Frame Relay allows you to interconnect
your remote sites in a variety of ways. Example topologies, as shown
in the Figure, include the following:
- A star topology, also known
as a hub-and-spoke configuration, is the most popular Frame
Relay network topology. In this topology, remote sites are
connected to a central site that generally provides a service or
application. This is the
least expensive topology because it requires the fewest PVCs. In
this scenario, the central router provides a multipoint
connection because it is typically using a single interface to
interconnect multiple PVCs.
- In a full-mesh topology,
all routers have VCs to all other destinations. This method,
although costly, provides direct connections from each site to
all other sites, and allows for redundancy. When one link goes
down, a router at site A can reroute traffic through site C, for
example. As the number of nodes in the full-mesh topology
increases, the topology becomes increasingly more expensive. The
formula to calculate the total number of VCs with a fully meshed
WAN is [n(n - 1)]/2.
- In a partial-mesh topology,
not all sites have direct access to each other.
By default, interfaces that support
Frame Relay are multipoint connection types. This type of connection
is not a problem when only one PVC is configured on an interface.
However, it is a problem when multiple PVCs are configured on a
single interface. However, it is a problem when multiple PVCs are supported
by a single interface. In this situation, routing updates received
by the central router cannot be broadcast to the other remote sites.
Depending on the traffic patterns in
your network, you may want to have additional PVCs connect to remote
sites that require heavy data traffic. In any case, when a
single interface must be used to interconnect multiple sites, you
may have reachability issues because of the nonbroadcast multiaccess
(NBMA) nature of Frame Relay. With Frame Relay running multiple PVCs
over a single interface, the primary issue is with split horizon.
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