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Chapter 10

Wide Area Networking Protocols

In Network 3, configuring subinterfaces actually works to subdivide the

Frame Relay network into smaller subnetworks--each with its own net-
work number. So locations A, B, and C connect to a fully meshed network,
while locations C and D, and D and E, are connected via point-to-point
connections. Locations C and D connect to two subinterfaces and forward
packets.

Subinterfaces also solve the problem with routing protocols that use split

horizon. As you may recall, split horizon protocols do not advertise routes
out the same interface they received the route update on. This can cause a
problem on a meshed Frame Relay network. However, by using subinter-
faces, routing protocols that receive route updates on one subinterface can
send out the same route update on another subinterface.

Creating Subinterfaces

You define subinterfaces with the int s0.subinterface number command
as shown below. You first set the encapsulation on the serial interface, then
you can define the subinterfaces.

RouterA(config)#int s0

RouterA(config)#encapsulation frame-relay

RouterA(config)#int s0.?

<0-4294967295> Serial interface number

RouterA(config)#int s0.16 ?

multipoint Treat as a multipoint link

point-to-point Treat as a point-to-point link

RouterA(config)#int s0.16 point-to-point

You can define an almost limitless number of subinterfaces on a given

physical interface (keeping router memory in mind). In the above example,
we chose to use subinterface 16 because that represents the DLCI number
assigned to that interface. However, you can choose any number between
0 and 4,292,967,295.

There are two types of subinterfaces:

Point-to-point Used when a single virtual circuit connects one router to
another. Each point-to-point subinterface requires its own subnet.

Multipoint Used when the router is the center of a star of virtual cir-
cuits. Uses a single subnet for all routers' serial interfaces connected to the
frame switch.

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