arasu]

                   Hi

1) wrt ospf default information originate command can we configure the 'default information originate' command in only one router in a ospf area/process where in we have defined default route or in multiple routers.

2) default information originate should be configure only on ABR/ASBR or any non ABR/ASBR also.

3) Without a default route in a router where in we are advertising the default toute with " default information originate always "  command what route will be inserted as a default route with the next hop.

Please clarify.

Thanks & Regards,

Naveen

[Rolf Fischer]

Hi,

1) You can inject a default route on multiple routers. They are external routes, so you can inject them either as E1 or E2 (default). The selection process is the same as for other external routes. As the flooding-scope of external routes is AS-wide, the topology database then will contain all the injected default routes (exception: Internal routers in Stub-Areas).

2) Normally you would inject the default route on an ASBR, but this is not a requirement (always-keyword). However, this router should know how to reach the outside world.

3) The default route is advertised as a Type-5 LSA, so other routers can reach it through the originating ASBR.

Without the always-keyword (when a default-route exists), the Forwarding-Address in the Type-5 can be set to a non-zero address for path optimization under certain conditions (next-hop is reachable through a multiaccess-interface and an interface in that network is OSPF-enabled). But in the majority of cases, even with an non-zero forwarding address set, the next-hop of the path to the advertising ASBR will be used for the next-hop.

HTH

Rolf

[EDIT]: I don't know how much you already know about the treatment of external routing information in OSPF. Please feel free to ask further if my posting is not clear.

[ncnaveen_arasu]

Hi Rolf,

Thanks for your explaination.

Still am confused with Question no.3

What is non zero forwarding address and without default route in ASBR what default route will be injected to AS and wjhat will be the nexthop.

Please clarify.

[Rolf Fischer]

Naveen,

you're welcome!

After I've read Joseph's #3 answer (which is of course correct too), I think that I perhaps misunderstood the question.

We have to distinguish the router which injects the default route and other routers that receive updates with the Type-5 LSA for this default route.

The originating router won't install an OSPF default route, it is supposed to already know how to reach the outside world.So it advertises the default information to the other routers within the domain and those routers will normally calculate the next-hop by evaluating the path towards the originating ASBR and instal the resulting default route into their routing tables.

A special case is a non-zero forwarding address. This is a path-optimization mechanism to avoid unnecessary hops through the originating ASBR when a shorter path (in terms of hops, not routing metric) exists. However, the result of the next-hop calculation will be the same for most routers, especially when they are further away from the origin.

The "Description of OSPF Forwarding Address" section in the linked document provides a very good description:

Common Routing Problem with OSPF Forwarding Address

Sorry for the confusion.

HTH

Rolf

P.S.:

Joseph pointed out another good point in #2: As soon as you configure the default-information origiante command, the router will announce external routing information, so technically it is an ASBR then.

[Joseph W. Doherty]

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Posting

#1 Multiple allowed

#2 I believe the moment you inject an OSPF default, it logically "makes" the router an ASBR (if not already one).  As with any ASBR, it doesn't also need to be an ABR.

#3 I believe it won't - i.e. it will black hole default traffic