This document provides the formula for calculating the maximum theoretical number of Data-Link Connection Identifiers (DLCIs) that can be advertised over an interface, based on the Local Management Interface (LMI) type. The method the formula was derived from is listed as well as debug examples.
This document is not restricted to specific software and hardware versions.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Notice that the DLCI is two bytes long, and the entire packet is 10 bytes long plus a variable amount of data for the Information Elements (IEs). We can view the IE portion of the Permanent Virtual Circuit (PVC) full status packets using the debug frame-relay lmi command. (These are only the full status messages from the frame switch; you also see regular status messages using this debug command.)
Notice that in all three cases, the Report Type (RT) IE is one byte long and the KeepAlive (KA) IE is two bytes long. For the ANSI and Q933a LMIs, the PVC information IE is 3 bytes long, whereas for the Cisco LMI it is 6 bytes long due to the additional "bw" (for BandWidth) value. The "bw" value represents the Committed Information Rate (CIR); the actual bw value will only be seen if the frame relay switch is configured to forward this information. For detailed information on the values shown, refer to the Command Reference for debug frame-relay lmi.
The static overhead in all three cases is 13 bytes [Entire LMI packet minus IEs (10 bytes) + RT (1 byte) + KA (2 bytes)]. We can subtract this number from the Maximum Transmission Unit (MTU) to get the total available bytes for DLCI information. We then divide that number by the length of the PVC IE (5 bytes for ANSI and Q933a, 8 bytes for Cisco) to get the maximum theoretical number of DLCIs for the interface:
For ANSI or Q933a, the formula is: (MTU - 13) / 5= max DLCIs.
For Cisco, the formula is (MTU - 13) / 8= max DLCIs.
Note: It is possible to share the flag between frames, which would decrease the static overhead to 12 bytes.