port speed of the link. For example, having four PVCs each with a 512 Kbps CIR is possible on a T1, even though the total of all the CIRs is 2,048 Kbps. Some providers will allow this, while others will not. The burst rate has no bearing on the oversubscription of a link. Careful planning should always be done to ensure that the CIRs of your PVCs total no more than the port speed of your physical link. I use spreadsheets to keep me honest, but any form of documentation will do. Often, simple charts like the one in Figure 22-8 are the most effective. 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 22-8. Subscription of a T1 using frame-relay PVCs There are no technical limitations preventing oversubscription. During the Internet boom, small ISPs often debated the ethics of oversubscription. Many ISPs routinely oversubscribed their frame-relay links to save money and thereby increase profits. Oversubscription is never good for customers, though: eventually, usage will increase to the point where packets are dropped, even though you've committed to delivering them. 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. Chapter 22: Frame Relay Local Management Interface (LMI) 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 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 use- ful enhancements that LMI provides is the exchange of status messages regarding vi