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responsibility of distributing routing information from the Autonomous System Border Router in order
for routers in the network to get and maintain routes to other Autonomous Systems.
Network link-state updates are generated by a router elected as the Designated Router on a multi-
access segment (with more than one attached router). These updates describe all of the routers on the
segment and their network connections.
External link-state updates carry routing information to networks outside the Autonomous System. The
Autonomous System Border Router is responsible for generating and distributing these updates.
OSPF Authentication
OSPF packets can be authenticated as coming from trusted routers by the use of predefined pass-
words. The default for routers is to use not authentication.
There are two other authentication methods - simple password authentication (key) and Message
Digest authentication (MD-5).
Simple Password Authentication
A password (or key) can be configured on a per-area basis. Routers in the same area that participate
in the routing domain must be configured with the same key. This method is possibly vulnerable to
passive attacks where a link analyzer is used to obtain the password.
Message Digest Authentication (MD-5)
MD-5 authentication is a cryptographic method. A key and a key-ID are configured on each router.
The router then uses an algorithm to generate a mathematical [message digest] that is derived from the
OSPF packet, the key and the key-ID. This message digest (a number) is then appended to the packet.
The key is not exchanged over the wire and a non-decreasing sequence number is included to prevent
replay attacks.
Backbone and Area 0
OSPF limits the number of link-state updates required between routers by defining areas within which
a given router operates. When more than one area is configured, one area is designated as area 0 - also
called the backbone.
The backbone is at the center of all other areas - all areas of the network have a physical (or virtual)
connection to the backbone through a router. OSPF allows routing information to be distributed by
forwarding it into area 0, from which the information can be forwarded to all other areas (and all other
routers) on the network.
In situations where an area is required, but is not possible to provide a physical connection to the
backbone, a virtual link can be configured.
Virtual Links
Virtual links accomplish two purposes:
1. Linking an area that does not have a physical connection to the backbone.
2. Patching the backbone in case there is a discontinuity in area 0.
Areas Not Physically Connected to Area 0
All areas of an OSPF network should have a physical connection to the backbone, but is some cases it
is not possible to physically connect a remote area to the backbone. In these cases, a virtual link is
configured to connect the remote area to the backbone. A virtual path is a logical path between two
