FRAME RELAY

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CCNA 4 Chapter 6

• FRAME RELAY

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What is FRAME RELAY ?

• Frame Relay is a WAN technology that provides
  LAN-to-LAN connectivity
• Frame Relay is a way of sending information over
  a WAN by dividing data into packets
• It operates at the Physical and Data Link layers
  of the OSI reference model
• It relies on upper-layer protocols such as TCP
  for error correction
• Frame Relay is a switched data link-layer
  protocol that handles multiple virtual circuits
  using (HDLC) encapsulation
• Frame Relay interface can be either a
  carrier-provided public network or a network of
  privately owned equipment, serving a single
  enterprise

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Benefits of FRAME RELAY

• Reduced internetworking costs
• Lower Equipment Costs
• Lower cost than dedicated leased lines
• Increased performance reduced network
  complexity
• Reduces the amount of processing (as compared to
  X.25)
• Frame relay can improve performance and response
  times of applications by efficiently utilizing
  high speed digital transmission lines.
• Increased interoperability via international
  standards
• Frame relay can be implemented over existing
  technology
• Access devices often require only software
  changes or simple hardware modifications to
  support the interface standard
• Existing packet switching equipment and T1/E1
  multiplexers often can be upgraded to support
  frame relay over existing backbone networks.

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Carrier-Provided Public Network vs. Network of
Privately-Owned Equipment

• Public Frame Relay network - the Frame Relay
  equipment is located in the central offices of a
  telecommunications carrier/service provider.
• Subscribers charged based on network usage but
  relieved from administering and maintaining the
  equipment and service.
• Private Frame Relay network - equipment,
  administration and maintenance are the
  responsibility of the private company.
• Note A majority of todays Frame Relay networks
  are public carrier-provided networks.

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Frame Relay Networks

• A typical Frame Relay network consists of a
  number of DTE devices, such as routers, connected
  to remote ports on a multiplexer via traditional
  point-to-point services such as T1, fractional
  T1, or 56kbps circuits.

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Frame Relay Networks

• Frame Relay is an ideal, cost effective solution
  for networks with bursty traffic that require
  connections to multiple locations and where a
  certain degree of delay is acceptable.
• Frame Relay is called a fast packet technology.

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LAN-to-LAN Connections

• Without Frame Relay, a multiport router and
  separate CSU/DSUs are required for each LAN.

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LAN-to-LAN Connections

• Using Frame Relay, only a single access is needed
  into the Frame Relay cloud

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LAN-to-LAN Connections

• Another way of looking at it
• Private vs.
  Public

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FRAME RELAY Overview

• Packet Switched technology
• Uses Virtual Circuits (Connection-Oriented
  Service)
• A logical connection is created between two
  (DTE) devices across a Frame Relay
  packet-switched network (PSN)

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FRAME RELAY Technology

• Access rate
• The clock speed (port speed) of the connection
  (local loop) to the Frame Relay cloud
• Data-link connection identifier (DLCI)
• DLCI number identifies the end point in a Frame
  Relay network
• Each Virtual Circuit is uniquely identified by a
  DLCI number
• The Frame Relay switch maps the DLCIs between a
  pair of routers to create a permanent virtual
  circuit
• Local management interface (LMI)
• A set of enhancements to the basic Frame Relay
  Specification
• includes a keepalive mechanism which verifies
  that data is flowing.

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Establishing Connections

• Frame Relay must first establish a connection
  before two routers can communicate.
• Frame Relay relies on Permanent Virtual Circuits
  (PVCs) instead of establishing and maintaining a
  permanent, dedicated link.
• Thus PVCs establish a logical connection between
  two sites, instead of a physical one.

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PVC Identification using DLCIs

• Frame Relay PVCs are identified by DLCIs.
• DLCI (Data Link Connection Identifier)
• DLCIs identify a particular PVC endpoint.
• Subinterfaces are required to configure multiple
  DLCIs on a single router interface.
• DLCIs have local significance.

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More on DLCIs

• A DLCI is a channel number which is attached to a
  data frame to tell the network how to route the
  data.
• Frame Relay is statistically multiplexed which
  means that only one frame can be transmitted at a
  time but many logical connections can co-exist on
  a single physical line.
• The DLCI allows the data to be logically tied to
  one of the logical connections.

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DLCIs Frame Relay Addressing

• Frame Relay DLCIs have local significance
• The values themselves are not unique in the Frame
  Relay WAN
• Two DTE devices connected by a virtual circuit
  might use a different DLCI value to refer to the
  same connection

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LMI (Local Management Interface)

• LMI
• is a set of enhancements to basic Frame Relay
• determines the operational status of the various
  PVCs
• transmits keepalive packets to ensure that the
  PVC stays up and does not shut down due to
  inactivity

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LMI EXTENSIONS

• LMI offers a number of features (called
  extensions) for managing complex internetworks
• Key Frame Relay LMI extensions include
• Virtual-circuit status messages
• Multicasting
• Global Addressing

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LMI EXTENSIONS/FEATURES

• Virtual circuit status messages (common)
• PVC integrity information about new and
  existing PVC.
• Multicasting (optional)
• Allows a sender to transmit a single frame but
  have it delivered to multiple recipients.
• Global addressing (optional)
• Gives DLCIs global rather than local
  significance
• allows them to be used to identify a specific
  interface to the Frame Relay network.
• makes the Frame Relay network resemble a LAN in
  terms of addressing.

