Frame Relay

Cisco Jan 5, 2025

Packet Switched Networks

X.25
Frame Relay (1984)
ATM
MPLS

Definations

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, or Q.933A)
DLCI - Data Link Connection Identifier, equivalent to MAC address (16-1007)

⚠️

DLCIs are locally significant.

PVC Status

INACTIVE - DLCI is valid, but the other side is not working
ACTIVE - DLCI is up
DELETED - service provider has no record of DLCI

Encapsulation

encapsulation frame-relay (Cisco - Default)


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
DE - Discard Eligibility, set when frame is over the CIR
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 is received, reduce sending by 25%

Local Management Interface

	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 is 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

On by default. Inverse ARP - Layer 2 (DLCI/MAC) to IP Address.

no frame-relay inverse-arp ip [DLCI]

Frame Relay PVC Design

Hub and Spoke

Partial Mesh

Full Mesh

Multipoint

All routers are on the same subnet
Multiple DLCI numbers mapped to the same 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

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
show frame-relay map