Type-4 LSA and NSSA

Hi group,

can someone please give a hint on this?

I have problems to understand why OSPF LSA Type-4 should be allowed within
NSSA's.

According to Routing TCP/IP, Vol. 1, page 389, Type-4 LSA's are allowed to
be sent into NSSA's.
Cisco (
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml)
gives a similar description:
After you define Area 1 as an NSSA, it must have these characteristics:
- No Type 5 LSAs are allowed in Area 1

Cisco further defines Type-3 and Type-4 LSA's are not sent into Totally
NSSA's.

Does it make sense?
As we know, LSA Type-4 describe ASBR's; but if the information about
external routes is blocked, there's no need to have Type-4 LSA's.

When I Lab this up, the NSSA router indeed has NO Type-4 LSA pointing to an
ASRB within Area 0.

Thus, in which scenario will I ever see Type-4 LSA's within a NSSA?

Thanks in advance.

Mike

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Received on Sat May 09 2009 - 01:37:22 ART