ccna4 module 5

8/14/2019 Ccna4 Module 5

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Frame Relay

Module Overview

Frame Relay was originally developed as an extension of ISDN. It was designed toenable the circuit-switched technology to be transported on a packet-switchednetwork. The technology has become a stand-alone and cost-effective means ofcreating a WAN.

Frame Relay switches create virtual circuits to connect remote LANs to a WAN.The Frame Relay network exists between a LAN border device, usually a router,and the carrier switch. The technology used by the carrier to transport the databetween the switches is not important to Frame Relay.

The sophistication of the technology requires a thorough understanding of theterms used to describe how Frame Relay works. Without a firm understanding ofFrame Relay, it is difficult to troubleshoot its performance.

Frame Relay has become one of the most extensively used WAN protocols. Onereason for its popularity is that it is inexpensive compared to leased lines.Another reason Frame Relay is popular is that configuration of user equipment ina Frame Relay network is very simple.

This module explains how to configure Frame Relay on a Cisco router. FrameRelay connections are created by configuring routers or other devices tocommunicate with a Frame Relay switch. The service provider usually configuresthe Frame Relay switch. This helps keep end-user configuration tasks to aminimum.

Students completing this module should be able to:

* Explain the scope and purpose of Frame Relay

* Discuss the technology of Frame Relay

* Compare point-to-point and point-to-multipoint topologies

* Examine the topology of a Frame Relay network

* Configure a Frame Relay Permanent Virtual Circuit (PVC)

* Create a Frame Relay Map on a remote network

* Explain the issues of a non-broadcast multi-access network


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* Describe the need for subinterfaces and how to configure them

* Verify and troubleshoot a Frame Relay connection

5.1 Frame Relay Concepts

5 . 1 . 1 Introducing Frame Relay

Frame Relay is an International Telecommunication Union TelecommunicationsStandardization Sector (ITU-T) and American National Standards Institute (ANSI)standard. Frame Relay is a packet-switched, connection-oriented, WANservice.It operates at the data link layer of the OSI reference model.Frame Relay uses a subset of the high-level data-link control (HDLC)protocol called Link Access Procedure for Frame Relay (LAPF). Frames carrydata between user devices called data terminal equipment (DTE), and the data

communications equipment (DCE) at the edge of the WAN.

Originally Frame Relay was designed to allow ISDN equipment to have access to apacket-switched service on a B channel. However, Frame Relay is now a stand-alone technology.

A Frame Relay network may be privately owned, but it is more commonlyprovided as a service by a public carrier. It typically consists of many

geographically scattered Frame Relay switches interconnected by trunk lines.

Frame Relay is often used to interconnect LANs. When this is the case, arouter on each LAN will be the DTE. A serial connection, such as a T1/E1leased line, will connect the router to a Frame Relay switch of the carrier at thenearest point-of-presence for the carrier. The Frame Relay switch is a DCEdevice. Frames from one DTE will be moved across the network and delivered toother DTEs by way of DCEs.

Computing equipment that is not on a LAN may also send data across a FrameRelay network. The computing equipment will use a Frame Relay access device(FRAD) as the DTE.

5 . 1 . 2 Frame Relay terminology

The connection through the Frame Relay network between two DTEs iscalled a virtual circuit (VC). Virtual circuits may be established dynamically bysending signaling messages to the network. In this case they are called switchedvirtual circuits (SVCs). However, SVCs are not very common. Generallypermanent virtual circuits (PVCs) that have been preconfigured by the carrier areused. A VC is created by storing input-port to output-port mapping in the memoryof each switch and thus linking one switch to another until a continuous pathfrom one end of the circuit to the other is identified.


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Because it was designed to operate on high-quality digital lines, Frame Relayprovides no error recovery mechanism. If there is an error in a frame, as detectedby any node, it is discarded without notification.

The FRAD or router connected to the Frame Relay network may have multiplevirtual circuits connecting it to various end points. This makes it a very cost-effective replacement for a mesh of access lines. With this configuration, eachend point needs only a single access line and interface. More savings arise as thecapacity of the access line is based on the average bandwidth requirement of thevirtual circuits, rather than on the maximum bandwidth requirement.

The various virtual circuits on a single access line can be distinguished because

each VC has its own Data Link Connection Identifier (DLCI).

The DLCI is stored in the address field of every frame transmitted. The DLCIusually has only local significance and may be different at each end of a VC.

