CCNA – Semester4

Module 5Frame Relay

Objectives

• Components of a Frame Relay network

• The technology of Frame Relay and topology of a Frame Relay network

• Configuring Frame Relay

• Issues of a non-broadcast multi-access network

Frame Relay Concepts

Introducing Frame Relay

• An ITU-T and ANSI standard.

• A packet-switched, connection-oriented, WAN service.

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

• Uses a subset of HDLC protocol LAPF.

Frame Relay operation

• Frames carry data between user devices called DTE, and the DCE at the edge of the WAN.

FRAD

• Computing equipment that is not on a LAN may also send data across a Frame Relay network.

• The computing equipment will use a Frame Relay access device (FRAD) as the DTE.

Frame Relay Toll network

• May be privately owned, but it is more commonly provided as a service by a public carrier.

• Typically consists of many geographically scattered Frame Relay switches interconnected by trunk lines.

Frame Relay terminology

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

• VCs established dynamically by sending signaling messages to the network are SVCs.

• PVCs are preconfigured by the carrier

Terminologies: Access Rate

• The clock speed of the connection (local loop) to the Frame Relay cloud.

• It is the rate at which data travels into or out of the network.

Terminologies: DLCI

• Data-link connection identifier.

• A number that identifies the end point in a Frame Relay network.

• Significant only to the local network.

• The Frame Relay switch maps the DLCIs between a pair of routers to create a permanent virtual circuit.

Terminologies: LMI

• Local management interface.

• A signaling standard between the CPE device and the Frame Relay switch.

• Responsible for managing the connection and maintaining status between the devices.

Terminologies: CIR

• Committed information rate (in bps).The average rate at which you want to transmit in periods of noncongestion.

• The CIR is the guaranteed rate, that the service provider commits to providing.

• While a frame is being transmitted, each bit will be sent at the port speed.

Terminologies: Tc

• Committed Rate Measurement Interval. The time interval over which the rates are calculated is called the committed time.

• The time interval shouldn’t exceed 125 ms, almost always 125 ms.

Terminologies: Bc

• The number of committed bits in Tc is the committed burst.

• Bc=CIR x Tc

Terminologies: Excess burst

• The maximum number of uncommitted bits that the switch attempts to transfer beyond the CIR.

• Dependent on the service offerings available by the vendor, but is typically limited to the port speed of the local access loop.

Terminologies

MinCIR

Frame Relay flow control

• The switch maintains a bit counter for each VC.

• An incoming frame is marked DE if it puts the counter over Bc.

• An incoming frame is discarded if it pushes the counter over Bc + Be.

• At the end of each Tc seconds the counter is reduced by Bc.

Terminologies: FECN

• Forward explicit congestion notification.

• When a switch recognizes congestion in the network, it sends a FECN packet to the destination device.

Terminologies: BECN

• Backward explicit congestion notification.

• When a switch recognizes congestion in the network, it sends a BECN packet to the source router, instructing the router to reduce the rate at which it is sending packets.

Frame Relay congestion

Terminologies: DE

• Discard eligibility indicator.

• A set bit that indicates the frame may be discarded in preference to other frames if congestion occurs.

• The DE bit is set on the oversubscribed traffic.

Frame Relay bandwidth

• The serial connection or access link to the Frame Relay network is normally a leased line.

• The speed of the line is the access speed or port speed.

• Port speeds are typically between 64 kbps and 4 Mbps. Some providers offer speeds up to 45 Mbps.

Frame Relay frame format

• DLCI: Indicates the DLCI value. Consists of the first 10 bits of the Address field.

• Congestion Control: The last 3 bits in the address field. These are the FECN, BECN, and discard eligible (DE) bits.

Frame Relay addressing

• DLCI address space is limited to 10 bits. possible 1024 DLCI addresses.

• The usable portion of these addresses are determined by the LMI type: – The Cisco LMI type supports a range of DLCI addresses from DLCI

16-1007. – The ANSI/ITU LMI type supports the range of addresses from DLCI

16-992.

• The remaining DLCI addresses are reserved for vendor implementation.

Frame Relay Topology

Frame Relay LMI functions

• The heartbeat mechanism, which verifies that a VC is operational

• The multicast mechanism

• The flow control

• The ability to give DLCIs global significance

• The VC status mechanism

LMI types

• The LMI type configured on the router must match the type used by the service provider.

• Three types of LMIs are supported by Cisco routers: – Cisco – The original LMI extensions – Ansi – Corresponding to the ANSI standard T1.617

Annex D – q933a – Corresponding to the ITU standard Q933 Annex

A

LMI frame format

• LMI messages are sent in frames distinguished by an LMI-specific DLCI.

• Cisco specification as DLCI 1023.• The LMI frame contains 4 mandatory bytes:

– The 1st bytes has the same format as the LAPB unnumbered information (UI) frame indicator, with the poll/final bit set to zero.

– The 2nd byte is referred to as the protocol discriminator, which is setto a value that indicates LMI.

