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jey.theepan
Beginner

NSSA default route

NSSA ABR still generate a default route even if it doesn't have a default route in his routing table but, it has area 0 present. when I shut down the area 0 interface it stops generating default route to that area.

is this normal for nssa?

is there any work around?

Thanks...Jey

7 REPLIES 7
Richard Burts
Hall of Fame Guru

Could you be specific about how you have configured OSPF on this router? In my experience NSSA does not generate a default unless you have done something to ask for it.

I do not think we can say whether this is normal or not until we know the specifics of your configuration.

HTH

Rick

correct,

- the ABR is configured with "nssa default-information-originate"

- or nssa no-summary

When you configure the ABR with default-information-originate you are asking the router to generate a default route and advertise it into the NSSA area. As long as the router has access to area 0 it will generate the default route and if it looses its access to area 0 it withdraws the default route. This is normal behavior for OSPF NSSA.

In OSPF NSSA the default route is optional while in OSPF normal stub areas the default route is automatic from the ABR. So why is it automatic in stub and optional in NSSA?

In normal stub areas there are no external routes at all in the stub area and the only way out of the stub area is through the ABR. So think especially of how things work on one of the routers within the stub area: we know that there are some destinations which are not represented in the routing table, so how do we get to those destinations that are not in the routing table - we must go through the ABR to get to them. So it makes sense for the ABR to automatically generate a default route and advertise it into the stub area. In effect the ABR is telling the routers: for any destination you want to get to that is not in your routing table, you should send it to me.

The NSSA is different. While the external routes from other areas (including area 0) are not advertised into the NSSA, there may be externals within the NSSA (which come from redistribution on one or more of the routers within the NSSA). So the path from a router in the NSSA to external destinations mighrt be through the ABR or it might be through the redistributing router. In effect the default route may be through the ABR or it may be through the redistributing router. So the ABR should not automatically generate the default route. It should generate the default route only if you ask it to (by configuring default-information-originate.

HTH

Rick

when "nssa no-summary" is configured it does generate default automatically...no need to configure default information generate. This have the same issue that it still generate default regardless it has default route in his routing table or not (as long as it has an area 0). if this is normal..looks that way.

do know any work around that it should stop generating the default as soon as there is no default route in his routing table.

That is correct. no-summary configured on the ABR instructs the ABR that not only are type 5 externals not advertised into the areaa but type 3 LSAs from other OSPF areas (including area 0) are not advertised into the area. The effect of no-summary is that the OSPF routers in the NSSA area have no visibility to any routes from anywhere else in the OSPF domain unless they have a default route from the ABR. So with no-summary generation of a default route by the ABR is automatic (as long as the ABR has access to area 0).

So let me try to summarize:

- "area 4 NSSA" means the ABR will advertise LSA type 3 from other areas into the area but not advertise any externals LSA type 5 into the area. The assumption here is that the default route will be through some router within the area which is redistributing.

- "area 4 NSSA default-information-originate" means that the ABR will advertise LSA type 3 from other areas into the area but not advertise any external LSA type 5 into the area, and will generate a default route. The assumption is that the default route to outside destinations is through the ABR.

- "area 4 NSSA no-summary" means that the ABR will not advertise any LSA type 3 from other OSPF areas, will not advertise any LSA type 5 externals from other areas, and will advertise a default route. The assummption is that the default route from the ABR is required for the routers in the area to access anything within the OSPF domain.

So the choice of how to configure NSSA depends on the topology of your network and how you want routing to work (how do you get to unknown destinations).

HTH

Rick
samsam_wang
Beginner

try

did you miss something?