Frame Relay

Packet Switched Networks

X.25

Frame Relay (1984)

MPLS

A leased line is expensive as dedicated bandwidth and not flexible.

Frame Relay Consortium (1990) - Cisco, Stratacom, Nortel, DEC - added extensions and standardized Frame Relay

DTE - Data Terminal Equipment (End device and interface device)

DCE - Data Communication Equipment (carrier switches)

SVC - Switched virtual circuit, the connection is created on demand when required

PVC - Permanent virtual circuit, connection from one site to another

CIR - committed information rate, minimum bandwidth on link

LAR - local access rate, physical max speed of connection

LMI - Local Management Interface, signaling between router and service provider (Cisco, ANSI, Q.933A)

DLCI - Data Link Connection Identifier, equivalent to MAC address (16-1007)

⚠️

DLCIs are locally significant.

PVC Status

INACTIVE - DLCI is valid, but other side is not working

ACTIVE - DLCI is up

DELETED - service provider has no record of DLCI

Encapsulation

encapsulation frame-relay (default Cisco)

Flag

Address

Packet Type

Data

FCS

Flag

encapsulation frame-relay ietf (Other vendors - RFC 1490 / RFC 2427)

Flag

Address

Ctrl

PAD

NLPID

Data

FCS

Flag

Address (DLCI) 10 Bits

High Bits (6)	C/R	EA
Low Bits (4)	DE	EA

C/R - comand or responce

EA - Extended address bit

FECN - Forward Explicit Congestion Notification

BECN - Backward Explicit Congestion Notification

Frame relay switch sets FECN bit to 1, on next frame to the source it sets BECN bit to 1

When BECN received, reduce sending by 25%

DE - Discard Eligibility, set when frame is over the CIR

Local Management Interface

ㅤ	AKA	Router	DLCI Mgmt
Cisco	Cisco	Cisco	1023
ITU Q.933A	Annex A	Q.933A	0
ANSI T1.617D	Annex D	ANSI	0

frame-relay lmi-type ?

iOS > 11.2 can auto detect lmi type. Default Cisco or auto sense

Extensions

VC Status Message - PVC Status

Optional Extensions

Global Addressing

By default frame relay has no unique layer 2 addressing

Enables ARP to function.

Multicasting

Frame relay is Non broadcast multiple access (NBMA)

m-cast DLCI: 1019-1022

Used for routing protocols. Status messages can also be sent.

Mapping

Manual


frame-relay [IP/IPX] [Address] [Local DLCI] {broadcast} {encapsulation type}

Dynamic

Inverse ARP - Layer 2 (DLCI/MAC) to IP Address

On by default

no frame-relay inverse-arp ip [DLCI]

Frame Relay PVC Design

Full Mesh

Partial Mesh

Hub and Spoke

Multipoint

All routers on same subnet

Multiple DLCI numbers mapped to interface

Causes problems with split horizon

Configuring Frame Relay

Multipoint Configuration


#R1
interface serial 0/0
	ip address 192.168.1.1 255.255.255.0
	encapsulation frame-relay
	no ip split-horizon eigrp 1
	no shut

#R2
interface serial 0/0
	ip address 192.168.1.2 255.255.255.0
	encapsulation frame-relay
	frame-relay map ip 192.168.1.3 201
	no shut
	
#R3
interface serial 0/0
	ip address 192.168.1.3 255.255.255.0
	encapsulation frame-relay
	frame-relay map ip 192.168.1.2 301
	no shut


show frame-relay map

show frame-relay pvc

Point-to-Point Configuration


#R1
interface serial 0/0
	encapsulation frame-relay
	no shut
	
interface serial 0/0.102 point-to-point
	ip address 192.168.1.1 255.255.255.0
	frame-relay interface-dlci 102
	
interface serial 0/0.103 point-to-point
	ip address 192.168.2.1 255.255.255.0
	frame-relay interface-dlci 103
	
#R2
interface serial 0/0
	encapsulation frame-relay
	no shut
	
interface serial 0/0.201 point-to-point
	ip address 192.168.1.2 255.255.255.0
	frame-relay interface-dlci 201

	
#R3
interface serial 0/0
	encapsulation frame-relay
	no shut
	
interface serial 0/0.301 point-to-point
	ip address 192.168.2.2 255.255.255.0
	frame-relay interface-dlci 301

Frame Relay Switch Configuration


frame-relay switching

interface serial 0/0
	no ip address
	encapsulation frame-relay
	clock rate 2000000
	frame-relay intf-type dce
	frame-relay route 102 interface Serial0/1 201
	frame-relay route 103 interface Serial0/2 301

interface serial 0/1
	no ip address
	encapsulation frame-relay
	clock rate 2000000
	frame-relay intf-type dce
	frame-relay route 201 interface Serial0/0 102
	
interface serial 0/2
	no ip address
	encapsulation frame-relay
	clock rate 2000000
	frame-relay intf-type dce
	frame-relay route 301 interface Serial0/0 103

show frame-relay route

Monitoring Frame Relay

show interface serial (Physical)

Up, Down, Admin Down
Control Signals

DCD - Data Carrier Detect
DSR - Data Set Ready
DTR - Data Terminal Ready
RTS - Request to send
CTS - Clear to send

show frame-relay lmi (Protocol)

LMI Statistics
Type
Counters (Invalid, Valid)

show frame-relay pvc (Logical)

DLCI
DLCI Usage
PVC Status (Active, Inactive, Deleted)
Congestion Control

Debug commands

debug frame-relay events

debug frame-relay packet

debug frame-relay lmi