Configure nssa in the ospf non-pure Stub Area

The NSSA region is similar to the OSPF stub region and does not spread from the trunk type 5 external link status announcement (ISA) to the region. However, the AS external route can be introduced in a limited form within the region.
　　
NSSA can introduce type 7 AS external routing in The NSSA region by means of excessive allocation, and then translate type 7 LSA into type 5 LSA, which is then distributed to the entire routing selection domain by The NSSA Region Border Router (ABR. Translation can be summarized and filtered.
　　
For Internet Service Providers (ISPs) or network administrators, they must use OSPF to connect a central site to a remote site using different routing protocols. Therefore, using NSSA makes management easier.
　　
Before NSSA, because the routes to the remote site cannot be re-allocated to the stub area, the connections between the common site border router and the remote router cannot be managed in the OSPF stub area. Generally, a simple protocol similar to RIP is used to handle the redistribution problem, which means that two routing protocols must be maintained. Define the common router and remote router as NSSA, and extend OSPF to overwrite the remote connection.
　　
If necessary, in router configuration mode, specify the following regional parameters and set the NSSA of OSPF:
　　
Command | role
Bytes ------------------------------------------------------------------------------------
Area-id nssa [no-redistribution] [default-information-originate] | defines the area as NSSA.
　　
In the configuration mode of the ABR (Regional Border Router) router, execute the following command to convert LSA 7 (link state notice) to LSA 5 (link state notice) and summarize and filter
　　
Command | role
-------------------------------------------------------------------------------
Summary address prefix mask [notadvertise] [tag] | aggregate filtering during conversion (optional ).
　　
When setting this feature, consider the following factors:
　　
You can set a default route of Type 7 to achieve external destination sites. In this way, The nssa abr router generates a default 7 route to The NSSA.
　　
Any vro in the same region must support the NSSA region. Otherwise, the vrouters cannot communicate with each other.
　　
Note: Try not to use explicit redistribution on nssa abr. Because the packets converted through this vro are easy to confuse. Article entry: csh responsible editor: csh