Frame Relay

1
Frame Relay

• Frame Relay is an ITU-T and ANSI standard that
  defines the process for sending data over a
  public-switched network (PSN).
• Typical speeds range from 56 kbps up to 2 mbps,
  although higher speeds are possible. The network
  providing the Frame Relay service can be either a
  carrier-provided public network or a privately
  owned network.

2
Frame Relay

• Frame Relay defines the interconnection process
  between the customer's DTE (e.g. router) and the
  service provider's DCE (Frame Relay switch).

3
Frame Relay

• Frame Relay does not define the way the data is
  transmitted within the service provider's network
  once the traffic reaches the provider's switch.
  Thus, a Frame Relay provider could use a variety
  of technologies, such as ATM or PPP, to move data
  from one end of its network to another.

4
Frame Relay Devices

• Devices attached to a Frame Relay WAN fall into
  two general categories data terminal equipment
  (DTE) and data circuit-terminating equipment
  (DCE).
• DTEs generally are considered to be terminating
  equipment for a specific network and typically
  are located on the premises of a customer.

5
Frame Relay Devices

• Examples of DTE devices are routers and Frame
  Relay Access Devices (FRADs). A FRAD is a
  specialized device designed to provide a
  connection between a LAN and a Frame Relay WAN.
• DCEs are carrier-owned internetworking devices.
  The purpose of DCE equipment is to provide
  clocking and switching services in a network.

6
Frame Relay Devices

• In most cases, these are packet switches, which
  are the devices that actually transmit data
  through the WAN.

7
Frame Relay Operation

• Generally, the greater the distance covered by a
  leased line, the more expensive the service.
• Packet-switched networks provide a means for
  multiplexing several logical data conversations
  over a single physical transmission link.
• Packet-switched networks use virtual circuits
  (VCs) to deliver packets from end-to-end over a
  shared infrastructure.

8
Frame Relay Operation

• A packet-switched service such as Frame Relay
  requires that a customer maintain only one
  circuit, typically a T1, to the provider's
  Central Office (CO).
• Frame Relay provides tremendous
  cost-effectiveness, since one site can connect to
  many geographically distant sites using a single
  T1 (and single CSU/DSU) to the local CO.

9
Frame Relay Operation

• Frame Relay networks, like X.25 networks, support
  both permanent virtual circuits (PVCs) and
  switched virtual circuits (SVCs).
• The PVC is the most common type of Frame Relay
  virtual circuit. PVCs are permanently established
  connections that are used when there is frequent
  and consistent data transfer between DTE devices
  across a Frame Relay network.

10
Frame Relay Operation

• SVCs are temporary connections, used when there
  is only sporadic data transfer between DTE
  devices across the Frame Relay network.
• Because they are temporary, SVC connections
  require call setup and termination for each
  connection.
• Many Frame Relay providers only support PVCs.

11
Frame Relay Operation

• In a packet-switched network, such as Frame
  Relay, each end of the virtual circuit is
  assigned a connection identifier.
• The service provider's switching equipment
  maintains a table that maps these connection
  identifiers to outbound ports.
• When a frame is received, the Frame Relay switch
  analyzes the connection identifier and delivers
  the frame to the appropriate outbound port.

12
DLCI

• In Frame Relay networks, a data-link connection
  identifier (DLCI) identifies the virtual circuit
  between the DCE and the Frame Relay switch.
• The Frame Relay switch maps the DLCIs between
  each pair of routers to create a PVC.
• DLCIs have local significance.

13
DLCI

• A locally significant DLCI does not reference the
  other end of the PVC. In other words, two DTE
  devices connected by a virtual circuit may use a
  different DLCI value to refer to the same
  connection.
• In order for the router to know which PVC to use,
  Layer 3 addresses must be mapped to DLCI numbers.

14
DLCI

• Cisco routers support two types of Frame Relay
  headers a 4-byte header (cisco) and a 2-byte
  header (ietf) that conforms to the IETF
  standards Internet Engineering Task Force.
• The 4-byte header is Cisco proprietary and cannot
  be used if the router is connected to another
  vendor's equipment across a Frame Relay network.
  The default is the 4-byte header cisco.

