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Frame Relay

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Frame Relay

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Cisco CCNA ICND Study Guide

By Cbrzana

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Frame Relay

Frame Relay Protocols

Frame Relay networks are Non-Broadcast Multiaccess Networks


Local Management Interface: protocol that defines keepalive messages and other messages.

Router's identify the Virtual Circuit by encapsulating that data in a Data-Link Connection Identifier (DLCI).

Frame Relay Terms

Term Description
Virtual Circuit (VC) Logical path that connects routers
Permanent Virtual Circuit (PVC) A predefined VC (equated to leased line)
Switched Virtual Circuit (SVC) Dial connection in concept
Data Terminal Equipment (DTE) Devices connected to Frame Relay Service
Data Communications Equipment (DCE) Frame Relay switches, typically in service provider's network.
Access Link Leased line between DTE and DCE
Access Rate (AR) Speed at which access link is clocked.
Data-link Connection Identifier (DLCI) Frame Relay address used in headers to identify the VC
Nonbroadcast Multiaccess (NBMA) Broadcasts not supported, but more than 2 devices can be connected
Local Management Interface Used between DCE and DTE, manages the connection (signaling messages, keepalive messages).

Frame Relay Standards

What specification defines ITU Document ANSI Document
Data-link specifications (LAPF header/trailer) Q.922-A T1.618
PVC Management, LMI Q.933-A T1.617-D
SVC Signaling Q.933 T1.617
Multiprotocol encapsulation Q.933-E T1.617-F

LMI and Encapsulation Types

LMI is between Router (DTE) and Frame Relay Switch (DCE).

DLCI is between Router (DTE) and Router (DTE)  encapsulation

LMI status inquiry messages perform two key functions:

  • Perform keepalive function between the DTE and DCE ( implies link is down)
  • Signal when PVC is active or inactive

There are three protocol types for LMI:

  • Cisco (proprietary)
  • ANSI (T1.617-D)
  • ITU (Q.933-A)

DTE needs to know which LMI type to use (must be the same as the DCE type)

 Autosense feature detects it automatically

 To configure it manually, frame-relay lmi-type [type]

Can use either cisco, ansi, or q933a as the type

Encapsulation defined by LAPF (Q.922-A)

LAPF Header
Information LAPF Trailer

 Doesn't provide protocol type field, can't support multiple protocols.

Two solutions developed to overcome lack of Protocol Type field.

1. Additional header created, 2-byte protocol type field

2. RFC 1490 (superseded by RFC 2427)

a. "Multiprotocol Interconnect over Frame Relay"

Protocol type field between LAPF header and L3 packet

LAPF Header Cisco or RFC 1490 Packet LAPF Trailer

If Cisco used, type = cisco If other standard used, type = ietf

DLCI Addressing Details

DLCI's are locally significant, meaning they must have a unique value only on the local access link.


There can only be one 150 ProProfs Avenue, Rochester, NY.

At the same time, every other city in the US can have a 150 ProProfs Avenue address.

Global Addressing makes the DLCI's appear as if they were unique LAN addresses, makes it easier to understand DLCI addressing.

  • Switches change the DLCI before the receiver gets it
  • Sender treats the DLCI as the destination address
  • Receiver treats the DLCI as the source address
Network Layer Concerns with Frame Relay

* Choices for L3 Addresses on Frame Relay interfaces
* Handling of broadcasts

3 Different Options for handling L3 packets:
1. One subnet for all DTEs
* Usually used in a full mesh topology

2. One subnet per VC
* Usually used in a partially meshed topology
* Wastes some IP addresses (unless mask of is used)

3. A hybrid of the 1'st 'two options
* Used if you can create a sub-full mesh network between some router, but only a partial between others
* Uses subinterfaces (logical interfaces on the same physical interface, with different IPs)

Broadcast Handling: Cisco IOS sends copies of the broadcasts across each VC. To reduce lag in network, these are placed in different output queue than the one for user traffic, and you can limit the amount of bandwidth this consumes


Frame Relay Service Interworking

Most use Asynchronous Transfer Mode (ATM) within the core of the Frame Relay Network.
* 53-byte cells
* Better Quality of Service (QoS)
* Service Interworking is the use of ATM between Frame Relay switches
* FRF.5 is the specification that defines how to use ATM in Frame Relay
* FRF.8 is the specification that defines how two routers communicate when one is using ATM, and the other is using Frame Relay

Frame Relay Configuration

Command Description
cisco] Frame relay encapsulation type
q933a | cisco} LMI type configuration
bandwidth num Configure bandwidth in kbps
Statically defines mapping between L3 address and a DLCI
keepalive sec Defines whether and how long LMI keepalives should be sent

Creates a subinterface or references already existing one.
[voice-cir cir] [ppp virtual-template-name]
Links or correlates a DLCI to the subinterface.

Show Commands

show interfaces

show frame-relay pvc [interface interface] [dlci]

show frame-relay lmi [type number]

Fully-Meshed Network with Single Subnet

interface serial0
encapsulation frame-relay
ip address [address][mask]

* Check that all serial interfaces are in the same subnet
* LMI type automatically sensed
* Encapsulation is cisco
* PVC DLCIs are learned via LMIs
* Inverse ARP enabled by default

More criteria given:
* Router 1 requires IETF encapsulation (it is not a Cisco router)
* Router 2's LMI type should be ANSI, no autosense used

Router 1
interface serial0
encapsulation frame-relay
frame-relay lmi-type ansi
frame-relay interface-dlci 53 ietf
ip address [address][subnet]

Router 2
interface serial0
encapsulation frame-relay ietf
ip address [address][subnet]

Frame Relay Address Mapping

Creates a correlation between L3 address and corresponding L2 address
* Statically configure the mapping
* Dynamically learn mapping through Inverse ARP

Inverse ARP: Announces L3 addresses as soon as the LMI signals the PVCs are up.

Static Configuration

no frame-relay inverse-arp
frame-relay map ip [address][DLCI #] broadcast

Partially Meshed Network with 1 IP per VC

Step 1: Configure Encapsulation
interface serial0
encapsulation frame-relay

Step 2: Configure sub-interfaces
interface serial 0.1 point-to-point
frame-relay interface-dlci [dlci #]

Step 3: Configure Individual VCs
interface serial0
encapsulation frame-relay

interface serial 0.1 point-to-point
ip address [address] [subnet]
frame-relay interface-dlci [dlci #]

Step 4: Verify Connectivity
show frame-relay map
show frame-relay pvc
debug frame-relay lmi

Partially Meshed Network with some Fully Meshed Parts

Step 1: Configure Encapsulation
interface serial0
encapsulation frame-relay

Step 2: Configure multipoint sub-interface(s)
interface serial 0.1 multipoint
ip address [address] [subnet mask]
frame-relay interface-dlci [dlci #]  first interface
frame-relay interface-dlci [dlci #]  second interface

Step 3: Configure point-to-point interface(s)
interface serial 0.2 point-to-point
ip address [address] [subnet mask]
frame-relay interface-dlci [dlci #]

Step 4: Verify Connectivity

show frame-relay map
debug frame-relay events -> Inverse ARP IP values given in HEX

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