5 . 1 . 3 Frame Relay stack layered support

Frame Relay functions by doing the following:

Takes data packets from a network layer protocol, such as IP or IPX

Encapsulates them as the data portion of a Frame Relay frame

Passes them to the physical layer for delivery on the wire

The physical layer is typically EIA/TIA-232, 449 or 530, V.35, or X.21. The FrameRelay frame is a subset of the HDLC frame type. Therefore it is delimited with flag

fields. The 1-byte flag uses the bit pattern 01111110. The Frame Check Sequence(FCS) is used to determine if any errors in the layer 2 address field occurred


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during transmission. The FCS is calculated prior to transmission and the result isinserted in the FCS field. At the distance end, a second FCS value is calculatedand compared to the FCS in the frame. If the results are the same, the frame isprocessed. If there is a difference, the frame is discarded. No notification is sentto the source when a frame is discarded. Error control left to the upper layers ofthe OSI model.

5 . 1 . 4 Frame Relay bandwidth and flow control

The serial connection or access link to the Frame Relay network is normally aleased line. The speed of the line is the access speed or port speed. Port speedsare typically between 64 kbps and 4 Mbps. Some providers offer speeds up to45 Mbps.

Usually there are several PVCs operating on the access link with each VC havingdedicated bandwidth availability. This is called the committed information rate

(CIR). The CIR is the rate at which the service provider agrees to acceptbits on the VC.

Individual CIRs are normally less than the port speed. However, the sum of theCIRs will normally be greater than the port speed. Sometimes this is a factor of 2or 3. Statistical multiplexing accommodates the bursty nature of computercommunications since channels are unlikely to be at their maximum data ratesimultaneously.

While a frame is being transmitted, each bit will be sent at the port speed. Forthis reason, there must be a gap between frames on a VC if the average bit rateis to be the CIR.

The switch will accept frames from the DTE at rates in excess of the CIR. Thiseffectively provides each channel with bandwidth on demand up to a maximumof the port speed. Some service providers impose a VC maximum that is lessthan the port speed. The difference between the CIR and the maximum, whetherthe maximum is port speed or lower, is called the Excess Information Rate (EIR).

The time interval over which the rates are calculated is called the committedtime (Tc). The number of committed bits in Tc is the committed burst (Bc).

The extra number of bits above the committed burst, up to the maximum speedof the access link, is the excess burst (Be).

Although the switch accepts frames in excess of the CIR, each excess frame ismarked at the switch by setting the Discard Eligible (DE) bit to "1" in the addressfield.


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The switch maintains a bit counter for each VC. An incoming frame is markedDE if it puts the counter over Bc. An incoming frame is discarded if itpushes the counter over Bc + Be. At the end of each Tc seconds the counteris reset. The counter may not be negative, so idle time cannot be saved up.

Frames arriving at a switch are queued or buffered prior to forwarding. As in anyqueuing system, it is possible that there will be an excessive buildup of frames ata switch. This causes delays. Delays lead to unnecessary retransmissions thatoccur when higher-level protocols receive no acknowledgment within a set time.In severe cases this can cause a serious drop in network throughput.

To avoid this problem, Frame Relay switches incorporate a policy of dropping

frames from a queue to keep the queues short. Frames with their DE bit set willbe dropped first.

When a switch sees its queue increasing, it tries to reduce the flow of frames toit. It does this by notifying DTEs of the problem by setting the ExplicitCongestion Notification (ECN) bits in the frame address field.

The Forward ECN (FECN obvestilo cilju) bit is set on every frame that theswitch receives on the congested link. The Backward ECN (BECN opozorilo

izvoru) bit is set on every frame that the switch places onto the congested link.DTEs receiving frames with the ECN bits set are expected to try to reduce theflow of frames until the congestion clears.

If the congestion occurs on an internal trunk, DTEs may receive notification eventhough they are not the cause of the congestion.

The DE, FECN and BECN bits are part of the address field in the LAPF frame.

5 . 1 . 5 Frame Relay address mapping and topology

When more than two sites are to be connected, consideration must be given tothe topology of the connections between them.

Frame Relay is unlikely to be cost-effective when only two sites areinterconnected with a point-to-point connection. Frame Relay is more cost-effective where multiple sites must be interconnected.


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WANs are often interconnected as a star topology. A central site hosts the primaryservices and is connected to each of the remote sites needing access to theservices.

In a hub and spoke topology the location of the hub is chosen to give the lowestleased line cost. When implementing a star topology with Frame Relay, each

remote site has an access link to the frame relay cloud with a single VC. The hubhas an access link with multiple VCs, one for each remote site.

Because Frame Relay tariffs are not distance related, the hub does not need to bein the geographical center of the network.

