8.1 Frame Relay
8.1.2 Frame Relay operation
Frame Relay provides a means for statistically multiplexing many logical data conversations (referred to as virtual circuits [VCs]) over a single physical transmission link by assigning connection identifiers to each pair of DTE devices. The service provider's switching equipment constructs a table that maps connection identifiers to outbound ports. When a frame is received, the switching device analyzes the connection identifier and delivers the frame to the pre-established associated outbound port.

The VCs can be either permanent VCs (PVCs) or switched VCs (SVCs). PVCs are permanently established connections that are used when there is frequent and consistent data transfer between DTE devices across a Frame Relay network.

With ANSI T1.617, ITU-T Q.933 (Layer 3), and Q.922 (Layer 2), Frame Relay now supports SVCs. SVCs are temporary connections, used when there is only sporadic data transfer between DTE devices across the Frame Relay network. Because they are temporary, SVC connections require call setup and termination for each connection. Cisco IOS® Release 11.2 or later supports Frame Relay SVCs. You will need to determine whether your carrier supports SVCs before implementing them.

A data-link connection identifier (DLCI) identifies the logical VC between the CPE and the Frame Relay switch. The Frame Relay switch maps the DLCIs between each pair of routers to create a PVC. DLCIs have local significance in that the identifier references the point between the local router and the Frame Relay switch to which it is connected. Your Frame Relay provider sets up the DLCI numbers to be used by the routers for establishing PVCs.

Local Significance

DLCIs have local significance; that is, the end devices at two different ends of a connection may use a different DLCI to refer to that same connection. The Figure provides an example of one VC identified at each end by two different DLCI numbers.

Because the DLCIs have only local significance, the only real restriction on the use of DLCIs is that they are not used for more than one destination from the same port.

You configure an available DLCI number to map this provided Frame Relay number to a network address. For example, an administrator might map to an IP address of the interface on Router A in the Figure. This mapping in the router points to a static route, which is the PVC to that remote router. For example, the administrator can configure a Frame Relay map for 172.16.11.3 by using the PVC identified as DLCI 500. Frame Relay uses VCs identified with DLCI numbers. The DLCI numbers have only local significance.

The address mapping can be either configured manually or dynamically. With dynamic address mapping, Frame Relay Address Resolution Protocol (ARP) provides a given DLCI and requests next-hop protocol addresses for a specific connection. The router then updates its mapping table and uses the information in the table to forward packets on the correct route. Frame Relay ARP is commonly known as inverse ARP.

When packets are sent across the network, the intermediate switches will look up the DLCI in the map table and do the following:

  • If the DLCI is defined on the link, the switch will forward packets toward their destination.
  • If the DLCI is not defined on the link, the switch will discard the frame.

DLCI Numbering Scheme

The Frame Relay service provider will assign the DLCI numbers for your WAN. Usually, DLCIs 0 to 15 and 1008 to 1023 are reserved for special purposes. Therefore, service providers typically assign DLCIs in the range of 16 to 1007.

Multicasts can use DLCI 1019 and 1020. Cisco Local Management Interface (LMI) uses DLCI 1023 and ANSI/ITU-T uses DLCI 0 (there is a discussion on LMI in the following section).