Virtual Links - D-Link DGS-3324SRi User Manual

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.
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
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:
Linking an area that does not have a physical connection to the backbone.
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 border routers that have a common area, with one border router
connected to the backbone.
Partitioning the Backbone
OSPF also allows virtual links to be configured to connect the parts of the backbone that are discontinuous. This is the
equivalent to linking different area 0s together using a logical path between each area 0. Virtual links can also be added for
redundancy to protect against a router failure. A virtual link is configured between two border routers that both have a
connection to their respective area 0s.
Neighbors
Routers that are connected to the same area or segment become neighbors in that area. Neighbors are elected via the Hello
protocol. IP multicast is used to send out Hello packets to other routers on the segment. Routers become neighbors when
they see themselves listed in a Hello packet sent by another router on the same segment. In this way, two-way
communication is guaranteed to be possible between any two neighbor routers.
Any two routers must meet the following conditions before the become neighbors:
Area ID Two routers having a common segment
segment. Of course, the interfaces should belong to the same subnet and have the same subnet mask.
Authentication
segment and belonging to the same area m ust also have the same OSPF password before they can become
neighbors.
Hello and Dead Intervals
that a router sends on an OSPF interface. The dead interval is the number of seconds that a router's Hello packets
have not been seen before its neighbors declare the OSPF router down. OSPF routers exchange Hello packets on
each segment in order to acknowledge each other's existence on a segment and to elect a Designated Router on
multi-access segments. OSPF requires these intervals to be exactly the same between any two neighbors. If any of
these intervals are different, these routers will not become neighbors on a particular segment.
Stub Area Flag
become neighbors.
xStack Stackable Gigabit Layer 3 Switch Manual
all areas of the network have a physical (or virtual) connection to the
OSPF allows for the configuration of a password for a specific area. Two routers on the same
The Hello interval specifies the length of time, in seconds, between the hello packets
Any two routers also have to have the same stub area flag in their Hello packets in order to
their interfaces have to belong to the same area on that
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