Chapter3 frame relay

Frame Relay

Accessing the WAN – Chapter 3

ITE I Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 1

Historical Background
 Frame Relay is a high-performance WAN protocol
that operates at the physical and data link layers of
the OSI model.
 Eric Scace, an engineer at Sprint International,
invented Frame Relay as a simpler version of the
X.25 protocol to use across ISDN interfaces.
 Network providers commonly implement Frame Relay
for voice and data as an encapsulation technique,
used between LANs over a WAN. Each end user gets
a private line (or leased line) to a Frame Relay node.
 Frame Relay has become one of the most extensively
used WAN protocols, primarily because it is
inexpensive compared to dedicated lines.

ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 2

Introducing Frame Relay
 Frame Relay has become the most widely used WAN
technology in the world primarily because of its price
and flexibility.
 Frame Relay reduces network costs by using less
equipment, less complexity, and an easier
implementation. Moreover, Frame Relay provides
greater bandwidth, reliability, and resiliency than
private or leased lines.
 With increasing globalization and the growth of one-
to-many branch office topologies, Frame Relay offers
simpler network architecture at a lower cost of
ownership.


Describe the Fundamental Concepts of Frame
Relay Technology
 Describe how Frame Relay is used to provide WAN
services to the Enterprise


Relay Technology
 Frame Relay Operation


Relay Technology
 Virtual circuit (VC) - connection through a Frame Relay network
between two DTEs; maybe SVCs or PVCs
 VCs are identified by DLCIs. DLCI values are assigned by the
service provider; DLCIs have local significance
 DLCIs 0 to 15 and 1008 to 1023 are reserved for special purposes.


Relay Technology


Relay Technology
 Describe the types of topologies that are used for
implementing Frame Relay in different environments

In mesh networks, total VCs
required is [n(n-1)/2]


Relay Technology
 Frame Relay Address Mapping
 Dynamic address mapping - relies on Inverse ARP
(IARP) to resolve a next hop network protocol address
to a local DLCI value.
 On Cisco routers, Inverse ARP is enabled by default for
all protocols
 One can choose to override dynamic Inverse ARP
mapping by supplying a manual static mapping for
the next hop protocol address to a local DLCI.
 You cannot use Inverse ARP and a frame-relay map
statement for the same DLCI and protocol.


Relay Technology
 LMI - a keepalive mechanism that provides status
information about FR connections between the router
(DTE) and the FR switch (DCE)
 Do not confuse LMI with encapsulation. LMI defines
messages used between the DTE and the DCE.
Encapsulation defines the headers used by a DTE to
communicate information to the DTE at the other end of
a VC.
 Three types of LMIs are supported by Cisco routers:
Cisco, Ansi and q933a.
 Starting with Cisco IOS 11.2, the default LMI autosense
feature detects the LMI type supported by the directly
connected Frame Relay switch.


Relay Technology
 LMI status messages combined with Inverse ARP
messages allow a router to associate network layer and
data link layer addresses.


Configure a Basic Frame Relay PVC
 Configure a basic Frame Relay PVC on a router serial
interface
 Verify encapsulation through sh int serial 0/0/0


Configure a Basic Frame Relay PVC
 Configure a static Frame Relay map
 Frame Relay is an NBMA network which does not support
broadcast traffic, using the broadcast keyword is a simplified way
to forward routing updates.


Describe Advanced Concepts of Frame Relay
Technology

Split Horizon prohibits routing
updates received on an interface
from exiting that same interface.

 Using a hub and spoke topology, the split horizon rule reduces the
chance of a routing loop with distance vector routing protocols.
 It prevents a routing update received on an interface from being
forwarded through the same interface.
 If the Central router learns about Network X from Branch A, that update
is learned via S0/0.
 According to the split horizon rule, Central could not update Branch B or
Branch C about Network X.
 This is because that update would be sent out the S0/0 interface, which
is the same interface that received the update.

Technology
 Frame Relay can allow customers to dynamically
access this extra bandwidth and "burst" over their CIR
for free.

•The duration of a burst
transmission should be short.
•BE is the term used to describe
the bandwidth available above
the CBIR.
•Unlike the CBIR, it is not
negotiated.


Technology
• Forward Explicit Congestion Notification (FECN) – When a
Frame Relay switch recognizes congestion in the network, it sends
an FECN packet to the destination device.
– This indicates that congestion has occurred.
• Backward Explicit Congestion Notification (BECN) – When a
Frame Relay switch recognizes congestion in the network, it sends
a BECN packet to the source router.
– This instructs the router to reduce the rate at which it is
sending packets.


Configure an Advanced Frame Relay PVC
 Explain the steps to configure point-to-point
subinterfaces on a physical interface


 Describe the commands used for verifying Frame Relay
operation


 The show frame-relay lmi command displays LMI
traffic statistics showing the number of status
messages exchanged between the local router and the
Frame Relay switch.


• The show frame-relay pvc command displays the
status of each configured connection, as well as traffic
statistics.
• This command is also useful for viewing the number of
BECN and FECN packets received by the router.
• The command show frame-relay pvc shows the status
of all PVCs configured on the router.


• To clear dynamically created Frame Relay maps, which
are created using Inverse ARP, use the clear
frame-relay-inarp command.


 Use the debug frame-relay lmi command to
determine whether the router and the Frame Relay
switch are sending and receiving LMI packets properly.



 The possible values of the status field are as follows:
 0x0 – Added/inactive means that the switch has this DLCI programmed but for
some reason it is not usable. The reason could possibly be the other end of the
PVC is down.
 0x2 – Added/active means the Frame Relay switch has the DLCI and everything
is operational.
 0x4 – Deleted means that the Frame Relay switch does not have this DLCI
programmed for the router, but that it was programmed at some point in the
past. This could also be caused by the DLCIs being reversed on the router, or
by the PVC being deleted by the service provider in the Frame Relay cloud.


Summary
 Frame relay is the most widely used WAN technology
because it:
–Provides greater bandwidth than leased line
–Reduces cost because it uses less equipment
–Easy to implement

 Frame relay is associated with layer 2 of the OSI model
and encapsulates data packets in a frame relay frame
 Frame relay is configured on virtual circuits
–These virtual circuits may be identified by a DLCI

 Frame relay uses inverse ARP to map DLCI to IP
addresses


Summary

 Configuring frame relay requires
–Enable frame relay encapsulation
–Configuring either static or dynamic mapping
–Considering split horizon problems that develop when multiple
VCs are placed on a single physical interface
 Factor affecting frame relay configuration
–How service provider has their charging scheme set up
 Frame relay flow control
–DE
–FECN
–BECN


Summary

 The following commands can be used to help verify
frame relay configuration
–Show interfaces
–Show frame-relay lmi
–Show frame-relay pvc ###
–Show frame-relay map

 Use the following command to help troubleshoot a
frame relay configuration
–Debug frame-relay lmi


Chapter3 frame relay

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