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FRAME RELAY Technology

• Committed information rate (CIR)
• The CIR is the guaranteed rate, in bits per
  second, that the service provider commits to
  providing.
• Committed burst
• The maximum number of bits that the switch
  agrees to transfer during a time interval.
• Excess burst
• Maximum number of uncommitted bits that the
  Frame Relay switch attempts to transfer beyond
  the CIR
• Typically limited to the port speed of the local
  access loop
• Discard eligibility (DE) indicator
• A set bit that indicates the frame may be
  discarded in preference to other frames if
  congestion occurs. When the router detects
  network congestion, the Frame Relay switch will
  drop packets with the DE bit set first. The DE
  bit is set on the oversubscribed traffic
  (Anything over the CIR).

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FRAME RELAY Congestion

• Forward explicit congestion notification (FECN)
  A bit set in a frame that notifies a DTE that
  congestion avoidance procedures should be
  initiated.
• When a Frame Relay switch recognizes congestion
  in the network, it sends a FECN packet to the
  destination device, indicating that congestion
  has occurred.
• Backward explicit congestion notification (BECN)
  A bit set in a frame that notifies a DTE
  that congestion avoidance procedures should be
  initiated.
• When a Frame Relay switch recognizes congestion
  in the network, it sends a BECN packet to the
  source router, instructing the router to reduce
  the rate at which it is sending packets.
• If the router receives any BECNs during the
  current time interval, it decreases the transmit
  rate by 25.

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FRAME RELAY Congestion
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FRAME RELAY Multiplexing

• Statistical Time Division Multiplexing (STDM)
• Multiplexes multiple virtual circuits, through a
  shared physical medium by assigning DLCIs to each
  DTE/DCE pair of devices

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FRAME RELAY Mapping

• Frame relay maps - bind next router hop IP
  addresses to DLCIs
• work together with standard routing tables
• can be statically configured, or can be
  dynamically created by the invocation of inverse
  ARP

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INVERSE ARP

• Allows the router to automatically build the
  Frame Relay map
• The router learns the DLCIs that are in use from
  the switch during the initial LMI exchange.
• The router then sends an Inverse ARP request to
  each DLCI for each protocol configured on the
  interface if the protocol is supported.
• The return information from the Inverse ARP is
  then used to build the Frame Relay map.

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FRAME RELAY Switching tables

• The Frame Relay switching table consists of four
  entries
• 2 for incoming port and DLCI
• 2 for outgoing port and DLCI

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Subinterfaces

• A single physical interface can be split into
  multiple logical interfaces
• Subinterfaces can resolve split horizon issues
• Routing updates can be sent out subinterfaces as
  if they were separate physical interfaces
• Overall cost of implementing a Frame Relay
  network can be reduced.

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One-to-One or One-to-Many

• Point-to-Point Multipoint are connection types
  that are supported on an interface in Frame Relay

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Multipoint Frame Relay Connection

• A single subinterface is used to establish
  multiple PVC connections to multiple physical
  interfaces or subinterfaces on remote routers
• Only ONE subnet is required on a multipoint Frame
  Relay connection

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Point-to-Point vs. Multipoint

• Multipoint can be used if you want the router to
  forward broadcasts and routing updates (all
  routers in the same subnet) .

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Split Horizon

• Split horizon reduces routing loops by not
  allowing a routing update received on one
  physical interface to be sent back out that same
  interface.

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The Frame Relay Frame

• The flag field is used to indicate the beginning
  and end of the frame

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Basic FRAME RELAY Configuration

• BASIC FRAME RELAY CONFIGURATION ASSUMES THAT
• Configure Frame Relay on one or more physical
  interfaces
• LMI and Inverse ARP are supported by the remote
  router(s)

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Basic FRAME RELAY Configuration

• Select the interface (S0, S1) get into the
  interface configuration mode
• Configure network layer address (IP address)
• Configure the encapsulation type (cisco is
  default, ietf is used if connecting to non-cisco
  routers)
• Cisco IOS release 11.1 or earlier, specify the
  LMI type used by the Frame relay switch ansi
  cisco q933a
• Cisco IOS 11.2 or later, the LMI type is
  autosensed
• Configure bandwidth for the link (Affects many
  routing protocols which use it for a metric)
• Inverse ARP is on by default

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Verifying FRAME RELAY Operation

• After configuring Frame Relay, you can verify
  that the
• connections are active by using the show commands
• Show interface serial (Displays DLCI used on the
  configured interface, LMI DLCI used for the LMI)
• Show frame-relay pvc (Displays status of each
  configured connection shows the number of BECN
  FECN packets received by the router)
• Show frame-relay map (Displays the IP address
  associated DLCI for each remote destination to
  which the router is connected to)
• Show frame-relay lmi (Displays LMI traffic
  statistics- it will show the number of status
  messages between the router the FR Switch)

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Configuring Subinterfaces

• Select interface get into interface
  configuration mode
• Remove any existing network-layer address
  assigned to the physical interface
• Configure Frame Relay encapsulation
  router(config-if)encapsulation frame-relay
• Select the subinterface you want to configure
  router(config-if)interface serial 0.1
  multipoint point to point
• Configure the network-layer address on the
  subinterface
• Configure the DLCI for the subinterface to
  distinguish it from the physical interface
  router(config-if)frame-r
  elay interface-dlci dlci-number

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THE END

• Remember to check out the links listed at the
  bottom of each section in Chapter 6 of the
  On-Line Curriculum
• before you take your test.
• Good Luck!