Information Technology Reference
In-Depth Information
There is also an NSSA totally stub area. The difference is that the default NSSA has no de-
fault route unless the ABR is explicitly configured to advertise one. The NSSA totally stub
area does receive a default route.
Virtual Links
OSPF requires that all areas be connected to a backbone router. Sometimes, WAN link
provisioning or failures can prevent an OSPF area from being directly connected to a
backbone router. You can use virtual links to temporarily connect (virtually) the area to
the backbone.
As shown in Figure 11-6, Area 4 is not directly connected to the backbone. A virtual link
is configured between Router A and Router B. The flow of the virtual link is unidirectional
and must be configured in each router of the link. Area 2 becomes the transit area through
which the virtual link is configured. Traffic between Areas 2 and 4 does not flow directly
to Router B. Instead, the traffic must flow to Router A to reach Area 0 and then pass
through the virtual link.
Area 3
Area 0
Virtual Link
Area 2
Area 1
Router A
Router B
Area 4
Figure 11-6
OSPF Virtual Link
OSPFv2 Router Authentication
OSPFv2 supports the authentication of routes using 64-bit clear text or cryptographic
message digest 5 (MD5) authentication. Authentication can be performed on a per-area or
per-interface basis. Plaintext authentication passwords do not need to be the same for the
routers throughout the area, but they must be the same between neighbors.
MD5 authentication provides higher security than plaintext authentication. As with plain-
text authentication, passwords do not have to be the same throughout an area, but they do
need to be same between neighbors.
OSPFv2 Summary
OSPFv2 is used in large enterprise IPv4 networks. The network topology must be hierar-
 
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