
QTECH
Software Configuration Manual
8-98
Another application of virtual links is to provide redundant links. If the backbone area cannot maintain
internal connectivity due to a physical link failure, configuring a virtual link can guarantee logical connectivity in the
backbone area, as shown below.
Virtual link application 2
The virtual link between the two ABRs acts as a point-to-point connection. Therefore, you can configure
interface parameters such as hello packet interval on the virtual link as they are configured on physical interfaces.
The two ABRs on the virtual link exchange OSPF packets with each other directly, and the OSPF routers in
between simply convey these OSPF packets as normal IP packets.
8.1.2.4 (Totally) Stub area
The ABR in a stub area does not distribute Type-5 LSAs into the area, so the routing table size and amount
of routing information in this area are reduced significantly.
You can configure the stub area as a totally stub area, where the ABR advertises neither the destinations in
other areas nor the external routes.
Stub area configuration is optional, and not every area is eligible to be a stub area. In general, a stub area
resides on the border of the AS.
The ABR in a stub area generates a default route into the area.
Note the following when configuring a (totally) stub area :
·
The backbone area cannot be a (totally) stub area.
·
The stub command must be configured on routers in a (totally) stub area.
·
A (totally) stub area cannot have an ASBR because AS external routes cannot be distributed into the stub
area.
·
Virtual links cannot transit (totally) stub areas.
8.1.2.5 NSSA area
Similar to a stub area, an NSSA area imports no AS external LSA (Type-5 LSA) but can import Type-7
LSAs that are generated by the ASBR and distributed throughout the NSSA area. When traveling to the NSSA ABR,
Type-7 LSAs are translated into Type-5 LSAs by the ABR for advertisement to other areas.
In the following figure, the OSPF AS contains three areas : Area 1, Area 2 and Area 0. The other two ASs
employ the RIP protocol. Area 1 is an NSSA area, and the ASBR in it translates RIP routes into Type-7 LSAs and
advertises them throughout Area 1. When these LSAs travel to the NSSA ABR, the ABR translates Type-7 LSAs to
Type-5 LSAs for advertisement to Area 0 and Area 2.
On the left of the figure, RIP routes are translated into Type-5 LSAs by the ASBR of Area 2 and distributed
into the OSPF AS. However, Area 1 is an NSSA area, so these Type-5 LSAs cannot travel to Area 1.