ubscription. Many ISPs routinely oversubscribed their Frame Relay links to save money and thereby increase profits. Oversubscription PVC 101 - 256k CIR PVC 102 - 256k CIR PVC 103 - 256k CIR 1.536 Mbps port speed PVC 104 - 256k CIR PVC 105 - 256k CIR Available - 256k Figure 23-8. Subscription of a T1 using Frame Relay PVCs Oversubscription is never good for customers, though: eventually, usage will increase to the point where packets are dropped. Be careful when ordering links from telecom providers. There are some providers who only provide 0 Kbps CIR Frame Relay links. They do this to provide links at lower costs than their competitors. The drawback is that all data sent over these links will be discard-eligible. Be specific when ordering, and know what you are buying. Local Management Interface In 1990, Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Cor- poration developed a set of enhancements to the Frame Relay protocol called the Local Management Interface (LMI). LMI provides communication between the data terminal equipment, or DTE devices (routers, in our examples), and the data communication equipment, or DCE devices (telecom switches, in our examples). One of the most useful enhancements in LMI is the exchange of status messages regarding VCs. These mes- sages tell routers when a Frame Relay PVC is available. LMI messages are sent on a predefined PVC. You can see the LMI type and PVC in use with the show interface command: Router-A#sho int so/o I inc LMI LMI enq sent 85, LMI stat recvd 86, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE Three forms of LMI are configurable on Cisco routers: cisco, ansi, and q933a (Annex A). The DCE device (telecom switch) usually determines the type of LMI. The default LMI type on Cisco routers is cisco. You can change the LMI type with the frame-relay lmi-type interface command: Router-A(config-if)#frame-relay lmi-type ? cisco Chapter 23: Frame Relay q933a Congestion Avoidance