nssa (OSPF Area)
Function
The nssa command configures an NSSA.
The undo nssa command cancels the configuration of an NSSA.
By default, no OSPF area is configured as an NSSA.
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 Type7 LSAs on the ASBR and then advertises
them to the NSSA. NOTE:
The ABR generates a default NSSA LSA (Type7 LSA) automatically and advertises it in the NSSA. Type 7 LSAs carrying the default route will be generated only when the default route 0.0.0.0/0 exists in the routing table on the ASBR. |
- |
| 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 exists 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 | Indicates 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 an NSSA. | - |
| no-summary | Indicates that an ABR is prohibited from sending summary LSAs to the NSSA. | - |
| set-n-bit | Sets the N-bit in DD packets. | - |
| suppress-forwarding-address | Sets the FA of the Type 5 LSAs translated from Type 7 LSAs by the NSSA ABR to 0.0.0.0. | - |
| translator-always | Specifies an ABR in an NSSA as an all-the-time 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 40. |
| zero-address-forwarding | Sets the FA of the generated NSSA LSAs to 0.0.0.0 when external routes are imported by 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 switches in the NSSA must be configured with NSSA attributes using the nssa command.
The default-route-advertise parameter is configured to advertise Type 7 LSAs carrying the default route on the ASBR to the NSSA.
Regardless of whether the default route 0.0.0.0/0 exists in the routing table on the ABR, Type 7 LSAs carrying the default route will be generated. However, Type 7 LSAs carrying the default route will be generated on the ASBR only when the default route 0.0.0.0/0 exists in the routing table on the ASBR.
When the area to which the ASBR belongs is configured as an NSSA, invalid Type 5 LSAs from other switches in the area where LSAs are flooded will be reserved. These LSAs will be deleted only when the aging time reaches 3600s. The switch performance is affected because the forwarding of a large number of LSAs consumes the memory resources. To resolve such a problem, you can set the parameter flush-waiting-timer to the maximum value 3600s for Type 5 LSAs. This ensures that the invalid Type 5 LSAs from other switches can be deleted in time.
When the LS age field value (aging time) in the header of an LSA reaches 3600s, the LSA is deleted.
If an ASBR also functions as an ABR, flush-waiting-timer does not take effect. This prevents Type 5 LSAs in the non-NSSAs from being deleted.
- If an ASBR also functions as an ABR, the no-import-route parameter is configured to prevent external routes imported using the import-route command from being advertised to the NSSA.
- The no-summary parameter is configured
on an ABR to reduce the number of LSAs that are transmitted to the
NSSA. This implementation prevents the ABR from transmitting Type
3 LSAs to the NSSA. After the nssa default-route-advertise backbone-peer-ignore no-summary command is run, the ABR generates default Type 7 and Type 3 LSAs as long as an interface that is Up exists in the backbone area. The default Type 3 LSAs preferentially take effect.
- After the set-n-bit parameter is configured, the N-bit is set in the database description (DD) packets during the synchronization between the switch and neighboring switches.
- If multiple ABRs are deployed in the NSSA, the system automatically selects an ABR (generally the switch with the largest router ID) as a translator to convert Type 7 LSAs into Type 5 LSAs. You can configure the translator-always parameter on an ABR to specify the ABR as an all-the-time translator. To specify two ABRs for load balancing, configure the translator-always parameter on the chosen ABRs to specify the ABRs as all-the-time translators. You can use this command to pre-configure a fixed translator to prevent LSA flooding caused by translator role changes.
- The translator-interval parameter is used to ensure uninterrupted services when translator roles change. The value of interval-value must be greater than the flooding period.
Configuration Impact
Configuring or deleting NSSA attributes will trigger routing updates in the area. A second configuration of NSSA attributes can be implemented or canceled only after a routing update is complete.
Precautions
It is recommended that a loopback address be configured for an switch in the NSSA so that the loopback address can be automatically selected as the FA. If other switches have routes of the same cost to the switch in the NSSA, load balancing is performed.
When the last ordinary area (other than a stub area or NSSA) under an OSPF process is deleted, useless Type 5 LSAs originated by the local switch in the area where LSAs are flooded will be deleted immediately. The local switch still reserves useless Type 5 LSAs from other switches. These useless Type 5 LSAs will be deleted only when the aging time reaches 3600s.