the ABR is a Cisco router. To configure an area as
totally stubby, use the following steps: Step 1
Configure OSPF. Step 2 Define the area as a stub area by
issuing the area area-id stub command to
all routers within the area. Step 3 At the ABR only, add
the no-summary keyword to the area
area-id stub command.
Content 3.7
Stub, Totally Stubby, and Not-So-Stubby Areas
3.7.6 Totally Stubby Area Configuration Example
Figure displays an example of a totally stubby area
configuration. All routes advertised into area 1 (from area 0
and the external autonomous system) default to 0.0.0.0. The
default route cost is set to 5 on router 2, and to 10 on router
4. Both default routes are advertised into area 1. However, the
default route from router 2 is advertised with a lower cost to
make it preferable if the internal cost from router 3 to router
4 is the same as the internal cost from router 3 to router 2.
Notice that router 3 requires the area 1 stub command,
yet the no-summary extension is not required. Only ABRs
use no-summary to keep summary LSAs from being
propagated into another area.
Content 3.7 Stub,
Totally Stubby, and Not-So-Stubby Areas 3.7.7
Interpreting Routing Tables Figure displays how the routing
table of an OSPF router in a standard area might look.
Intra-area, interarea, and external routes are all maintained
in a standard area. Figure displays how the same routing table
looks if the area is configured as a stub. Intra-area and
interarea routes are all maintained. External routes are not
visible in the routing table, but are accessible via the
intra-area default route. Figure displays how the same routing
table looks if the area is configured as a stub and
summarization is performed on the ABR. Intra-area and
summarized interarea routes are all maintained. External routes
are not visible in the routing table, but are accessible via
the intra-area default route. (Summarization is covered in the
next lesson.) Figure displays how the same routing table looks
if the area is configured as totally stubby. Notice that the
routers in the totally stubby area have the smallest routing
tables. Intra-area routes are maintained. Interarea and
external routes are not visible in the routing table, but are
accessible via the intra-area default route.
Content
3.7 Stub, Totally Stubby, and Not-So-Stubby
Areas 3.7.8 Configuring Not-So-Stubby
Areas The OSPF NSSA feature is described by RFC 3101 and
was first introduced in Cisco IOS Software Release 11.2. It is
a nonproprietary extension of the existing stub area feature,
which allows the injection of external routes in a limited
fashion into the stub area. Redistribution into an NSSA creates
a special type of LSA (type 7), which can exist only in an
NSSA. An NSSA ASBR generates this LSA, and an NSSA ABR
translates it into a type 5 LSA, which then gets propagated
into the OSPF domain. – The NSSA includes the other stub area
features, such as the ABR sends a default route into the NSSA
instead of external routes from other ASBRs. The type 7 LSA is
described in the routing table as an O N2 or O N1 (N stands for
NSSA). N1 means that the metric is calculated like an external
type 1; N2 means that the metric is calculated like an external
type 2. The default is O N2. To configure an area as an NSSA,
use the following steps: Step 1 Configure OSPF. Step
2 Define the area as an NSSA by issuing the area
area-id nssa command to all routers within the
area. All routers in the NSSA must have this command
configured. Routers cannot form an adjacency unless both are
configured as NSSA. Figure lists the parameters of this
command.
Content 3.7 Stub, Totally Stubby, and
Not-So-Stubby Areas 3.7.9 Configuring
Not-So-Stubby Area Example In Figure , router 1 is the ASBR
that is redistributing RIP routes into area 1, the NSSA. Router
2 is the NSSA ABR and converts LSA type 7 into type 5 for
advertisement into the backbone area 0. Router 2 is also
configured to summarize the type 5 LSAs that originate from the
RIP network. The 172.16.0.0 subnets are summarized to
172.16.0.0/16 and advertised into area 0.To cause router 2 (the
NSSA ABR) to generate an O *N2 default route (O *N2 0.0.0.0/0)
into the NSSA, use the default-information-originate
option of the area area-id nssa command on
router 2.
Content 3.7 Stub, Totally Stubby, and
Not-So-Stubby Areas 3.7.10 Configuring an NSSA
Totally Stubby Area Example In Figure , the ABR (router 2)
is using the area 1 nssa no-summary command. This
command works exactly the same as the totally stubby technique.
A single default route replaces both inbound-external (type 5)
LSAs and summary (type 3 and 4) LSAs into the area. The NSSA
ABR, which is router 2, automatically generates the O *N2
default route into the NSSA area with the no-summary
option configured at the ABR, so the
default-information-originate option is not required.
All other routers in the NSSA area require the area 1
nssa command only. The NSSA totally stubby configuration is
a Cisco proprietary feature like the totally stubby area
feature.
Content 3.7 Stub, Totally Stubby, and
Not-So-Stubby Areas 3.7.11 Verifying All Stub
Area Types The show commands in Figure are used to
display information about the different types of stub
areas.
Content 3.8 Virtual Links
3.8.1 OSPF Virtual Links The two-tiered area
hierarchy of OSPF requires that all areas connect directly to
the backbone area. A virtual link is a link that allows
discontiguous area 0s to be connected or a disconnected area to
be connected to area 0 via a transit area. You should use thee
OSPF virtual link feature only in very specific cases, such as
for temporary connections or backup after a failure. Virtual
links should not be used as a primary backbone design feature
and should be avoided if possible. Note
Virtual
links are part of the OSPF open standard and have been a part
of Cisco IOS software since Cisco IOS Software Release 10.0. In
Figure , area 3 cannot be directly connected to area 0.
Therefore, area 1 is used as a transit area to connect area 3
to area 0. Figure is displaying two area 0s that have become
partitioned, for example, if two OSPF networks were merged. The
disconnected area 0s can be linked through the common area
(area 3). If a common area does not already exist, one can be
created to become the transit area. The hello protocol works
over virtual links as it does over standard links in 10-second
intervals. However, LSA updates work differently on virtual
links. An LSA usually refreshes every 30 minutes. LSAs learned
through a virtual link have the DoNotAge (DNA) option set so
that the LSA does not age out. DNA is required to prevent
excessive flooding over the virtual link.
Content
3.8 Virtual Links 3.8.2 Configuring
Virtual Links Use the area area-id
virtual-link router-id router configuration command,
along with any necessary optional parameters, to define an OSPF
virtual link. To remove a virtual link, use the no form
of this command.The area virtual-link command includes
the router ID of the far-end router. To find the router ID in
the far-end router, use the show ip ospf, show ip
ospf interface, or show ip protocol commands on that
remote router, as illustrated in Figure .
Content
3.8 Virtual Links 3.8.3 Virtual Link
Configuration Example In Figure , area 0 is discontiguous
because of a network failure. A virtual link is used as a
backup strategy to temporarily reconnect Area 0. Area 1 is used
as the transit area.Router A builds a virtual link to router B,
and router B builds a virtual link to router A. Each router
points at the router ID of the other router.
Content