nssa (OSPF)
Function
The nssa command configures an not-so-stubby area (NSSA) area.
The undo nssa command cancels the configuration.
By default, no OSPF area is configured as the NSSA area.
Format
nssa [ { default-route-advertise [ backbone-peer-ignore ] | suppress-default-route } | flush-waiting-timer interval-value | no-import-route | no-summary | set-n-bit | suppress-forwarding-address | translator-always | translator-interval interval-value | zero-address-forwarding | translator-strict ] *
undo nssa [ flush-waiting-timer interval-value ]
Parameters
| Parameter | Description | Value |
|---|---|---|
| default-route-advertise | Generates default Type-7 LSAs on the ASBR and then advertises
them to the NSSA. NOTE:
The ABR generates a default NSSA
LSA (Type-7 LSA) automatically and advertises it in the NSSA. |
- |
| backbone-peer-ignore | Prevents the ABR from checking the neighbor status when the ABR generates default Type 7 LSAs and advertises them to the NSSA. Specifically, the ABR generates default Type 7 LSAs and advertises them to the NSSA as long as an interface that is Up exist in the backbone area. | - |
| suppress-default-route | Generates default Type-7 LSAs on the ASBR or ABR and then not advertises them to the NSSA. | - |
| flush-waiting-timer interval-value | Specifies the interval for an ASBR to send aged Type-5 LSAs. The parameter takes effect only when it is set. | The value is an integer that ranges from 1 to 40, in seconds. |
| no-import-route | Indicates that no external route is imported to the NSSA. | - |
| no-summary | Indicates that the ABR is prevented from transmitting summary LSAs to the NSSA. | - |
| set-n-bit | Indicates that the N-bit is set in DD packets. | By default, N-bit is not set in DD packets. |
| suppress-forwarding-address | Sets the forwarding address (FA) of the Type-5 LSAs translated from Type-7 LSAs by the NSSA ABR to 0.0.0.0. | - |
| translator-always | Specifies the ABR in an NSSA as the translator. Multiple ABRs in an NSSA can be configured as translators. | - |
| translator-interval interval-value | Specifies the timeout period of a translator. | The value is an integer ranging from 1 to 120, in seconds. The default value is 40s. |
| zero-address-forwarding | Sets the FA of the generated NSSA LSAs to 0.0.0.0 when external routes are imported to the ABR in an NSSA. | - |
| translator-strict | Configures the translator to perform strict check on the P-bit flag. The translator determines whether to translate Type-7 LSAs into Type-5 LSAs based on the P-bit flag. | - |
Usage Guidelines
Usage Scenario
An NSSA is configured in the scenario where AS external routes are to be imported but not forwarded to save system resources. AS external routes can be imported to an NSSA and transmitted to the entire NSSA.
All FWs in the NSSA must be configured with NSSA attributes using the nssa command. This command is applicable to the following scenarios:
When the area to which the ASBR belongs is configured as an NSSA, invalid Type-5 LSAs are still reserved on other FWs in the area where LSAs are flooded. These LSAs are deleted only when the aging time reaches 3600 seconds. The performance of the device is deteriorated because a large number of LSAs consume the memory of the router. By setting flush-waiting-timer, you can set the aging time to a maximum value of 3600 seconds to delete invalid Type-5 LSAs on other FWs.
When the LS age field (aging time) in the LSA header reaches 3600 seconds, this LSA is deleted.
When the ASBR also functions as an ABR, the flush-waiting-timer parameter does not take effect. This prevents Type-5 LSAs in the non-NSSA from being deleted.
When an ASBR also functions as an ABR, the no-import-route parameter enables OSPF to import external routes using the import-route command but not to advertise them to the NSSA.
If the device is not an ABR, then after the nssa default-route-advertise command is configured, the device generates an LSA that describes a default route only when the local routing table contains an active default route that is not an OSPF or BGP route.
If the local routing table contains an active default BGP route, the default route can be injected to the OSPF routing table based on the following situations:- Whether the ospf process-id vpn-instance vpn-instance-name command is run together with
the vpn-instance-capability simple command:
- If the two commands are executed together, to inject an active default EBGP route into the OSPF routing table, run the nssa default-route-advertise command. To inject an active default IBGP route to the OSPF routing table, run the import-route bgp permit-ibgp command before you run the default-route-advertise command.
- If only the ospf process-id vpn-instance vpn-instance-name command is run, to inject an active default EBGP or IBGP route into the OSPF routing table, run the nssa default-route-advertise command.
- If the ospf process-id vpn-instance vpn-instance-name command is not run, to inject an active default EBGP route into the OSPF routing table, run the default-route-advertise command. To inject an active default IBGP route to the OSPF routing table, run the import-route bgp permit-ibgp command before you run the nssa default-route-advertise command.
Injecting an IBGP route into the OSPF routing table may cause a routing loop. Exercise caution when you perform this step.- Whether the ospf process-id vpn-instance vpn-instance-name command is run together with
the vpn-instance-capability simple command:
To reduce the number of LSAs being transmitted to the NSSA, configure the no-summary attribute for an ABR. This prevents the ABR from transmitting summary LSA (Type-3 LSA) to the NSSA.
After the keyword set-n-bit is set, the FW re-establishes neighbor relationship with FWs directly connected to it in the NSSA.
If multiple ABRs are deployed in the NSSA, the system automatically selects an ABR (generally the one with the largest router ID) to translate Type-7 LSAs into Type-5 LSAs. You can configure the translator-always parameter on an ABR to specify the ABR as a translator. To specify two ABRs for load balancing, configure the translator-always parameter to specify the ABRs as all-time-translators. This prevents LSA flooding caused by translator role changes.
The translator-interval parameter is used to ensure uninterrupted services when translator roles change. The interval-value parameter is used to ensure the value of the default interval is larger than the flooding period.
Configuration Impact
Configuring or deleting NSSA attributes may trigger routing update in the area. NSSA attributes can be re-configured or deleted only after the last update of NSSA attributes is complete.
Precautions
Configuring a loopback address for a device in the NSSA is recommended. The loopback address can be automatically selected as the FA. If other devices have paths of the same cost to the device in the NSSA, load balancing is performed.