8.3 Frame Relay Topologies
8.3.4 Configuring Frame Relay Subinterfaces
You can configure subinterfaces to support the following connection types:
  • Point to point - A single subinterface is used to establish one PVC connection to another physical or subinterface on a remote router. In this case, the interfaces would be in the same subnet, and each interface would have a single DLCI. Each point-to-point connection is its own subnet. In this environment, broadcasts are not a problem because the routers are point-to-point and act like a leased line.
  • Multipoint - A single subinterface is used to establish multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. In this case, all the participating interfaces would be in the same subnet, and each interface would have its own local DLCI. In this environment, because the subinterface is acting like a regular NBMA Frame Relay network, broadcast traffic is subject to the split-horizon rule.

Figure provides the visual representation for example of subinterface configuration.

To configure subinterfaces on a physical interface, perform the following steps:

1. Select the interface that you want to create subinterfaces on, and enter the interface configuration mode.

2. Remove any network-layer address that is assigned to the physical interface. If the physical interface has an address, frames will not be received by the local subinterfaces.

3. Configure Frame Relay encapsulation, as discussed in the “Configuring Basic Frame Relay” section.

4. Select the subinterface you want to configure :

Router(config-if)#interface serial number subinterface-number
{multipoint | point-to-point}

5. Configure a network-layer address on the subinterface. If the subinterface is point to point and you are using IP, you can use the ip unnumbered command:

Router(config-if)#ip unnumbered interface

If you use this command, it is recommended that the interface be the loopback interface because the Frame Relay link will not work if this command is pointing to an interface that is not fully operational, and a loopback interface is less likely to fail. When using ip unnumbered, it should have two ends on the same major network.

6. If you configured the subinterface as multipoint or point-to-point, you must configure the local DLCI for the subinterface to distinguish it from the physical interface :

Router(config-if)#frame-relay interface-dlci dlci-number

This command is required for all point-to-point subinterfaces. It is also required for multipoint subinterfaces for which inverse ARP is enabled. It is not required for multipoint subinterfaces that are configured with static route maps. Do not use this command on physical interfaces.

If you defined a subinterface for point-to-point communication, you cannot reassign the same subinterface number to be used for multipoint communication without first rebooting the router. Instead, you must avoid using that subinterface number and use a different subinterface number.

Lab Activity    
  You have been contracted by a small bank to install a Frame Relay network to connect their offices in Atlanta, Orlando, and Houston. Each office needs to connect to the other two. You have ordered Frame Relay service from a national Frame Relay service provider and have received the DLCIs in the above diagram. Routing should be handled using IGRP.