TMS zl Management and Configuration Guide ST.1.2.100916

9-30
Routing
OSPF
a non-local area network to the ABR that advertised the summary for that
area.When this traffic arrives in Area 0, the ABRs route it toward the correct
area. When the traffic arrives in the new area, internal routers use intra-area
routing to direct it to its destination.
Autonomous system border routers (ASBRs) support external traffic (in
networks with one area or with multiple areas.) An ASBR connects to an
external network and runs both OSPF and the external network’s routing
protocol. It then injects the external routes, or a default route for external
traffic, into the OSPF network. An ASBR is often in the network backbone,
but it can also be in a stub area that connects to an external network. When a
stub area connects to an external network, it is called a not-so-stubby area
(NSSA).
Figure 9-8 shows several types of OSPF areas.
Figure 9-8. OSPF Areas
Stub Areas and Stub Routers
A stub network is a network in which traffic terminates. The network receives
traffic destined for its hosts, but it does not pass any traffic to another network.
A stub area is an extension of the idea of a stub network. A stub area is a group
of networks that:
■ Does not forward traffic from one area to another
■ Connects to only one other area—the network backbone or Area 0