H3C S3610-28P Operation Manual page 375

Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
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.
Figure 3-4 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.
IV. (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.
V. 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
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Chapter 3 OSPF Configuration