Lecture Week 3 Frame Relay Accessing the WAN

3.1 Basic Frame Relay Concepts Accessing the WAN

Basics Frame Relay is a data link layer packet-switching protocol that uses digital circuits. It is used for medium to longer distances and for longer connectivity. Leased lines also provide longer connectivity but a physical circuit is used to make connection between 2 sites and the same circuit path is used always. Frame Relay connections use logical circuits to make connections between 2 sites. These logical circuits are referred to as Virtual Circuits(VCs). Multiple VCs can exist on the same physical connection. VCs are Full duplex.

Advantages of Frame Relay VCs overcome the scalability problems of leased lines by providing multiple logical circuits over the same physical connection. Only one serial interface of a router is needed to handle the VC connections to multiple sites Whereas using leased lines multiple serial interfaces are needed to connect to multiple sites. VCs provide full connectivity at a much lower price compared to leased lines.

Sub-interfaces Uses Shared bandwidth Local Management interface(LMI): –used between the Frame relay DTE(eg.Router) and the Frame Relay DCE(eg. Frame Relay switch) –Defines how the DTE interacts with the DCE –Locally significant –Provides VCs status information(a keep-alive mechanism) –LMI standards : Cisco, ANSI, Q933a The DTE and DCE must have the same LMI signaling type Frame Relay Terminology

Data Link Connection Identifier(DLCI) : –used to identify each VC on a physical interface (i.e.) Each VC has a unique local address called a DLCI number. –switch will map to the destination depending on the DLCI number –Inverse ARP is used to map DLCIs to next hop addresses. –Mapping can also be done manually. –Its Locally significant. –These numbers are given by the Frame relay service providers, Service providers assign DLCIs in the range of 16 to Frame Relay Terminology

Virtual circuits are of two types: –Permanent Virtual Circuits – PVCs –Switched Virtual Circuits –SVCs Permanent Virtual Circuit : –similar to a dedicated leased line, permanent connection. –used when constant data is being generated. Switched Virtual Circuit : –also called as Semi-permanent virtual circuit –similar to a circuit switched connection where the VC is dynamically built and then torn down once the data has been sent. –used when data has to be sent in small amounts and at periodic intervals. Frame Relay Terminology

Committed Information Rate(CIR) : –Average data rate measured over a fixed period of time that the carrier guarantees for a VC. –committed bandwidth Burst Rate(BR) : –Average data rate provider guarantees for a VC. –Excess bandwidth Frame Relay Terminology

FECN and BECN : –Forward Explicit Congestion Notification –Backward Explicit Congestion Notification When congestion occurs switch marks the FECN and BECN bits in the frame header. FECN is sent to the destination BECN is sent to the source Thereby notifying both source and destination about the congestion. FECN = 0 and BECN =0 implies no congestion. Frame Relay Terminology

Frame Relay: An Efficient and Flexible WAN Technology Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation.

The Frame Relay WAN Frame Relay provides access to a network, delimits and delivers frames in proper order, and recognizes transmission errors through a standard Cyclic Redundancy Check.

Virtual Circuits The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC).

The Frame Relay Encapsulation Process

Frame Format

Frame Relay Topologies

Frame Relay address maping Dynamic Mapping – Inverse ARP

Frame Relay address mapping Static Mapping

Local Management Interface LMI is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE)

LMI extensions VC status messages - Provide information about PVC integrity by communicating and synchronizing between devices. Multicasting - Allows a sender to transmit a single frame that is delivered to multiple recipients. Global addressing - Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network. Simple flow control - Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface.

LMI status messages

3.2 Configure a Basic FR Accessing the WAN

Configuring FR

Configuring FR (contd.)

Configure a static Frame Relay map

3.3 Advanced FR Concepts Accessing the WAN

Frame Relay NBMA topology

Frame Relay Subinterfaces

Bandwidth control

Implementing flow control

3.4 Configuring Advanced FR Accessing the WAN

Point-to-point subinterfaces

Use the frame-relay interface-dlci for local DLCI on the subinterface

Verifying FR configuration.

Troubleshooting FR.

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

Thank You