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Chapter 13: AI2524 Frame Relay Configuration Steps
August 1997 Page 13-19
2524UM
Configuring Frame Relay subinterfaces ensures that a single physical
interface is treated as multiple virtual interfaces. This capability allows
us to overcome split horizon rules. Packets received on one virtual in-
terface can now be forwarded out another virtual interface, even if they
are configured on the same physical interface.
Subinterfaces address the limitations of Frame Relay networks by pro-
viding a way to subdivide a partially meshed Frame Relay network
into a number of smaller, fully meshed (or point-to-point) subnet
works. Each subnetwork is assigned its own network number and ap-
pears to the protocols as if it is reachable through a separate interface.
Note: Point-to-point subinterfaces can be unnumbered for us
with IP, reducing the addressing burden that might
otherwise result.
For example, suppose you have a 5-node Frame Relay network (see
Figure 13-2
) that is partially meshed (Network A). If the entire net-
work is viewed as a single subnetwork (with a single network number
assigned), most protocols assume that node A can transmit a packet di-
rectly to node E, when in fact it must be relayed through nodes C and
D. This network can be made to work with certain protocols (for ex-
ample, IP) but will not work at all with other protocols because nodes
C and D will not relay the packet out the same interface on which it
was received. One way to make this network function fully is to creat
a fully meshed network (Network B), but doing so requires a larg
number of PVCs, which may not be economically feasible.
Using subinterfaces, you can subdivide the Frame Relay network into
three smaller subnetworks (Network C) with separate network num-
bers. Nodes A, B, and C are connected to a fully meshed network, and
nodes C and D, as well as nodes D and E are connected via point-to
point networks. In this configuration, nodes C and D can access two
subinterfaces and can therefore forward packets without violating split
horizon rules. If transparent bridging is being used, each subinterfac
is viewed as a separate bridge port.