 in Frame Relay Frame Relay includes provisions for congestion avoidance. Included in the Frame Relay header are two bits titled Forward-Explicit Congestion Notification (FECN, pronounced FECK-en), and Backward-Explicit Congestion Notification (BECN, pronounced BECK- en). These flags are used to report congestion to the DTE devices (your routers). The DCE devices (telecom switches) set the FECN bit when network congestion is found. When the receiving DTE device (your router) receives the FECN, it can then execute flow control, if SO configured. The Frame Relay cloud does not perform any flow control; this is left up to the DTE devices on each end. The Frame Relay switches set the BECN bit in frames when FECNs are found in frames traveling in the opposite direction. This allows the sending DTE device to know about congestion in the frames it is sending. Figure 23-9 shows a Frame Relay network where congestion is occurring. A PVC exists between Router A and Router B. The PVC traverses the topmost Frame Relay switches in the drawing. Halfway through the cloud, a switch encounters congestion in Router B's direction. The switch marks packets moving forward (toward Router B) with FECNs, and packets moving in the opposite direction (toward Router A) with BECNs. Router B Congestion found here Router A Router C Figure 23-9. FECN and BECN example Local Management Interface Configuring Frame Relay Once you understand how Frame Relay works, the mechanics of configuration are not very difficult to grasp. There are some interesting concepts, such as subinterfaces, that may be new to you; we'll cover those in detail here. Basic Frame Relay with Two Nodes Figure 23-10 shows a simple two-node Frame Relay network. Router A is connected to Router B using Frame Relay over a T1. The port speed is 1.536 Mbps, the CIR is 512 Kbps, and the burst rate is 2x (1,024 Kbps). 192.168.1.1/30 192.168.1.2/30 Router A DLCI 102 DLCI 201 Router B Figure 23-10. Two-node Frame Relay network The first step in configuring Frame Relay is to configure Fr