Chapter3 frame relay

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1

Frame Relay

Accessing the WAN – Chapter 3

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 2

Historical Background Frame Relay is a high-performance WAN protocol

that operates at the physical and data link layers of the OSI model.

Eric Scace, an engineer at Sprint International, invented Frame Relay as a simpler version of the X.25 protocol to use across ISDN interfaces.

Network providers commonly implement Frame Relay for voice and data as an encapsulation technique, used between LANs over a WAN. Each end user gets a private line (or leased line) to a Frame Relay node.

Frame Relay has become one of the most extensively used WAN protocols, primarily because it is inexpensive compared to dedicated lines.


Introducing Frame Relay Frame Relay has become the most widely used WAN

technology in the world primarily because of its price and flexibility.

Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation. Moreover, Frame Relay provides greater bandwidth, reliability, and resiliency than private or leased lines.

With increasing globalization and the growth of one-to-many branch office topologies, Frame Relay offers simpler network architecture at a lower cost of ownership.


Describe the Fundamental Concepts of Frame Relay Technology Describe how Frame Relay is used to provide WAN

services to the Enterprise


Describe the Fundamental Concepts of Frame Relay Technology Frame Relay Operation


Describe the Fundamental Concepts of Frame Relay Technology Virtual circuit (VC) - connection through a Frame Relay network

between two DTEs; maybe SVCs or PVCs

VCs are identified by DLCIs. DLCI values are assigned by the service provider; DLCIs have local significance

DLCIs 0 to 15 and 1008 to 1023 are reserved for special purposes.


Describe the Fundamental Concepts of Frame Relay Technology


Describe the Fundamental Concepts of Frame Relay Technology Describe the types of topologies that are used for

implementing Frame Relay in different environments

In mesh networks, total VCs required is [n(n-1)/2]


Describe the Fundamental Concepts of Frame Relay Technology Frame Relay Address Mapping

Dynamic address mapping - relies on Inverse ARP (IARP) to resolve a next hop network protocol address to a local DLCI value.

On Cisco routers, Inverse ARP is enabled by default for all protocols

One can choose to override dynamic Inverse ARP mapping by supplying a manual static mapping for the next hop protocol address to a local DLCI.

You cannot use Inverse ARP and a frame-relay map statement for the same DLCI and protocol.


Describe the Fundamental Concepts of Frame Relay Technology LMI - a keepalive mechanism that provides status

information about FR connections between the router (DTE) and the FR switch (DCE)

Do not confuse LMI with encapsulation. LMI defines messages used between the DTE and the DCE. Encapsulation defines the headers used by a DTE to communicate information to the DTE at the other end of a VC.

Three types of LMIs are supported by Cisco routers: Cisco, Ansi and q933a.

Starting with Cisco IOS 11.2, the default LMI autosense feature detects the LMI type supported by the directly connected Frame Relay switch.


Describe the Fundamental Concepts of Frame Relay Technology LMI status messages combined with Inverse ARP

messages allow a router to associate network layer and data link layer addresses.


Configure a Basic Frame Relay PVC Configure a basic Frame Relay PVC on a router serial

interface

Verify encapsulation through sh int serial 0/0/0


Configure a Basic Frame Relay PVC Configure a static Frame Relay map

Frame Relay is an NBMA network which does not support broadcast traffic, using the broadcast keyword is a simplified way to forward routing updates.


Describe Advanced Concepts of Frame Relay Technology

Using a hub and spoke topology, the split horizon rule reduces the chance of a routing loop with distance vector routing protocols.

It prevents a routing update received on an interface from being forwarded through the same interface.

If the Central router learns about Network X from Branch A, that update is learned via S0/0.

According to the split horizon rule, Central could not update Branch B or Branch C about Network X.

This is because that update would be sent out the S0/0 interface, which is the same interface that received the update.

Split Horizon prohibits routing updates received on an interface from exiting that same interface.


Describe Advanced Concepts of Frame Relay Technology Frame Relay can allow customers to dynamically

access this extra bandwidth and "burst" over their CIR for free.

•The duration of a burst transmission should be short.

•BE is the term used to describe the bandwidth available above the CBIR.

•Unlike the CBIR, it is not negotiated.


Describe Advanced Concepts of Frame Relay Technology• Forward Explicit Congestion Notification (FECN) – When a

Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device.

– This indicates that congestion has occurred. • Backward Explicit Congestion Notification (BECN) – When a

Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router.

– This instructs the router to reduce the rate at which it is sending packets.


Configure an Advanced Frame Relay PVC Explain the steps to configure point-to-point

subinterfaces on a physical interface


Configure an Advanced Frame Relay PVC Describe the commands used for verifying Frame Relay

operation


Configure an Advanced Frame Relay PVC The show frame-relay lmi command displays LMI

traffic statistics showing the number of status messages exchanged between the local router and the Frame Relay switch.


Configure an Advanced Frame Relay PVC • The show frame-relay pvc command displays the

status of each configured connection, as well as traffic statistics.

• This command is also useful for viewing the number of BECN and FECN packets received by the router.

• The command show frame-relay pvc shows the status of all PVCs configured on the router.


Configure an Advanced Frame Relay PVC

• To clear dynamically created Frame Relay maps, which are created using Inverse ARP, use the clear frame-relay-inarp command.


Configure an Advanced Frame Relay PVC Use the debug frame-relay lmi command to

determine whether the router and the Frame Relay switch are sending and receiving LMI packets properly.


Configure an Advanced Frame Relay PVC

The possible values of the status field are as follows:

0x0 – Added/inactive means that the switch has this DLCI programmed but for some reason it is not usable. The reason could possibly be the other end of the PVC is down.

0x2 – Added/active means the Frame Relay switch has the DLCI and everything is operational.

0x4 – Deleted means that the Frame Relay switch does not have this DLCI programmed for the router, but that it was programmed at some point in the past. This could also be caused by the DLCIs being reversed on the router, or by the PVC being deleted by the service provider in the Frame Relay cloud.


Summary Frame relay is the most widely used WAN technology

because it:–Provides greater bandwidth than leased line

–Reduces cost because it uses less equipment

–Easy to implement

Frame relay is associated with layer 2 of the OSI model and encapsulates data packets in a frame relay frame

Frame relay is configured on virtual circuits–These virtual circuits may be identified by a DLCI

Frame relay uses inverse ARP to map DLCI to IP addresses


Summary

Configuring frame relay requires–Enable frame relay encapsulation

–Configuring either static or dynamic mapping

–Considering split horizon problems that develop when multiple VCs are placed on a single physical interface

Factor affecting frame relay configuration–How service provider has their charging scheme set up

Frame relay flow control–DE

–FECN

–BECN


Summary

The following commands can be used to help verify frame relay configuration

–Show interfaces

–Show frame-relay lmi

–Show frame-relay pvc ###

–Show frame-relay map

Use the following command to help troubleshoot a frame relay configuration

–Debug frame-relay lmi


Chapter3 frame relay

Documents

frame relay frame relay

frame relay network

invented frame relay

frame relay node

cisco systems

cisco public

basic frame relay pvc

framerelay map statement