3Com 10014299 manual Frame Relay over Isdn Operation Process and Fundamentals

270CHAPTER 17: CONFIGURING FRAME RELAY

After configuring the Frame Relay route through the fr dlci-switch interface tunnel command, two routes will be added to the Frame Relay routing table on the router. One route takes the tunnel interface as the incoming interface and the serial interface as the outgoing interface. On the contrary, the other route takes the serial interface as the incoming interface and the tunnel interface as the outgoing interface.

After the Frame Relay route is configured through the fr dlci-switch interface tunnel command, a PVC will be created on the tunnel interface and assigned with a DLCI number out-dlci. When implementing configuration, make sure that the same DLCI number (that is, out-dlci) should be used on the tunnel interfaces at two ends of the GRE tunnel.

Frame Relay over ISDN Nowadays, Frame Relay technique has gained wide applications, in which, most devices are accessed to the Frame Relay networks via leased lines. To shorten the time for users to access Frame Relay networks and lower the cost of leased lines, ISDNs and the related devices can be used to access Frame Relay networks, the so-called Frame Relay over ISDN.

With the Frame Relay over ISDN technique, the cost of a leased line can be shared by the routers, so the overall cost is lowered. The users can access the Frame Relay networks much quicker and with lower cost. At the same time, ISDN can also be taken as a standby for Frame Relay accessing. Therefore, the Frame Relay over ISDN is mainly used in the following two aspects:

■The simplest application is to take Frame Relay over ISDN as the main communications method. That is, all the routers support Frame Relay over ISDN, and the individual routers can directly access the Frame Relay networks (without TA adapters) to communicate.

■Combined with BDR, Frame Relay over ISDN can be taken as the standby communication method for Frame Relay. In such applications, routers support Frame Relay over ISDN. Being the standby for a Frame Relay network, ISDN can be used to re-establish the connections for accessing the Frame Relay network, whenever a Frame Relay accessing line/device fails to work or the Frame Relay network is blocked.

Frame Relay over ISDN Operation Process and Fundamentals

The following figure shows a typical networking for Frame Relay accessing, in which all the routers support Frame Relay over ISDN:

Figure 89 Networking of a typical Frame Relay over ISDN application

Frame Relay	ISDN	Frame Relay
Router A		Router B

RouterA, RouterB and RouterC support Frame Relay over ISDN. Being DTE devices, they and RouterD transmit Frame Relay packets over ISDN B channels. RouterD, which works as a DCE device, supports both Frame Relay over ISDN and Frame Relay switching. A simplified working procedure is shown below:

■RouterA (DTE device) originates a call on the BRI interface to the PRI interface on RouterD (DCE device).