External default route in NSSA area

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By Ivan Pepelnjak

OSPF NSSA area accepts external routes redistributed into the OSPF domain as NSSA external (LSA type 7) routes that are later translated by an ABR into domain-wide external (LSA type 5) routes. In designs where the default exit from the OSPF domain (for example, Internet connection) resides in an NSSA area, a router in an NSSA area has to insert a default route into the NSSA area (a sample network is displayed in Figure 1).

Figure 1: Sample network with default exit in NSSA area

Cisco IOS uses the default-information originate router configuration command to insert a default route into an OSPF domain. The default route could be inserted conditionally (if the router itself has a non-OSPF default route) or unconditionally. You can also change the metric and metric type (External 1 or 2) of the default route.

The default-information originate command injects a default route only in regular OSPF areas (backbone area and other regular areas). To inject a default route into an NSSA area, you have to use the area number nssa default-information-originate router configuration command, which can also specify the metric with the metric keyword and the metric type (NSSA external 1 or 2) with the metric-type keyword.

Origination of NSSA default route is always conditional: a router configured with area nssa default-information-originate will insert a default route into an NSSA area only if it has a non-OSPF default route or if it’s an ABR (having at least one OSPF interface in the backbone area). Default routes originated by ABRs have the propagate (P) bit cleared and are thus not translated into type-5 external routes. Default routes originated by non-ABR routers due to presence of a non-OSPF default route have the P bit set and are thus translated by the ABRs into type-5 external routes that are propagated throughout the OSPF domain.

The following configuration is entered on the GW router in the sample network to generate default route in the area 22:

router ospf 1
 log-adjacency-changes
 area 22 nssa default-information-originate metric 100 metric-type 1
 network 0.0.0.0 255.255.255.255 area 22
!
ip route 0.0.0.0 0.0.0.0 Serial0/0

The GW router generates a type-7 default route, which can be inspected at ABR-A:

ABR-A#show ip ospf database nssa-external | exclude LS|Check

            OSPF Router with ID (10.0.1.9) (Process ID 1)

                Type-7 AS External Link States (Area 22)

  Options: (No TOS-capability, Type 7/5 translation, DC)
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: GW
  Length: 36
  Network Mask: /0
        Metric Type: 1 (Comparable directly to link state metric)
        Metric: 100
        Forward Address: 10.0.1.8
        External Route Tag: 0

ABR-A translates the type-7 default route (which has the P bit set, as indicated by the Type 7/5 translation text in the printout) into a type-5 default route which is propagated throughout the network. The following printout was taken on a router residing in area 66:

B1#show ip ospf database external | exclude LS|Check

            OSPF Router with ID (172.16.1.1) (Process ID 1)

                Type-5 AS External Link States

  Options: (No TOS-capability, DC)
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: ABR-A
  Length: 36
  Network Mask: /0
        Metric Type: 1 (Comparable directly to link state metric)
        Metric: 100
        Forward Address: 10.0.1.8
        External Route Tag: 0

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