Frame Relay

  • Data Terminal Equipment (DTE): Generally CPE (Customer Premises Equipment)
  • Data Communication Equipment (DCE): Carrier owned device, provides clocking and switching services in the network and transmit data through the WAN.
  • Local Access Rate: Clock speed (port speed)
  • Virtual Circuit: DTE to another device
  • Permanent Virtual Circuit: Permanently established connection
  • Switch Virtual Circuit: “On Demand” Circuit
  • Data-Link Connection Identifier (DLCI): The point between the local router and the local Frame Relay switch.
  • Committed Information Rate (CIR): The maximum average data rate under normal circumstances.
  • Inverse Address Resolution Protocol (ARP): Used to dynamically assign a DLCI to an IP address.
  • Local Management Interface (LMI): Signalling standard between the DTE and the DCE device.
  • Forward Explicit Congestion Notification (FECN): A bit in the address field which indicates that these frames experienced congestion in the path between source and destination.
  • Backward Explicit Congestion Notification (BECN): A bit in the address field which informs the DTE device that a particular path through the network is congested.

Frame Relay provides Non broadcast Multi Access (NBMA) between remote sites.

NBMA is like other broadcast environments eg: Ethernet where all routers are on the same subnet.

Split horizon is a main issue when frame relay is running multiple PVC’s over a single interface.

In a NBMA environment network hosts communicate directly to one another.  There are no broadcasts for multicast’s.

Solve Update Reachability by:

  • Turn off split horizon
  • Use fully meshed topology
  • Use sub-interfaces for PVC’s that share the same single physical interface

In frame relay the local DLCI must be mapped to a destination network layer address.

Routers can automatically discover their DLCI by using LMI protocol.

LMI Signals between the router and the frame relay switch.

Cisco routers support the following LMI types:

  • Cisco
  • ANSI (T1.617 Annex D)
  • Q.933A (ITU-T Q.933 Annex A)

LMI and VC Status States

  1. Active – VC connection active and routers can exchange information
  2. Inactive – Local to the frame relay switch is working but the remote router to frame relay switch is not.
  3. Deleted – Either no LMI from the frame relay switch or no service exists between the router and local frame relay switch.

Every 60 seconds router send inverse ARP’s on all active DLCI’s.

Every 10 seconds the router exchanges LMI info with the frame relay switch (keep alives).

Sub-interfaces can be configured in one of the following modes:

  • Point to Point: Each pair on different subnets
  • Multipoint: Each pair on the same subnet

Summary of Frame Relay WAN’s

  • Frame relay PVC’s are identified with DLCI’s and the status reported through LMI protocol
  • Point to Point sub interfaces require separate subnets for each PVC
  • Multipoint sub interfaces share a subnet with peers

Troubleshooting Frame Relay WAN’s

  • Troubleshoot the link
  • Troubleshoot the mapping from one router to another
  • Troubleshoot routing across the frame relay network
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