15
DLCI

• The technique of allowing multiple logical
  channels to transmit across a single physical
  circuit is called statistical multiplexing.
• Statistical multiplexing dynamically allocates
  bandwidth to active channels.
• Statistical multiplexing contrasts with
  time-division multiplexing (TDM). TDM is
  typically used over dedicated circuits (leased
  lines).

16
DLCI

• Using TDM, information from multiple channels can
  be allocated bandwidth on a single wire based on
  preassigned time slots. Unfortunately, TDM
  allocates bandwidth to each channel regardless of
  whether the station has data to transmit.

17
DLCI

• Frame Relay does not operate at Layer 3.
  Multiplexing is achieved at Layer 2, using a DLCI
  field.
• Your Frame Relay service provider will assign the
  DLCI numbers for your WAN. Usually, DLCIs 0 to 15
  and 1008 to 1023 are reserved for special
  purposes.
• Therefore, service providers typically assign
  DLCIs in the range of 16 to 1007. Multicasts can
  use DLCI 1019 and 1020.

18
DLCI

• In order to build a map of DLCIs to Layer 3
  addresses, the router must first know what VCs
  are available. Typically, the process of learning
  about available VCs and their DLCI values is
  handled by a signaling standard called Local
  Management Interface (LMI).

19
LMI

• Local Management Interface (LMI) is a signaling
  standard between the DTE and the Frame Relay
  switch. LMI is responsible for managing the
  connection and maintaining the status between
  devices.
• It includes support for the following

20
LMI

• A keepalive mechanism - This verifies that data
  is flowing.
• A status mechanism - These messages provide
  communication and synchronization between the
  network and the user device. VC status messages
  prevent the sending of data into black holes,
  that is, over PVCs that no longer exist.

21
LMI

• A multicast mechanism - Multicasting allows a
  sender to transmit a single frame, but have it
  delivered by the network to multiple recipients.
  Multicasting supports the efficient delivery of
  routing protocol messages and address-resolution
  procedures that typically must be sent to many
  destinations simultaneously.

22
LMI

• Global addressing - This gives connection
  identifiers global rather than local
  significance, which allows 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.

23
LMI

• There are three types of LMI, none of which are
  compatible with the other. Cisco, StrataCom,
  Northern Telecom, and Digital Equipment
  Corporation, collectively known as the "gang of
  four," released one type of LMI, while the ANSI
  and the ITU-T each released their own version.
  Cisco, ANSI, and Q933a.

24
LMI

• The LMI type used by the provider's Frame Relay
  switch and the customer's DTE must match. In
  Cisco IOS releases prior to 11.2, you must
  manually configure a Frame Relay interface to use
  the correct LMI type. Your provider will furnish
  you with this information.

25
LMI

• If you are using Cisco IOS Release 11.2 or later,
  the router will attempt to automatically detect
  the type of LMI being used by the provider's
  switch. This automatic detection process is
  called LMI autosensing.
• The Frame Relay switch uses LMI to report the
  status of each configured PVC. The three possible
  PVC states are
• Active state - indicates that the connection is
  active and that routers can exchange data.

26
LMI

• Inactive state - indicates that the local
  connection to the Frame Relay switch is working,
  but the remote router connection to the Frame
  Relay switch is not working.
• Deleted state - indicates that no LMI is being
  received from the Frame Relay switch, or that
  there is no service between the CPE router and
  Frame Relay switch.

27
Inverse ARP

• You can map DLCIs to Layer 3 addresses manually
  on a router using the appropriate configuration
  commands.
• Building static maps can require a great deal of
  administrative overhead in complex networks, and
  static maps cannot adapt to changes in the Frame
  Relay topology.
• Through the exchange of LMI, a Frame Relay switch
  may announce a new virtual circuit with its
  corresponding DLCI.

28
Inverse ARP

• Unfortunately, Layer 3 protocol addressing is not
  included in the announcement.
• Without a new configuration or a mechanism for
  discovering the protocol address of the other
  side, this new virtual circuit is unusable.
• Inverse ARP was developed to provide a mechanism
  for dynamic DLCI to Layer 3 address maps.

29
Inverse ARP

• Inverse ARP works much the same way Address
  Resolution Protocol (ARP) works on a LAN. Once
  the router learns from the switch about available
  PVCs and their corresponding DLCIs, the router
  can send an Inverse ARP request to the other end
  of the PVC.

