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
This document is not restricted to specific software and hardware
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
Reference for debug frame-relay lmi.
If you have the output of a show frame-relay
lmi command from your Cisco device, you can use to display potential issues and fixes. To use , you must be a registered customer, be
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.