A full mesh topology is chosen when services to be accessed aregeographically dispersed and highly reliable access to them is required. With fullmesh, every site is connected to every other site. Unlike with leased lineinterconnections, this can be achieved in Frame Relay without additional

hardware.

It is necessary to configure additional VCs on the existing links to upgrade fromstar to full mesh topology. Multiple VCs on an access link will generally makebetter use of Frame Relay than single VCs. This is because they take advantageof the built-in statistical multiplexing.

For large networks, full mesh topology is seldom affordable. This is because thenumber of links required for a full mesh topology grows at almost the square ofthe number of sites. While there is no equipment issue for Frame Relay, there is a

limit of less than 1000 VCs per link. In practice, the limit will be less than that,and larger networks will generally be partial mesh topology. With partial mesh,there are more interconnections than required for a star arrangement, but not asmany as for a full mesh. The actual pattern is very dependant on the data flowrequirements.

In any Frame Relay topology, when a single interface is used to interconnectmultiple sites, there may be reachability issues. This is due to the nonbroadcastmultiaccess (NBMA) nature of Frame Relay. Split horizon is a technique used byrouting protocols to prevent routing loops. Split horizon does not allow routing

updates to be sent out the same interface that was the source of the routeinformation. This can cause problems with routing updates in a Frame Relayenvironment where multiple PVCs are on a single physical interface.

Whatever the underlying topology of the physical network, a mapping is neededin each FRAD or router between a data link layer Frame Relay address and anetwork layer address, such as an IP address. Essentially, the router needs toknow what networks are reachable beyond a particular interface. The sameproblem exists if an ordinary leased line is connected to an interface. Thedifference is that the remote end of a leased line is connected directly to a single

router. Frames from the DTE travel down a leased line as far as a network switch,where they may fan out to as many as 1000 routers. The DLCI for each VCmust be associated with the network address of its remote router . This


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information can be configured manually by using map commands. The DLCI canalso be configured automatically using Inverse ARP.

5 . 1 . 6 Frame Relay LMI

Frame Relay was designed to provide packet-switched data transfer with minimal

end-to-end delays. Anything that might contribute to delays was omitted. Whenvendors implemented Frame Relay as a separate technology rather than as onecomponent of ISDN, they decided that there was a need for DTEs todynamically acquire information about the status of the network. This featurewas omitted in the original design. The extensions for this status transfer arecalled the Local Management Interface (LMI).

The 10-bit DLCI field allows VC identifiers 0 through 1023. The LMI extensionsreserve some of these identifiers. This reduces the number of permitted VCs. LMImessages are exchanged between the DTE and DCE using these reserved DLCIs.

The LMI extensions include the following:

The keepalive 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

There are several LMI types, each of which is incompatible with the others.The LMI type configured on the router must match the type used by the serviceprovider. 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 messages are carried in a variant of LAPF frames. This variant includes fourextra fields in the header so that they will be compatible with the LAPD framesused in ISDN. The address field carries one of the reserved DLCIs. Following thisare the control, protocol discriminator, and call reference fields that do notchange. The fourth field indicates the LMI message type.

There are one or more information elements (IE) that follow the header. EachIE consists of the following:

A one byte IE identifier

An IE length field


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One or more bytes containing actual data that typically includes thestatus of a DLCI

Status messages help verify the integrity of logical and physical links. Thisinformation is critical in a routing environment because routing protocols makedecisions based on link integrity.

5 . 1 . 7 Stages of Inverse ARP and LMI operation

LMI status messages combined with Inverse ARP messages allow a router toassociate network layer and data link layer addresses.

When a router that is connected to a Frame Relay network is started, it sends anLMI status inquiry message to the network. The network replies with an LMIstatus message containing details of every VC configured on the access link.

Periodically the router repeats the status inquiry, but subsequent responsesinclude only status changes. After a set number of these abbreviatedresponses, the network will send a full status message.

If the router needs to map the VCs to network layer addresses, it will send anInverse ARP message on each VC. The Inverse ARP message includes thenetwork layer address of the router, so the remote DTE, or router, can alsoperform the mapping. The Inverse ARP reply allows the router to make thenecessary mapping entries in its address to DLCI map table. If several networklayer protocols are supported on the link, Inverse ARP messages will be sent foreach.

5.2 Configuring Frame Relay

5 . 2 . 1 Configuring basic Frame Relay

his section explains how to configure a basic Frame Relay PVC.

Frame Relay is configured on a serial interface. The default encapsulation type isthe Cisco proprietary version of HDLC. To change the encapsulation to FrameRelay use the encapsulation frame-relay [cisco | ietf] command.

cisco

Uses the Cisco proprietary Frame Relay encapsulation. Use this option ifconnecting to another Cisco router. Many non-Cisco devices also support thisencapsulation type. This is the default.

ietf


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Sets the encapsulation method to comply with the Internet Engineering TaskForce (IETF) standard RFC 1490. Select this if connecting to a non-Cisco router.

