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IP Configuration 285
Chapter 8, "WAN Protocols and Design," covers issues relating to the design choices of when
to use subinterfaces and what type to use. This current section discusses how to assign IP
addresses after those choices are made.
The use of subinterfaces and the type of subinterface implies the number of subnets used for
Frame Relay. A point-to-point subinterface terminates one VC and has an IP address assigned
to it; the router on the other end of the VC uses an IP address in the same subnet. These two IP
addresses are the only two addresses in the subnet. Each separate instance of a pair of routers
on opposing ends of a VC, each with a point-to-point subinterface configuration, implies the
use of yet another subnet, with only two host addresses in the subnet.
The use of no subinterfaces and the use of multipoint subinterfaces are identical from the
perspective of how to assign IP addresses. Multipoint subinterfaces are used when multiple VCs
terminate at the subinterface; this subinterface, along with all subinterfaces on other routers at
the other end of these VCs, are configured to be in the same subnet. With no subinterfaces used,
all the routers attached to the Frame Relay network are also considered to be in the same subnet.
Most often, point-to-point subinterfaces are used when a partial mesh of VCs is used.
Conversely, multipoint subnets are used when a full mesh is used. However, both types of
subinterface are allowed in the same router. Figure 5-28 shows a Frame Relay configuration
requiring three different subnets over a Frame Relay cloud.
Figure 5-28
Frame Relay Subnets with Point-to-Point and Multipoint Subinterfaces
A
S0.3 (Multipoint)
S0.1
S0.2
Subnet
150.10.3.0
D
E
C
B
Subnet 150.10.1.0
Subnet 150.10.2.0
ch05.fm Page 285 Monday, March 20, 2000 5:06 PM