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Connection
to a frame relay network is done with a local loop from the serial
interface of a router to one of a service provider’s frame relay
switches. Communication across a frame relay network uses virtual
circuits, which are built by a service provider from a router’s
serial interface, through a collection of frame relay switches, to
another router’s serial interface. Virtual circuits that are
programmed into a service provider’s network to stay active all the
time are called permanent virtual circuits (PVCs). IOS also
supports switched virtual circuits (SVCs), which become active
only when they are used; however, SVCs are not yet widely available from
frame relay service providers. We use only PVCs in this chapter.
Many PVCs can be built on a single local loop. PVCs are addressed with Data
Link Connection Identifiers (DLCIs) at layer 2. From our
perspective, each PVC has two DLCIs—one at each end. From a router’s
perspective, each PVC needs only one DLCI—the local one. When a router
wants to transmit a packet to another router across a PVC, the router
must know the local DLCI of the PVC on which the packet is to be
transmitted. For this reason, some people say that DLCIs are locally
significant.
Figure
1 shows a basic frame relay network. There are three routers—Dallas,
FortWorth, and Austin. Each router has a local loop to the frame relay
network. There are two PVCs—one from Dallas to FortWorth and one from
Dallas to Austin. Let us take a closer look at the PVC between Dallas
and FortWorth. The Dallas end of the PVC has DLCI 100 and the FortWorth
end has DLCI 101. These DLCIs, since they are on different local loops,
do not have to be different, but they usually are, anyway. When Dallas
wants to send a packet to FortWorth, Dallas must transmit the packet out
the serial interface that contains the PVC, and the frame header must
contain the local DLCI, 100. The switch knows that the path of the PVC
with DLCI 100 on the Dallas side is supposed to go to the switch
connected to FortWorth. The switch connected to FortWorth knows the path
of the PVC extends across the local loop to FortWorth and the DLCI of
the PVC on the local loop is 101. The switch puts the DLCI 101 into the
frame header so the FortWorth router knows, when it receives the frame,
that the frame came in on the PVC with local DLCI 101.
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Figure 1 - Basic
Frame Relay Network
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Dallas
has two PVCs coming in on the same local loop; therefore, these PVCs
must have different local DLCIs. The frame relay topology shown in
Figure 1 is called a partial-meshed network because not all of
the routers have PVCs to all of the other routers. We could also call
this particular topology a hub-and-spoke network because there is
one router (the hub) that has a connection to each of the other routers
(the spokes), and traffic from a spoke router must go through the hub to
reach another spoke router. A fully meshed network has PVCs
running between all of the router pairs; to make the Figure 1 frame
relay topology into a fully meshed network, we would have to add a PVC
between FortWorth and Austin.
Routers
and switches maintain contact with each other using Local Management
Interface (LMI). About every 10 seconds, routers and switches send
an LMI keepalive across the local loop. If a router is receiving LMI
keepalives from a switch, the router makes the line protocol of its
interface up so the interface state will be up/up. If a Cisco router
interface is connected to a frame relay network and the state of the
interface is up/up, the router has a communication path to a frame relay
switch. This has nothing to do with being able to reach another router
on the other end of a PVC. By default, a Cisco router uses LMI to
request a status report from the switch every six keepalives (about once
a minute). The status report contains a list of each of the local loop’s
PVCs, their DLCIs, and their status.
There
are two types of LMI that are widely used between routers and switches:
Annex D and Gang of Four. Annex D is from the American National
Standards Institute (ANSI). The Gang of Four LMI was jointly
developed by Cisco, DIGITAL, Intel, and Stratacom. The router and the
local switch must agree on the type of LMI they will use between them.
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