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262 Chapter 9: Frame Relay Connection Controlling Traffic Flow
Theoretically, almost any number of subinterfaces can be placed on a single major interface.
The limit lies in the number of Interface Descriptor Blocks (IDBs) available to the router. This
number varies depending on the router series and Cisco IOS Software release.
A logical subinterface should be treated almost the same as a physical interface. It can be
configured for multiple protocols, shut down, or anything else common on other interfaces with
one notable exception--you cannot set encapsulation on a subinterface. Encapsulation must be
defined at the major interface.
The configuration of subinterfaces solves the split horizon issue by creating multiple logical
interfaces where before existed only one physical interface. Split horizon has no problem with
updates leaving through interfaces other than the one through which they were received.
Example 9-1 shows the necessary commands for creating a subinterface.
Figure 9-6 depicts the same network scenario from Figure 9-5, but with subinterfaces
implemented.
Figure 9-6
Frame Relay Subinterfaces
Note that the routing updates can now flow easily. However, using subinterfaces involves a
trade-off. In exchange for the proper function of split horizon as well as the passing of routing
updates, additional subnets are necessary. In addition, each interface has to have its own unique
subnet, no matter which protocol(s) are configured.
Example 9-1
Creating a Frame Relay Subinterface
RouterA(config)#interface serial 0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#interface serial 0.1 point-to-point
RouterA(config-subif)#ip address 10.1.1.1 255.255.255.0
RouterA(config-subif)#interface serial 0.2 point-to-point
RouterA(config-subif)#ip address 10.1.2.1 255.255.255.0
RouterA(config-subif)#interface serial 0.3 point-to-point
RouterA(config-subif)#ip address 10.1.3.1 255.255.255.0
S0.1
S0.2
S0.3
A
D
C
B
DLCI=18
DLCI=16
DLCI=17
Routing
update
Routing
update
Routing
update