Cisco's proprietary Frame Relay encapsulation uses a 4-byte header, with 2 bytes

to identify the data-link connection identifier (DLCI) and 2 bytes to identify thepacket type.

Set an IP address on the interface using the ip address command. Set thebandwidth of the serial interface using the bandwidth command. Bandwidth isspecified in kilobits per second (kbps). This command is used to notify the routingprotocol that bandwidth is statically configured on the link. The bandwidth valueis used by Interior Gateway Routing Protocol (IGRP), Enhanced Interior GatewayRouting Protocol (EIGRP), and Open Shortest Path First (OSPF) to determine themetric of the link.

The LMI connection is established and configured by the frame-relay lmi-type[ansi | cisco | q933a] command. This command is only needed if using CiscoIOS Release 11.1 or earlier. With IOS Release 11.2 or later, the LMI-type isautosensed and no configuration is needed. The default LMI type is cisco. The LMItype is set on a per-interface basis and is shown in the output of the showinterfaces command.

These configuration steps are the same, regardless of the network layer protocolsoperating across the network.

5 . 2 . 2 Configuring a static Frame Relay map

The local DLCI must be statically mapped to the network layer address of theremote router when the remote router does not support Inverse ARP. This is alsotrue when broadcast traffic and multicast traffic over the PVC must be controlled.

These static Frame Relay map entries are referred to as static maps.

Use the frame-relay map protocol protocol-address dlci [broadcast]

command to statically map the remote network layer address to the local DLCI.

5 . 2 . 3 Reachability issues with routing updates in NBMA

By default, a Frame Relay network provides non-broadcast multi-access (NBMA)connectivity between remote sites. An NBMA environment is viewed like othermultiaccess media environments, such as Ethernet, where all the routers are onthe same subnet. However, to reduce cost, NBMA clouds are usually built in ahub-and-spoke topology. With a hub-and-spoke topology, the physical topologydoes not provide the multi-access capabilities that Ethernet does.

The physical topology consists of multiple PVCs.


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A Frame Relay NBMA topology may cause two problems:

Reachability issues regarding routing updates

The need to replicate broadcasts on each PVC when a physical interface

contains more than one PVC

Split-horizon updates reduce routing loops by not allowing a routing updatereceived on one interface to be forwarded out the same interface. If Router B, aspoke router, sends a broadcast routing update to Router A, the hub router, andRouter A has multiple PVCs over a single physical interface, then Router A cannotforward that routing update through the same physical interface to other remotespoke routers. If split-horizon is disabled, then the routing update can beforwarded out the same physical interface from which it came. Split-horizon is nota problem when there is a single PVC on a physical interface. This would be apoint-to-point Frame Relay connection.

Routers that support multiple connections over a single physical interface havemany PVCs that terminate in a single router. This router must replicate broadcastpackets such as routing update broadcasts, on each PVC, to the remote routers.

The replicated broadcast packets can consume bandwidth and cause significantlatency to user traffic. It might seem logical to turn off split-horizon to resolve thereachability issues caused by split-horizon. However, not all network layerprotocols allow split-horizon to be disabled and disabling split-horizon increasesthe chances of routing loops in any network.

One way to solve the split-horizon problem is to use a fully meshed topology.However, this will increase the cost because more PVCs are required. Thepreferred solution is to use subinterfaces.

5 . 2 . 4 Frame Relay subinterfaces

To enable the forwarding of broadcast routing updates in a hub-and-spoke FrameRelay topology, configure the hub router with logically assigned interfaces. Theseinterfaces are called subinterfaces. Subinterfaces are logical subdivisions of aphysical interface.

In split-horizon routing environments, routing updates received on onesubinterface can be sent out another subinterface. In a subinterfaceconfiguration, each virtual circuit can be configured as a point-to-pointconnection. This allows each subinterface to act similarly to a leased line. Using aFrame Relay point-to-point subinterface, each pair of the point-to-point routers ison its own subnet.

Frame Relay subinterfaces can be configured in either point-to-point or multipointmode:


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Point-to-point - A single point-to-point subinterface is used to establish onePVC connection to another physical interface or subinterface on a remoterouter. In this case, each pair of the point-to-point routers is on its ownsubnet and each point-to-point subinterface would have a single DLCI. In apoint-to-point environment, each subinterface is acting like a point-to-pointinterface. Therefore, routing update traffic is not subject to the split-horizonrule.

