ospf,asbr in nssa and default route

[sarahr202]

Hi everybody

Please consider the following example:

rip-199.199.199.3 f0/0----R1----nssa area 1--------R2.

R1 is an asbr in nssa.

R1

router ospf 1

area 1 nssa

R1 routing table is shown below"

R 10.0.0.0

S  0.0.0.0 0.0.0.0 199.199.199.3

Even when i configured " redistribute static " under ospf on R1,  R1 does not redistribute default route into ospf.

Commnads " default-information originate/always" have no effect either on asbr (R1) in nssa.

Even we have default route on asbr(r1) in nssa as:

DEX 0.0.0.0 /0 via 199.199.199.3

And we configure " redistribute eigrp subnets" under ospf on R1,  still R1 does not redistribute default route into ospf.

1)Does it mean an asbr in nssa will not redistribute default route into ospf or I  am dealing with a bug in ios?

2)Commnads " default-information originate/always" have no effect either on R1 in nssa. Is this normal for asbr in nssa ?

=============================================================================================

  ----eigrp-----R1-----area 2-----R2-------area 0-----R3

Above area 2 is a normal area with R1 as asbr.

Let say we have following entrries in routing table on R1:

DEX 0.0.0.0/0 199.199.199.2

Now if we configure " redistribute eigrp 1 subnets" under ospf 1.  R1 still does not redistribute default route into ospf.

The only time asbr redistribute default route into ospf when we configure " default-information originate" if the asbr has default route present in its routing table or we configure " distribute-information originate always" which does not require default route be present in routing table of a asbr.

The concept I want to confirm is if we have a default route learned via eigrp and we want to redistribute it into ospf,  the command " redistribute eigrp 1 subnets" will not redistribute the default route into ospf.

Happy Hollidays!

[Peter Paluch · Accepted solution]

Hello Sarah,

Even when i configured " redistribute static " under ospf on R1,  R1 does not redistribute default route into ospf.

Yes, that is expected. OSPF (and IS-IS) in Cisco implementation never redistributes default route into itself. You may redistribute any other route into OSPF but the IOS code is deliberately written to not redistribute the default route into OSPF.

1)Does it mean an asbr in nssa will not redistribute default route into ospf or I  am dealing with a bug in ios?

No ASBR will redistribute a default route into OSPF, as explained earlier. The area type does not matter. In your particular scenario, however, the default-information originate command has no effect because it creates an LSA-5 carring the 0.0.0.0/0 route. Recall that NSSA areas do not accept nor process LSA-5, so the default route injected in form of LSA-5 will not be seen by an NSSA area. If you need to inject a default route into a NSSA area, use the area 1 nssa default-information-originate command that will inject the default route as LSA-7.

2)Commnads " default-information originate/always" have no effect either on R1 in nssa. Is this normal for asbr in nssa ?

Yes as explained above.

Now if we configure " redistribute eigrp 1 subnets" under ospf 1.  R1 still does not redistribute default route into ospf.

Correct. Remember the plain rule: you cannot redistribute a default route into OSPF. You have to inject it using specific commands or rely on the ABR to do that (ABRs inject default routes into stubby/totally stubby/NSSA totally stubby areas without any additional configuration).

The concept I want to confirm is if we have a default route learned via  eigrp and we want to redistribute it into ospf,  the command "  redistribute eigrp 1 subnets" will not redistribute the default route  into ospf. 

Yes, that is correct. Once again, a default route cannot be redistributed into OSPF. It is not a limitation of the OSPF protocol itself but rather a sanity check performed on redistributed routes by Cisco IOS. Different vendors may behave differently in this aspect.

Please feel welcome to ask further.

Best regards,

Peter

[sarahr202]

Hi Peter.

how are you doing?

area 1 nssa default-information-originate  

The above command is only valid  on abr connected to nssa.

However, If we have a asbr in nssa and we want this asbr to advertise default route in nssa using lsa 7.,we can not do so.

The only way to introduce a default route in nssa is to use " area x nssa default-information -originate command on abr connected to nssa.  Is my understanding correct ?

-----------------------------------------------------------------------------------------------

Abr redistribute default route into ospf using lsa5 .But it requires the use of" default-information originate/always " command. The default route  has a cost 1 and type E2.

Let say we have abr redistributing 0.0.0.0/0 as E2,cost=1 into ospf.

Can we change that i.e cost and type ?

Just curious, is this Holiday season in Slovakia ?

Thanks Peter.

[Peter Paluch · Accepted solution]

Hello Sarah,

I am fine - how are you? Hope you are enjoying this Christmas time... actually, I see you enjoying it by diving deep into OSPF

area 1 nssa default-information-originate   
The above command is only valid  on abr connected to nssa.

Why do you think so? You can use this command also on routers internal to area 1. Agreed that usually, this command is used on ABRs but apart from that, this command can be perfectly used on any router within a NSSA area. Please note that this command will inject a default route (as an LSA-7) only if at least one of the two following conditions is met:

• The router on which this command is entered is truly an ABR (i.e. has an active interface in area 0)
• The router on which this command is entered already has a default route installed in its routing table (perhaps from a different routing protocol or configured statically)

The only way to introduce a default route in nssa is to use " area x  nssa default-information -originate command on abr connected to nssa.   Is my understanding correct ?

As explained above - not entirely. You can inject a default route to a NSSA area on any ASBR within the NSSA area using the area X nssa default-information-originate and making sure that this ASBR already has a default route in its routing table. This requirement is very similar to the classic default-information originate command.

Let say we have abr redistributing 0.0.0.0/0 as E2,cost=1 into ospf.
Can we change that i.e cost and type ?

Yes, this is easily accomplished using the additional arguments to the default-information originate command, for example:

R4(config-router)#  default-information originate ?                   

  always       Always advertise default route

  metric       OSPF default metric

  metric-type  OSPF metric type for default routes

  route-map    Route-map reference

R4(config-router)#  default-information originate metric ?

  <0-16777214>  OSPF metric

R4(config-router)#  default-information originate metric 100 ?

  always       Always advertise default route

  metric-type  OSPF metric type for default routes

  route-map    Route-map reference

R4(config-router)#  default-information originate metric 100 metric-type ?

  <1-2>  OSPF Link State type

R4(config-router)#  default-information originate metric 100 metric-type 1 ?

  always     Always advertise default route

  route-map  Route-map reference

R4(config-router)#  default-information originate metric 100 metric-type 1

This is a quick transcript of me going through the help to the default-information originate command. I also believe that the attribute of the default route could be modified by referring to a route-map that would use the appropriate set commands.

Just curious, is this Holiday season in Slovakia ?

Oh, yes, it is! And I am enjoying it sooooooo much Honestly, needed it.

Best regards,

Peter