Use the Not-So-Stubby Area (NSSA) for Open Shortest Path First version 2 (OSPFv2) feature to simplify administration in a
network that connects a central site that uses OSPF to a
remote site that is using a different routing protocol.
the NSSA feature was not implemented, the connection between the border
device at the corporate site and the remote device was not established as an OSPF stub area due to
A protocol such as Routing Information Protocol (RIP) is run to
handle the redistribution.
- Routes for the remote site
were not redistributed into the stub area.
- Two routing protocols had
to be maintained.
By implementing NSSA, you can
extend OSPF to include the remote connection by defining the area between the
device at the corporate site and the remote device as an NSSA.
As with OSPF stub
areas, NSSA areas cannot be injected with distributed routes via Type 5 Link
State Advertisement (LSA). Route redistribution into an NSSA area is possible
only with Type 7 LSA. An NSSA Autonomous System Border Router (ASBR) generates the Type
7 LSA , and an NSSA Area Border Router (ABR)
translates the Type 7 LSA into a Type 5 LSA. These LSAs can be flooded throughout
the OSPF routing domain. Route summarization and filtering are supported during the
is the consolidation of advertised addresses. This feature enables an ABR to advertise a single
summary route to other areas. If the network numbers in an
area are assigned in a way such that they are contiguous, you can configure the
ABR to advertise a summary route that covers all the individual networks within
the area that fall into the specified range.
When routes from
other protocols are redistributed to OSPF area, each route is advertised
individually in an external LSA. However, you can configure the Cisco IOS
software to advertise a single route with a specified network address and mask for all the redistributed routes that are
covered by a specified network address and mask. Thus, the size of the OSPF
link-state database decreases.
RFC 3101 allows you
to configure an NSSA ABR device as a forced NSSA LSA translator.
Even a forced
translator might not translate all LSAs; translation depends on the content of
The figure below
shows a network diagram in which OSPF Area 1 is defined as the stub area. The
Enhanced Interior Gateway Routing Protocol (EIGRP) routes are not propagated
into the OSPF domain because routing redistribution is not allowed in the stub
area. However, once OSPF Area 1 is defined as an NSSA, an NSSA ASBR can include
the EIGRP routes to the OSPF NSSA by generating Type 7 LSAs.
Figure 1. OSPF NSSA
routes from the RIP device are not allowed into OSPF Area 1 because NSSA is an
extension to the stub area. The stub area characteristics still exist,
including the exclusion of Type 5 LSAs.
The figure below
shows the OSPF stub network with NSSA Area 1. The redistributed routes that
Device 4 is propagating from the two RIP networks is translated into Type
7 LSAs by NSSA ASBR Device 3. Device 2, which is configured to be the NSSA ABR,
translates the Type 7 LSAs back to Type 5 so that they can be flooded
through the rest of the OSPF stub network within OSPF Area 0.
Figure 2. OSPF NSSA Network with NSSA
ABR and ASBR Devices