NSSA

Understand

NSSA blocks incoming Type 5 LSAs but permits a local ASBR to originate Type 7 externals within the area. An elected/eligible ABR translates suitable Type 7 records to Type 5 for the rest of the domain. The P-bit requests propagation; translation also depends on forwarding address and other eligibility rules. Type 7 flooding itself remains NSSA-local.

A translated LSA can retain a nonzero forwarding address. Remote routers need internal reachability to that address, so filtering or summarizing it incorrectly can leave an external LSA visible but unusable. Do not suppress it blindly to conceal a reachability error.

An ordinary NSSA does not automatically get the ordinary-stub default. On this platform an NSSA ABR can explicitly originate a Type 7 default. An NSSA ASBR has a local-default condition for that mechanism. The no-summary ABR variant supplies a Type 3 default and suppresses ordinary Type 3 specifics; it still allows local Type 7 externals.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0, R3: area 1.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

configure terminal
hostname R3
router ospf 1
 router-id 10.255.0.3
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
 ip ospf 1 area 1
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

Apply

Start fresh with the border baseline, not with stub commands left over. Configure area 1 as NSSA on R2/R3. R3 imports a single discard static. This demonstrates advertisement, not delivery to a real external host.

R2 · ABR

configure terminal
router ospf 1
 area 1 nssa default-information-originate metric 10 metric-type 1
end

R3 · NSSA ASBR

configure terminal
ip route 203.0.113.0 255.255.255.0 Null0
ip prefix-list NSSA-IMPORT seq 5 permit 203.0.113.0/24
route-map NSSA-IN permit 10
 match ip address prefix-list NSSA-IMPORT
 set tag 300
 exit
router ospf 1
 area 1 nssa
 redistribute static subnets metric 20 metric-type 1 route-map NSSA-IN
end

Verify and interpret

R2# show ip ospf database nssa-external 203.0.113.0
  LS Type: AS External Link
  Link State ID: 203.0.113.0
  Advertising Router: 10.255.0.3
  Metric Type: 1
  Metric: 20
  Forward Address: 10.255.0.3
  External Route Tag: 300
R1# show ip ospf database external 203.0.113.0
  Advertising Router: 10.255.0.2
  Metric Type: 1
  Metric: 20
  Forward Address: 10.255.0.3
  External Route Tag: 300

This constructed example selects R3’s enabled loopback as forwarding address. Selection can vary by image; inspect the actual field. R2 is the sole ABR/translator and originates the Type 5.

R1# show ip route ospf | include 203.0.113
O E1 203.0.113.0/24 [110/41] via 192.0.2.2, GigabitEthernet1
R3# show ip route ospf | include 0.0.0.0
O*N1 0.0.0.0/0 [110/20] via 192.0.2.5, GigabitEthernet1

With forwarding address 10.255.0.3, R1’s internal cost is 21; external seed 20 makes 41. R3’s Type 7 default costs 10 to the ABR plus external seed 10.

For the no-summary alternative, remove R2’s explicit Type 7 default command, configure area 1 nssa no-summary and area 1 default-cost 5. R3 then uses a Type 3 default at cost 15. Local externals still appear as Type 7 in area 1 and translated Type 5 outside. Restore a fresh baseline before comparing normal, stub and NSSA behavior.

Technical references

OSPFv2 standard · Router configuration guide · NSSA specification