What is FRAME RELAY ?

1
What is FRAME RELAY ?

• 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
• Statistically multiplexed traffic from multiple
  sources over private backbone networks can reduce
  the number of circuits and corresponding cost of
  bandwidth
• Lower Equipment Costs
• Lower cost than dedicated leased lines
• Increased performance reduced network
  complexity
• Reduces the amount of processing (as compared to
  X.25)
• Efficiently utilizing high speed digital
  transmission lines, frame relay can improve
  performance and response times of applications.
• 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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FRAME RELAY Overview

• Packet Switched
• Uses Virtual Circuits (Connection Oriented
  Service)
• Logical connection 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)
• Signaling standard between the customer premises
  equipment (CPE) device and the Frame Relay
  switch. It includes
• 1. A keepalive mechanism, which verifies that
  data is flowing
• 2. Multicast mechanism, which conserves
  bandwidth

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FRAME RELAY Technology

• DLCI
• LMI

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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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Functions of LMI (Local Management Interface)

• Determines the operational status of the various
  PVCs that the router knows about
• To transmit keepalive packets to ensure that the
  PVC stays up and does not shut down due to
  inactivity
• Three LMI types can be invoked by the router
  ansi, cisco, and q933a

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LMI EXTENSIONS

• Are set of enhancements to the basic Frame Relay
  specifications
• It offers a number of features (called
  extensions) for managing complex internetworks
• Key Frame Relay LMI extensions include
• - Global addressing
• - Virtual-circuit status messages
• - Multicasting

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LMI EXTENSIONS

• Virtual circuit status messages (common)
• PVC Integrity and information about new and
  existing PVC.
• Multicasting (optional)
• Allows a sender to transmit a single frame but
  have it delivered by the network to multiple
  recipients.
• Global addressing (optional)
• Gives connection identifiers global rather than
  local significance, allowing them to be used to
  identify a specific interface to the Frame Relay
  network. Global addressing makes the Frame Relay
  network resemble a local-area network (LAN) in
  terms of addressing address resolution protocols
  therefore perform over Frame Relay exactly as
  they do over a LAN.
• Simple flow control (optional)
• Provides for an XON/XOFF flow control mechanism
  that applies to the entire Frame Relay interface.
  It is intended for devices whose higher layers
  cannot use the congestion notification bits and
  that need some level of flow control

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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
• The maximum number of uncommitted bits that the
  Frame Relay switch attempts to transfer beyond
  the CIR.
• Excess burst is typically limited to the port
  speed of the local access loop (Your routers
  connection to the Frame Relay Switch).
• 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 by the receiving device. 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 by the receiving device. 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
13
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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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 Mapping

• Frame relay maps (which bind next router hop IP
  addresses to DLCIs and 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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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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FRAME RELAY Implementation without subinterfaces

• Router (DTE device) have a WAN serial interface
  for every PVC

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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

1. Select the interface (S0, S1) get into the
   interface configuration mode
2. Configure network layer address (IP address)
3. Configure the encapsulation type (cisco is
   default, ietf is used if connecting to non-cisco
   routers)
4. Cisco IOS release 11.1 or earlier, specify the
   LMI type used by the Frame relay switch ansi
   cisco q933a
5. Cisco IOS 11.2 or later, the LMI type is
   autosensed
6. Configure bandwidth for the link (Affects many
   routing protocols which uses it for a metric)
7. 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 view 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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Multipoint Subinterfaces
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Point to point Subinterfaces