cisco ccna icnd2

cisco ccna icnd2.LMI is only locally meaningful, so individual Frame Relay DTEs in the network can use different types of LMI. for example, in Figure 25.4, Site 1 and Site 2 may be connected by a PVC, but the router at Site 1 may use LMI type ANSI, while the router at Site 2 may use LMI type Q.933O Although they are connected by a PVC, the LMI processing is local and therefore can be different. In practice, the LMI type usually varies with the carrier and the switch it uses. Most carrier switches support all three types, but some carrier switches do not. Even if the switches support all three types, carriers may standardize and use a specific type on each of these switches. Cisco routers support all three LMI standards.Site 2DTESite 4 Functions of the LMIThe primary function of LMI is to enable the Frame Relay DTE and DCE to exchange information about themselves and the status of the VC. To achieve this function, the Frame Relay DTE periodically sends LMI status inquiry (query) messages to its connected Frame Relay DCEs. If the DCE is turned on and configured with the same type of LMI, it will respond with status answer messages. These messages are equivalent to survivor messages and allow the two devices to determine the status of the other. Basically, the DTE asks the switch "still there?" and the switch replies "still there". By default, only the DTE sends survivor messages, while the DCE only responds.After sending many status queries, the Frame Relay DTE generates a special query message - a full status update. In such a message, the DTE requests a full status update from the DCE, which contains all DTE-related information, such as all VCs connected by the DTE and their addresses (DLCI), configurations (CIR,Bc, and BE), and states. For example, assuming that in Figure 25.4, Site 1 has a PVC to every other remote site and sends a full status update message to its connected DCE, the DCE will provide the following PVC information: Site I - Site 2, Site IT Site 3, and Site 1-> Site 4O Note that the DCE switch will not provide the following VCs: Site 2 -Site 3, Site 3- Site 4, and Site 2-> Site 4, because these VCs are not local to that DTE.Cisco supports status queries and full status update timers. Status query messages are sent every 10 seconds by default, and a full status update message is sent for every five status query messages sent by default.Table 25.2 LMI AddressesLMI StandardsChapter 25 Frame Relay 685