1 © 2003, Cisco Systems, Inc. All rights reserved. CCNA 4 v3.1 Module 5 Frame Relay Cisco Networking Academy

1© 2003, Cisco Systems, Inc. All rights reserved.

CCNA 4 v3.1 Module 5 Frame Relay

Cisco Networking Academy


Objectives

• Frame Relay concepts

• Configuring Frame Relay


Frame Relay

• Frame Relay is a packet-switched, connection-oriented, WAN service.

• Frame Relay operates at the data link layer of the OSI reference model.

• Frame Relay uses a subset of the high-level data-link control (HDLC) protocol called Link Access Procedure for Frame Relay (LAPF).


Frame Relay DTEs and DCEs

• In a Frame Relay topology the routers at the edge of each LAN are the DTEs.

The Frame Relay switch is a DCE device.

• A serial connection, such as a T1 leased line (local loop), will connect the router to a Frame Relay switch of the carrier at the nearest point-of-presence for the carrier.


Terminology

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

• Generally, permanent virtual circuits (PVCs) that have been preconfigured by the carrier are used.

• Virtual circuits may be established dynamically by sending signaling messages to the network. In this case they are called switched virtual circuits (SVCs).

–SVCs are far less common than PVCs

–SVCs are more common with X.25, the predecessor of FR


PVCs and DLCIs

• The Frame Relay router connected to the Frame Relay network may have multiple virtual circuits connecting it to various end points.

• The various virtual circuits on a single access line can be distinguished because each VC has its own Data Link Connection Identifier (DLCI).


Frame Relay Functions

1. Frame Relay receives a packet from the network layer protocol, such as IP.

2. Frame Relay wraps it with a layer 2 address field which contains the DLCI.

3. The frame is then passed to the physical layer and transmitted on the wire.


Frame Relay Operation


PVCs and DLCIs

• The Frame Relay router connected to the Frame Relay network may have multiple virtual circuits connecting it to various end points.

• The various virtual circuits on a single access line can be distinguished because each VC has its own Data Link Connection Identifier (DLCI).

• The 10-bit DLCI field of the Frame Relay frame allows VC identifiers 0 through 1023.

The LMI extensions reserve some of these identifiers.


Local Significance of DLCIs

The data-link connection identifier (DLCI) is stored in the Address field of every frame transmitted.

PVC

PVC


Frame Relay Switches

• Frame Relay switches basically route (switch) from one DLCI to another.

Each local serial interface is assigned a local DLCI.

• In order for layer 3 routed protocols to run over Frame Relay interfaces, each interfaces IP address must be mapped to a layer 2 DLCI.

• This mapping of the remote IP address to a local DLCI occurs at the local router’s interface via a ‘frame-relay map’ or through inverse ARP.

-if)#frame-relay map ip 172.31.254.2 201

Local DLCI


Link Management Interface (LMI)

• The purpose of LMI is for DTEs to dynamically acquire information about the status of the network. – Status messages help verify the integrity of logical and

physical links.

– LMI messages are exchanged between the DTE and DCE using reserved DLCIs

• Three types of LMIs are supported by Cisco routers:1. Cisco — The original LMI extensions (Cisco, Nortel and Digital)

2. Ansi — Corresponding to the ANSI standard T1.617 Annex D

3. q933a — Corresponding to the ITU standard Q933 Annex A

• The LMI type must be specified at the Frame Relay interface as the three LMI types are incompatible with one another. – Cisco routers, of course, default to Cisco.The LMI type can be dynamically learned – LMI Autosense

-if)# frame-relay lmi-type [cisco | ansi | q933a]


Inverse ARP and LMI

• LMI status messages combined with Inverse ARP allow a router to associate network layer and data link layer addresses.

• When a router that is connected to a Frame Relay network is started, it sends an LMI status inquiry message to the network.

• The network replies with an LMI status message containing details of every VC configured on the access link.

• If the router needs to map the VC’s DLCIs to network layer addresses, it will send an Inverse ARP message on each VC.

Inverse ARP is enabled by default once you configure the encapsulation type on the serial interface as Frame Relay.

If a static ‘frame-relay map’ is configured then Inverse ARP is disabled on that interface.

config)# int s0/0.102 point-to-point


Frame Relay LMI Types


Stages of Inverse ARP and LMI Operation #1

DLCI 101


Stages of Inverse ARP and LMI Operation #2

DLCI 101

172.16.0.1


Configuring Basic Frame Relay


Configuring a Static Frame Relay Map


Reachability Issues with Routing Updates in NBMA

By default, a Frame Relay network provides nonbroadcast multiaccess (NBMA) connectivity between remote sites.

An NBMA environment is treated like other multiaccess media environments, where all the routers are on the same subnet.

This causes a problem for split-horizon.192.168.1.0/24


Frame Relay and Split-Horizon

• Split-horizon will not accept a routing update about a network that it has sent an update for.– If RTA sends an update for network 192.168.1.0/24 out of

its s0/0 interface then it will not accept an update about 192.168.1.0/24 from RTB, RTC or RTD on that same interface.

• The solution is to either:

1. Use a separate physical interface for each PVC

2. Turn off split-horizon on the interface

3. Configure sub-interfaces


Subinterfaces

• To enable the forwarding of broadcast routing updates in a hub-and-spoke Frame Relay topology, configure the hub router with logically assigned interfaces.

• These interfaces are called subinterfaces.

Subinterfaces can configured as point-to-point or multipoint.

Point-to-point interfaces are all on separate subnets whereas multipoint interfaces are all on the same subnet.

Configuring subinterfaces is much more common because it creates a less complex routing protocol configuration (OSPF)


Frame Relay Subinterfaces


Configuring Point-to-Point Subinterfaces


Verifying Frame Relay

• The show interfaces command displays information regarding the encapsulation and Layer 1 and Layer 2 status. It also displays information about the following:

1. The LMI type

2. The LMI DLCI

3. The Frame Relay (DTE/DCE) type

4. Clockrate on DCE


The show frame-relay lmi Command


The show frame-relay pvc Command


The show frame-relay map Command


Troubleshooting Frame Relay

• Use the debug frame-relay lmi command to determine whether the router and the Frame Relay switch are sending and receiving LMI packets properly.


Frame Relay Congestion Notification

• There are two types of FR congestion notificationForward Explicit Congestion Notification (FECN)

Backward Explicit Congestion Notification (BECN)

• Network DCE devices (switches) change the value of the FECN bit to one on packets traveling in the same direction as the data flow.

This notifies an interface device (DTE) that congestion avoidance procedures should be initiated by the receiving device.

• BECN bits are set in frames that travel the opposite direction of the data flow to inform the transmitting DTE device of network congestion.

• In order for FECN and BECN bits to effect network congestion, the receiving frame relay DTE interfaces must be configured for frame relay traffic shaping


Frame Relay Concepts


Full-Mesh Topology


Frame Relay Mesh

Documents

1 © 2003, Cisco Systems, Inc. All rights reserved. CCNA 4 v3.1 Module 5 Frame Relay Cisco Networking Academy