Frame Relay Data Link Connection Identifiers
To connect two DTEs, an FR service uses a virtual circuit (VC) between pairs of routers. A router
can then send an FR frame with the appropriate (typically) 10-bit Data Link Connection Identifier
(DLCI) header field that identifies each VC. The intermediary FR switches forward the frame
based on its DLCI, until the frame eventually exits the FR service out the access link to the router
on the other end of the VC.
FR DLCIs are locally significant, meaning that a particular DLCI value only matters on a single
link. As a result, the DLCI value for a frame may change as the frame passes through the network.
The following five-step process shows the locally significant DLCI values for a VC in Figure 15-1:
1. Router A sends a frame with DLCI 41.
2. The FR service identifies the frame as part of the VC connecting Router A to Router B.
3. The FR service replaces the frame’s DLCI field with a value of 40.
4. The FR service forwards the frame to Router B.
5. Router B sees the incoming DLCI as 40, identifying it as being from Router A.
Figure 15-1 Comparing Local and Global Frame Relay DLCIs
In practice, some providers use a convention called global addressing. The global DLCI convention
simply allows humans to think of routers as having a single address, more akin to how MAC
addresses are used. However, the addresses are still local, and a VC’s DLCI may well change
values as it passes through the network. For instance, the same VC from Router A to Router B in
Figure 15-1 could use global addressing, listing Router A’s DLCI as 40, and Router B’s as 41.
The logic based on the global addresses works like LANs. For example, for Router A to send a
frame to Router B, Router A would send the frame to Router B’s global address (41). Similarly,
Router B would send frames to Router A’s global address of 40 to send packets to Router A.
Frame with DLCI 40
Frame with DLCI 42
Frame with DLCI 40Global
DLCI 41
Global
DLCI 42
Global
DLCI 40
Frame with DLCI 41
Frame with DLCI 40
Frame with DLCI 42
Frame with DLCI 40