An NSSA is a special type of stub network. It is only used in an area with an ASBR, and is typically implemented to reduce routing traffic and SPF calculations within that area while maintaining communications with external ASes. External routes injected into the NSSA by the ASBR use type 7 packets. When the type 7 packets hits the ABR, it is then translated to a type 5 packet for dissemination through the rest of the OSPF AS.

To configure an NSSA, use the following commands:

area [area-id] nssa [no-summary]

Usually you would want to use the no-summary keyword, which prevents type 3 and type 4 LSAs. This further reduces OSPF traffic and SPF recalculations. You may also wish to use route summarization on the ASBR to further reduce SPF calculations and routing table entries.

Summary-address [prefix mask] [not advertise] [tag tag]

The [not advertise] option is used to suppress routes that match the prefix/mask. This only works for OSPF routes. [tag] is used for policy routing.

Command	Description
show ip ospf border-routers	Display internal OSPF routing table entries to an ABR
show ip ospf virtual-links	Display virtual link parameters
show ip ospf [process-id]	display are information and router OSPF information
show ip ospf database	Show the topological database
show ip ospf [process-id area-id] database [router]	Show router link-state information
show ip ospf [process-id area-id] database [network]	Show network link-state information
show ip ospf [process-id area-id] database [summary]	Summary information about ABR link-states
show ip ospf [process-id area-id] database [asbr-summary]	Information on ASBR link states
show ip ospf [process-id area-id] database [external]	Information on AS external link states
show ip ospf [process-id area-id] database [database-summary]	display database summary and totals

LSA Type	Description
1. Router link entry	Internal to 1 area
2. Network Link entry	Internal to 1 area, sent by DR in multiaccess networks
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3. Summary Link Entry	ABR to ABR in an AS and to backbone. Describes links in AS
4. Summary Link Entry	ABR to ABR in an AS and to backbone. Describes links to ASBR(s).
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5. AS External Link	From ASBR to all except in NSSA and stubs. E1/E2.
7. AS External Link	From ASBR to ABR, only in an NSSA. N1/N2.
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6. Multicast	Not Used

Area Type	LSAs allowed
Standard multiaccess	1,2,3,4,5
Standard not-multiaccess	1,3,4,5
Backbone	1,2,3,4,5
Stub	1,2,3,4
Totally Stubby	1,2
Not So Stubby	1,2,3,4,7
Not So Stubby, no-summary	1,2,7

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