Frame Relay Virtual Circuits
As mentioned earlier, Frame Relay is a layer 2 (Data Link layer) connection-oriented protocol.
After a connection has been established, end devices can transmit data across the network. This
layer 2 connection across the packet-switched network is called a
virtual circuit
.
The end devices (in this case, routers) will act as
data terminal equipment (DTE), and the
Frame Relay switch will be the data communications equipment (DCE). The difference between
the two is that the DCE device will most likely be responsible for the clocking of the line and
also initiates LMI messages, which we will discuss later in this chapter.
NOTE:Two quick terms you will encounter later:
ingress
refers to Frame Relay frames
from an access device toward the Frame Relay network, and
egress
refers to
Frame Relay frames leaving a Frame Relay network toward the destination device.
From the point-of-view of the router, the virtual circuit is somewhat transparent. This means
that the router sees the virtual circuit, but only up to the Frame Relay switch, which is where
the term
locally significant
originates with regard to the DLCI. In other words, the router speaks
LMI and understands what a DLCI is. This isn’t all that transparent. The transparency comes
in when you consider that the router uses the DLCI like a MAC address for the remote router
to bind the virtual circuit to the protocol address. The previous statement, however, is not a
pure Frame Relay function. In pure Frame Relay terms, the router still has to know how to recognize
the existence of the ingress Frame Relay switch. Even though the circuit might traverse
many switches en route to its destination, the router simply sees its connection to the local
Frame Relay switch, which again is part of the VC.
Figure 29.1 shows how routers see the Frame Relay network. In the figure, notice that Frame
Relay is configured between the routers and the switching office.
FIGURE 2 9 . 1
Frame Relay operation
Frame actually traverses this
CO
PVC
Routers see this
Users only see this User Server
Hub or
switch
Hub or
switch
Router
DLCI 16
CSU/DSU Demarc Demarc CSU/DSU
Router
DLCI 17
There are two ways for Frame Relay to establish this connectivity. Either you can set up
a circuit that is enabled only when needed by using a switched virtual circuit, or you can set
up a dedicated circuit between the local router and remote router by using a permanent virtual
circuit (PVC). Each of these is discussed in more detail next.