In order to ensure that data reaches its proper destination through a packet-switched network, virtual circuits somehow need to be identified. On a Frame relay network, virtual circuits are identified using what are known as Data Link Connection Identifiers (DLCIs).
A DLCI is a 10-bit number that is stored in the header of a frame that traverses a Frame Relay network. DLCI numbers are issued by the service provider and uniquely identify a virtual circuit. These numbers are said to be only locally significant, in that they may not be the same (or consistent) throughout a Frame Relay network. This concept can be confusing and is best illustrated with an example.
Imagine that you were to provision a Frame Relay PVC between two locations from a service provider. The provider would issue two DLCI numbers – one for each location. While these numbers could be the same, there is no guarantee that will be the case, and it doesn’t really matter anyhow. The DLCI number is only used to ensure that the provider’s switching equipment can properly identify the customer’s virtual circuit at any point in the network. So, although your virtual circuit (VC) may be identified as DLCI 99 between your Toronto router and the first Frame Relay switch it encounters, intermediate switches may use different DLCI numbers to identify the same virtual circuit. At each switch, the DLCI stored in the header will be translated if necessary. As such, the very same virtual circuit may be identified using DLCI 101 at the New York office. In this way, the DLCI value stored in the header is only locally significant. This is illustrated in the figure below.
Figure: A single PVC is usually identified by different DLCI numbers across a Frame Relay network.
While less common, it is also possible for DLCI numbers to have global significance in a service provider’s network. When globally significant, each customer location would be identified by a single DLCI value. While this makes a Frame Relay network easier to manage, it also seriously limits the number of possible PVCs that can be configured.
Because a DLCI number is a 10-bit value, up to 1024 (210) DLCI numbers (0-1023) can be used to identify virtual circuits. However, some of these numbers are reserved for management functions. On a Cisco router, up to 992 DLCIs can be defined, ranging from DLCI 16 up to 1007.