Multipoint - A single multipoint subinterface is used to establish multiplePVC connections to multiple physical interfaces or subinterfaces on remoterouters. All the participating interfaces would be in the same subnet. Thesubinterface acts like an NBMA Frame Relay interface so routing updatetraffic is subject to the split-horizon rule.

The encapsulation frame-relay command is assigned to the physicalinterface. All other configuration items, such as the network layer address andDLCIs, are assigned to the subinterface.

Multipoint configurations can be used to conserve addresses that can beespecially helpful if Variable Length Subnet Masking (VLSM) is not being used.However, multipoint configurations may not work properly given the broadcasttraffic and split-horizon considerations. The point-to-point subinterface optionwas created to avoid these issues.

5 . 2 . 5 Configuring Frame Relay subinterfaces

The Frame Relay service provider will assign the DLCI numbers. These numbersrange from 16 to 992, and usually have only local significance. This number

range will vary depending on the LMI used. DLCIs can have global significance incertain circumstances.

In the figure, Router A has two point-to-point subinterfaces. The s0/0.110subinterface connects to router B and the s0/0.120 subinterface connects torouter C. Each subinterface is on a different subnet. To configure subinterfaces ona physical interface, the following steps are required:

Configure Frame Relay encapsulation on the physical interface using theencapsulation frame-relay command


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For each of the defined PVCs, create a logical subinterface

router(config-if)#interface serial number.subinterface-number [multipoint |

point-to-point]

To create a subinterface, use the interface serial command. Specify the portnumber, followed by a period (.), and then by the subinterface number. Usually,the subinterface number is chosen to be that of the DLCI. This makestroubleshooting easier. The final required parameter is stating whether thesubinterface is a point-to-point or point-to-multipoint interface. Either themultipoint or point-to-point keyword is required. There is no default. The followingcommands create the subinterface for the PVC to router B:

routerA(config-if)# interface serial 0/0.110 point-to-point

If the subinterface is configured as point-to-point , then the local DLCI for thesubinterface must also be configured in order to distinguish it from the physicalinterface. The DLCI is also required for multipoint subinterfaces for which InverseARP is enabled. It is not required for multipoint subinterfaces configured withstatic route maps. The frame-relay interface-dlci command is used to configurethe local DLCI on the subinterface

router(config-subif)#frame-relay interface-dlci dlci-number

5 . 2 . 6 Verifying the Frame Relay configuration

The show interfaces command displays information regarding theencapsulation and Layer 1 and Layer 2 status. It also displays information aboutthe following:


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The LMI type

The LMI DLCI

The Frame Relay data terminal equipment/data circuit-terminating

equipment (DTE/DCE) type

Normally, the router is considered a data terminal equipment (DTE) device.However, a Cisco router can be configured as a Frame Relay switch. The routerbecomes a data circuit-terminating equipment (DCE) device when it is configuredas a Frame Relay switch.

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

For example, this command demonstrates the number of status messagesexchanged between the local router and the local Frame Relay switch.


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, the show frame-relay pvc 100 command displays the status of only PVC 100.

Use the show frame-relay map command to display the current map entriesand information about the connections. The following information interprets the

show frame-relay map output that appears in Figure

:

10.140.1.1 is the IP address of the remote router, dynamically learned viathe Inverse ARP process

100 is the decimal value of the local DLCI number

0x64 is the hex conversion of the DLCI number, 0x64 = 100 decimal

0x1840 is the value as it would appear on the wire because of the way the

DLCI bits are spread out in the address field of the Frame Relay frame

Broadcast/multicast is enabled on the PVC

PVC status is active

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

5 . 2 . 7 Troubleshooting the Frame Relay configuration

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

The "out" is an LMI status message sent by the router. The "in" is a messagereceived from the Frame Relay switch. A full LMI status message is a "type 0". AnLMI exchange is a "type 1". The "dlci 100, status 0x2" means that the status ofDLCI 100 is active. 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 endof 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 DLCIprogrammed for the router, but that it was programmed at some point in thepast. This could also be caused by the DLCIs being reversed on the router, orby the PVC being deleted by the service provider in the Frame Relay cloud.


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Module Summary

An understanding of the following key points should have been achieved:

* The scope and purpose of Frame Relay

* The technology of Frame Relay

* Point-to-point and point-to-multipoint topologies

* The topology of a Frame Relay network

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

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

* Potential problems with routing in a non-broadcast multi-access network

* Why subinterfaces are needed and how they are configured

* How to verify and troubleshoot a Frame Relay connection

ccna4 module 5

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