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CCNA 4 v3.1 Module 5
Frame Relay
Cisco Networking Academy
© 2003, Cisco Systems, Inc. All rights reserved.
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Objectives
• Frame Relay concepts
• Configuring Frame Relay
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Frame Relay
• Frame Relay is a packet-switched, connectionoriented, WAN service.
• Frame Relay operates at the data link layer of the
OSI reference model.
• Frame Relay uses a subset of the high-level datalink control (HDLC) protocol called Link Access
Procedure for Frame Relay (LAPF).
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Frame Relay DTEs and DCEs
• In a Frame Relay topology the routers at
the edge of each LAN are the DTEs.
The Frame Relay switch is a DCE device.
• A serial connection, such as a T1 leased
line (local loop), will connect the router to
a Frame Relay switch of the carrier at the
nearest point-of-presence for the carrier.
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Terminology
• The connection through the Frame Relay
network between two DTEs is called a virtual
circuit (VC).
• Generally, permanent virtual circuits (PVCs) that
have been preconfigured by the carrier are used.
• Virtual circuits may be established dynamically
by sending signaling messages to the network.
In this case they are called switched virtual
circuits (SVCs).
–SVCs are far less common than PVCs
–SVCs are more common with X.25, the predecessor of FR
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PVCs and DLCIs
• The Frame Relay router connected to the
Frame Relay network may have multiple
virtual circuits connecting it to various
end points.
• The various virtual circuits on a single
access line can be distinguished because
each VC has its own Data Link Connection
Identifier (DLCI).
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Frame Relay Functions
1. Frame Relay receives a packet from the
network layer protocol, such as IP.
2. Frame Relay wraps it with a layer 2
address field which contains the DLCI.
3. The frame is then passed to the physical
layer and transmitted on the wire.
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Frame Relay Operation
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PVCs and DLCIs
• The Frame Relay router connected to the Frame
Relay network may have multiple virtual circuits
connecting it to various end points.
• The various virtual circuits on a single access
line can be distinguished because each VC has
its own Data Link Connection Identifier (DLCI).
• The 10-bit DLCI field of the Frame Relay frame
allows VC identifiers 0 through 1023.
The LMI extensions reserve some of these identifiers.
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Local Significance of DLCIs
The data-link connection identifier (DLCI) is stored
in the Address field of every frame transmitted.
PVC
PVC
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Frame Relay Switches
• Frame Relay switches basically route (switch)
from one DLCI to another.
Each local serial interface is assigned a local DLCI.
• In order for layer 3 routed protocols to run over
Frame Relay interfaces, each interfaces IP
address must be mapped to a layer 2 DLCI.
• This mapping of the remote IP address to a local
DLCI occurs at the local router’s interface via a
‘frame-relay map’ or through inverse ARP.
-if)#frame-relay map ip 172.31.254.2 201
Local DLCI
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Link Management Interface (LMI)
•
The purpose of LMI is for DTEs to dynamically acquire
information about the status of the network.
–
–
•
Three types of LMIs are supported by Cisco routers:
1.
2.
3.
•
Status messages help verify the integrity of logical and
physical links.
LMI messages are exchanged between the DTE and DCE
using reserved DLCIs
Cisco — The original LMI extensions (Cisco, Nortel and Digital)
Ansi — Corresponding to the ANSI standard T1.617 Annex D
q933a — Corresponding to the ITU standard Q933 Annex A
The LMI type must be specified at the Frame Relay interface
as the three LMI types are incompatible with one another. –
Cisco routers, of course, default to Cisco.
The LMI type can be dynamically learned – LMI Autosense
-if)# frame-relay lmi-type [cisco | ansi | q933a]
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Inverse ARP and LMI
• LMI status messages combined with Inverse ARP allow a
router to associate network layer and data link layer
addresses.
• When a router that is connected to a Frame Relay network is
started, it sends an LMI status inquiry message to the
network.
• The network replies with an LMI status message containing
details of every VC configured on the access link.
• If the router needs to map the VC’s DLCIs to network layer
addresses, it will send an Inverse ARP message on each VC.
Inverse ARP is enabled by default once you configure the
encapsulation type on the serial interface as Frame Relay.
If a static ‘frame-relay map’ is configured then Inverse ARP is
disabled on that interface.
config)# int s0/0.102 point-to-point
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Frame Relay LMI Types
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Stages of Inverse ARP and
LMI Operation #1
DLCI 101
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Stages of Inverse ARP and
LMI Operation #2
172.16.0.1
DLCI 101
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Configuring Basic Frame Relay
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Configuring a Static Frame Relay Map
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Reachability Issues with Routing Updates
in NBMA
By default, a Frame Relay network provides nonbroadcast
multiaccess (NBMA) connectivity between remote sites.
An NBMA environment is treated like other multiaccess media
environments, where all the routers are on the same subnet.
This causes a problem for split-horizon.
192.168.1.0/24
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Frame Relay and Split-Horizon
•
Split-horizon will not accept a routing update
about a network that it has sent an update for.
– If RTA sends an update for network 192.168.1.0/24 out of
its s0/0 interface then it will not accept an update about
192.168.1.0/24 from RTB, RTC or RTD on that same
interface.
•
The solution is to either:
1. Use a separate physical interface for each PVC
2. Turn off split-horizon on the interface
3. Configure sub-interfaces
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Subinterfaces
• To enable the forwarding of broadcast routing
updates in a hub-and-spoke Frame Relay
topology, configure the hub router with logically
assigned interfaces.
• These interfaces are called subinterfaces.
Subinterfaces can configured as point-to-point or multipoint.
Point-to-point interfaces are all on separate subnets whereas
multipoint interfaces are all on the same subnet.
Configuring subinterfaces is much more common because it
creates a less complex routing protocol configuration (OSPF)
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Frame Relay Subinterfaces
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Configuring Point-to-Point Subinterfaces
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Verifying Frame Relay
•
The show interfaces command displays
information regarding the encapsulation
and Layer 1 and Layer 2 status. It also
displays information about the following:
1. The LMI type
2. The LMI DLCI
3. The Frame Relay (DTE/DCE) type
4. Clockrate on DCE
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The show frame-relay lmi Command
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The show frame-relay pvc Command
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The show frame-relay map Command
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Troubleshooting Frame Relay
• Use the debug frame-relay lmi
command to determine whether the router
and the Frame Relay switch are sending
and receiving LMI packets properly.
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Frame Relay Congestion Notification
• There are two types of FR congestion notification
Forward Explicit Congestion Notification (FECN)
Backward Explicit Congestion Notification (BECN)
• Network DCE devices (switches) change the value of the
FECN bit to one on packets traveling in the same direction as
the data flow.
This notifies an interface device (DTE) that congestion avoidance
procedures should be initiated by the receiving device.
• BECN bits are set in frames that travel the opposite direction
of the data flow to inform the transmitting DTE device of
network congestion.
• In order for FECN and BECN bits to effect network
congestion, the receiving frame relay DTE interfaces must be
configured for frame relay traffic shaping
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Frame Relay Concepts
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Full-Mesh Topology
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Frame Relay Mesh
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