OSPF Stub Area and NSSA area..

[vipin kumar]

Hi All,

I am new on cisco and now reading the OSPF and have some  confusion in STUB area and OSPF NSSA area my confusion are as..

1. where is the exit point os stub area ( Always in Area 0 ). and if the OSPF stub area Exit point is not in Area 0 but to the external network what will happen ?

2. if external routes can be described through the LSA 5 then why we need the LSA 7...??

thanks in advance

[cadet alain]

Hi,

One of the stub and nssa areas role are to shorten the size of the routing table.To achieve this they forbid certain types of lsa from entering the area, for a stub these are lsa 4 and 5 and same goes for nssa so we are not permitting external routes into the area.In a stub there can't be no asbr but in an nssa it can and as lasa type 5 are denied the trick is to replace them with lsa type 7 which will be translated to lsa type 5 at the abr.

Regards.

Alain.

[Latchum Naidu · Accepted solution]

Hi,

The type 7 LSA that is generated by an NSSA ASBR. Type 5 LSAs are not allowed in NSSA areas, so the NSSA ASBR generates a type 7 LSA instead, which remains within the NSSA. This type 7 LSA gets translated back into a type 5 by the NSSA ABR.

Please rate the helpfull posts.
Regards,
Naidu.

[Giuseppe Larosa · Accepted solution]

Hello Vipin,

1) only an NSSA can have an exit point to a different routing domain and this is the reason to have both NSSA and stub areas. A stub area cannot have any ASBR node inside: no LSA type 5 are allowed.

This is by definition. The objective is to limit the size of the link state DB to the minimum required.

Totally stub areas block also all LSA type 3 from entering the area: only one O IA route exists in a totally stub area and it is a default route injected by the ABR(s).

2) because in a stub area no LSA type 5 can exist, so NSSA area allows two different tasks:

  it allows to have an exit point to another routing domain without the need to import all the external routes that are in area 0.0.0.0.

The area 0 to NSSA ABR acts as a firewall for LSA type 5 in area 0.0.0.0 and doesn-t allow them to enter the NSSA link state database.

LSA type 7 may be converted to LSA type 5 or not depending on the setting of the P bit from the NSSA ABR.

The P bit is set by the NSSA ASBR node.

so another use of NSSA areas is to have control on the propagation of external routes: if you need to have an external route injected from another routing domain only used within a single area you can use NSSA for this.

LSA type 7 lives within the original NSSA area only.

Hope to help

Giuseppe

[vipin kumar]

Thanks to all for such informative responce. My confusion is clear now..