– The 3rd byte (call reference) is always filled with zeros.– The final byte is the message type field:

• Status messages• Status enquiry messages

Frame Relay mapping

• Network address DLCI• The routing table is then used to supply the next-hop protocol

address or the DLCI for outgoing traffic.• The resolution is done through a data structure called a Frame Relay

map.• This data structure can be statically configured in the router, or the

Inverse ARP feature can be used for automatic setup of the map.

LMI operation

• LMI status messages combined with Inverse ARP messages 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 inquirymessage to the network.

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

• Subsequent responses include only status changes.

Frame Relay mapping

Frame Relay switching table

• The Frame Relay switching table consists of four entries: incoming port and DLCI, and outgoing port and DLCI.

• The DLCI may be remapped as it passes through each switch.

Configuring Frame Relay

Frame Relay encapsulation

• Frame Relay is configured on a serial interface and the default encapsulation type is the Cisco proprietary version of HDLC.

• To change the encapsulation to Frame Relay use the

encapsulation frame-relay [cisco | ietf]

LMI type

• The LMI connection is established and configured by the command:

frame-relay lmi-type [ansi | cisco | q933a]

• IOS Release 11.2 or later, the LMI-type is autosensed and no configuration is needed.

• The default LMI type is cisco. • The LMI type is set on a per-interface basis and is

shown in the output of the show interfacescommand.

Configuring basic Frame Relay

Configuring a static Frame Relay map

• The local DLCI must be statically mapped to the network layer address when:– Remote router does not support Inverse ARP. – Broadcast traffic and multicast traffic over the PVC must be

controlled. – Paritial-mesh Frame Relay topology.

router(conf-if)#frame-relay map protocol protocol-address dlci [broadcast]

• Static frame-relay map disables InverseARP, to turn it back on use the command:

router(conf-if)#frame-relay inverse-arp [protocol] [dlci]

Configuring a static Frame Relay map

Split Horizon

• When a single interface is used to interconnect multiple sites, there may be reachability issues as nonbroadcast multiaccess(NBMA) nature of Frame Relay .

• Split horizon does not allow routing updates to be sent out the same interface that was the source of the route information.

Frame Relay subinterfaces

• Use subinterfaces in Frame Relay:– To enable the forwarding of broadcast in a hub-and-spoke Frame

Relay topology.

– To subject problem regarding split-horizon

– To reduce overall cost of many physical interfaces

• Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode: – Point-to-point

– Multipoint

Frame Relay subinterfaces

• The encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network layer address and DLCIs, are assigned to the subinterface.

Configuring Frame Relay subinterfaces

• Configure encapsulation and no shut the physical interface without ip address.

router(config-if)#interface serial number.subinterface-number{multipoint | point-to-point}

• Using major interface as point-to-point, DLCI configuration is not required as it can be learned via LMI from Frame Relay switch.

• With subinterfaces, use this command configure the local DLCI:

router(config-subif)#frame-relay interface-dlci dlci-number [cisco|ietf]

Subinterface configuration sample

Configure Cisco router as Frame Relay switch

• Cisco router can be configured as Frame Relay switch using command:

Router(config)#frame-relay switching

• Then all connections should be DCE type and be specified with the command:

Router(conf-if)#frame-relay intf-type dce|dte

• Configure LMI type on Frame Relay switch:Router(conf-if)#frame-relay lmi-type {ansi | cisco | q933a}

• Configure Frame Relay routes to create switching table:Router(conf-if)#frame-relay route in_dlci interface out_interface out_dlci

Frame Relay routes

Verifying the Frame Relay configuration

• show interfaces command displays LMI type, LMI DLCI, Frame Relay DTE/DCE type

• show frame-relay lmi command to display LMI traffic statistics.

• show frame-relay pvc [interface interface] [dlci] command to display the status of each configured PVC as well as traffic statistics.

• show frame-relay map command to display the current map entries and information about the connections.

• show frame-relay route command on frame relay switch to display switching table.

Show interface

Show frame-relay lmi

Show frame-relay pvc

Show frame-relay map

Troubleshooting the Frame Relay configuration

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

• debug frame-relay events command to display frame-relay packets.

Debug frame-relay lmi

Summary

• The components of a Frame Relay network

• The technology of Frame Relay

• Point-to-point and point-to-multipoint topologies

• The topology of a Frame Relay network and potential problems

• How to configure a Frame Relay Permanent Virtual Circuit (PVC)

• How to create a Frame Relay Map on a remote network

• Why subinterfaces are needed and how they are configured

• How to verify and troubleshoot a Frame Relay connection

Lab1 Topology

1. PVC(21-42) and PVC(32-41) belong to 1 subnet (partial-mesh)2. All interfaces are multipoint subinterface (full-mesh)3. IP address is 192.168.1.0/24 . Each Loopback interface requires 30

IPs. Rouing protocol is EIGRP with AS 100

Lab2 Topology

Lab2 Requirements

• PVC(111-121), PVC(123-143), PVC(142-112) form 1 subnet

• PVC(122-131) form 1 subnet

• PVC(132-141) form 1 subnet

• Routing protocol is EIGRP, AS 200

• Network address is 172.30.0.0/16, each loopback interface requires 31 IPs.

CCNA4 – Module5