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 1007.

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 TechnologyFrame 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 CircuitsThe 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

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