| By default, a Frame
Relay network provides NBMA connectivity between remote sites. NBMA
connectivity means that although all locations can reach each other,
depending on the topology, routing update broadcasts received by one
router cannot be forwarded to all locations because Frame Relay
networks use split horizon to reduce the number of routing loops.
Split horizon reduces the
number of routing loops by not allowing a routing update received on
one interface to be forwarded through the same interface. As shown
in the Figure, central router interface S0 receives a routing update
from router Branch A. Central router is connecting three PVCs over a
single interface. Split horizon forbids the central router to send
out updates via the same interface that it received them. Therefore,
Branch B and Branch C routers never receive the update.
There are two inherent problems with
multiple PVCs terminating in one interface: split horizon and the
support for broadcast traffic.
Broadcasts are not a problem if there
is only a single PVC on a physical interface because this would be
more of a point-to-point connection type.
Another issue with routers that
support multipoint connections over a single interface is that when
many DLCIs terminate in a single router, that router must replicate
routing updates and service advertising updates on each DLCI to the
remote routers. The updates can consume access-link bandwidth and
cause significant latency variations in user traffic. The updates
can also consume interface buffers and lead to higher packet-rate
loss for both user data and routing updates.
The amount of broadcast traffic and
the number of VCs terminating at each router should be evaluated
during the design phase of a Frame Relay network. Overhead traffic,
such as routing updates, can affect the delivery of critical user
data, especially when the delivery path contains low-bandwidth (56-Kbps) links.
One common solution to split horizon is subinterfaces, which are
covered in the upcoming section.
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