30
Inverse ARP

• In effect, the Inverse ARP request asks the
  remote station for its Layer 3 address, while at
  the same time providing the remote system with
  the local system's Layer 3 address.
• The return information from the Inverse ARP is
  then used to build the Frame Relay map.

31
Inverse ARP

• On a Cisco router, Inverse ARP is on by default
  when you configure an interface to use Frame
  Relay encapsulation. If you configure a static
  mapping for a specific DLCI, Inverse ARP is
  automatically disabled for the specified protocol
  on the specified DLCI.

32
Configuring Frame-Relay

• To configure an interface for Frame Relay, you
  must specify frame relay encapsulation to
  encapsulate data traffic end to end. There are
  two possible Frame Relay encapsulations cisco
  and ietf.

33
Configuring Frame-Relay

• By default, an interface will use the Cisco Frame
  Relay encapsulation method. This method is Cisco
  proprietary, and should not be used if the router
  is connected to another vendor's equipment across
  a Frame Relay network.
• If you are using Cisco IOS release 11.1 or
  earlier, you must manually define the LMI type
  used by the Frame Relay switch.

34
Configuring Frame-Relay

• The default LMI type is cisco
• Router(config-if)encapsulation Frame-Relay
  cisco ietf
• Router(config-if)frame-relay lmi-type ansi
  cisco q933a

35
Configuring Frame-Relay Maps

• Dynamic address mapping is enabled by default for
  all protocols enabled on a physical interface. If
  your Frame Relay environment supports LMI
  autosensing and Inverse ARP, dynamic address
  mapping will take place automatically, therefore
  no static address mapping is required.
• If your environment does not support LMI
  autosensing and Inverse ARP, you may have to
  manually configure a Frame Relay map.

36
Configuring Frame-Relay Maps

• The frame-relay map command is used to configure
  static address mapping. Once you configure a
  static map for a given DLCI, Inverse ARP is
  disabled on that DLCI. The frame-relay map uses
  the following syntax.
• Router(config-if)frame-relay map protocol
  protocol-address dlci broadcast ietf cisco.

37
Configuring Frame-Relay Maps

• The broadcast keyword provides two functions it
  forwards broadcasts when multicasting is not
  enabled, and it simplifies the configuration of
  OSPF for nonbroadcast networks that will use
  Frame Relay.
• Using the broadcast keyword in frame relay
  configuration can assist OSPF in locating
  neighbors.
• This is done by requiring a designated router.

38
Configuring Frame-Relay Maps

• In previous releases, selection of a designated
  router required manual assignment in the OSPF
  configuration using the neighbor interface router
  command.

39
Configuring Frame-Relay Maps

• When the frame-relay map command (with the
  broadcast keyword) and the ip ospf network
  command (with the broadcast keyword) are
  configured, there is no need to configure any
  neighbors manually. OSPF will now automatically
  use the Frame Relay network as a broadcast
  network.

40
Verifying Frame-Relay

• After you configure Frame Relay, you can verify
  that the connections are active by using several
  different show commands
• show interfaces serial
• show frame-relay pvc
• show frame-relay map
• show frame-relay lmi

41
Verifying Frame-Relay

• A typical Frame Relay WAN is composed of numerous
  sites that are connected to the central office
  via a local loop. The Telco identifies these
  individual local loops with a circuit number,
  such as 05QHDQ101545-080TCOM-002. Telco
  technicians typically leave a label on the
  channel service unit/data service unit (CSU/DSU)
  with the circuit number.

42
Verifying Frame-Relay

• When you call the Telco for help with
  troubleshooting your Frame Relay WAN, you will be
  required to provide the circuit number.
• To simplify the management of your WAN, use the
  description command at the interface level to
  record the circuit number

43
Verifying Frame-Relay

• Atlanta(config)interface serial
  0Atlanta(config-if)description Cicuit-
  05QHDQ101545-080TCOM-002Atlanta(config-if)zAtl
  antashow interfaces serial 0.

44
Configuring Frame-Relay Maps

• OSPF can be configured to treat a nonbroadcast,
  multiaccess network such as Frame Relay in much
  the same way as it treats a broadcast network.