chap 3 – frame relay learning objectives

Chapter 31

Chap 3 – Frame Relay Learning Objectives

• Describe the fundamental concepts of Frame Relay technology in terms of Enterprise WAN services including Frame Relay operation, Frame Relay implementation requirements, Frame Relay maps, and LMI operation.

• Configure a basic Frame Relay PVC including configuring and troubleshooting Frame Relay on a router serial interface and configuring a static Frame Relay map.

• Describe advanced concepts of Frame Relay technology in terms of Enterprise WAN services including Frame Relay sub-interfaces, Frame Relay bandwidth and flow control.

• Configure an advanced Frame Relay PVC including solving reachability issues, configuring Frame Relay sub-interfaces, verifying and troubleshooting Frame Relay configuration.

Chapter 32

Requirement for Frame Relay

•A dedicated line provides little practical opportunity for a one-to-many connection without getting more lines from the network provider.

•Inefficient use of bandwidth.

•Cost increases are proportional to the distance between sites.

Chapter 33

Requirement for Frame Relay

•Frame Relay customers only pay for the local loop, and for the bandwidth they purchase from the network provider.

•Distance between nodes is not important.

• Efficient use of bandwidth as Frame Relay shares bandwidth across a larger base of customers.

Chapter 34

Frame Relay• Frame Relay is an ITU-T and ANSI standard.

• Frame Relay is a packet-switched, connection-oriented, WAN service, operating 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 carry data between user devices called data terminal equipment (DTE), and the data communications equipment (DCE) at the edge of the WAN

Chapter 35

Frame Relay Overview

Frame Relay is used between the customer premises equipment (CPE) device and the Frame Relay switch. It does not affect how packets get routed within the Frame Relay ‘cloud’.

Chapter 36

Frame Relay Overview

•Packet Switched•STDM•Uses Virtual Circuits through the network•Bandwidth is not allocated until required•Buffering and congestion control mechanisms•Relies on upper layer protocols (e.g. TCP) for error recovery•Supports data rates up to 45Mbps

Chapter 37

Terminology• The connection through the Frame Relay network

between two DTEs is called a VC (Virtual Circuit).

• Virtual circuits may be established dynamically by sending signaling messages to the network. In this case they are called SVC (Switched Virtual Circuits)

• Generally PVC (Permanent Virtual Circuits) that have been pre-configured by the carrier are used. The switching information for a VC is stored in the memory of the Frame relay switches and/or infrastructure switches.

Chapter 38

Data Link Connection Identifier (DLCI)

319 432

119

579

121A

B

C

D

•Virtual circuits sharing a single access line can be distinguished because each VC has a separate DLCI. •The DLCI is stored in the address field of every frame transmitted, and usually has only local significance.

0

Chapter 39

DLCI 100DLCI 300DLCI 200

Frame Relay Network

DLCI 301

DLCI 201

DLCI 101

•The Frame Relay Access Device (FRAD) or router connected to the Frame Relay network may have multiple VCs connecting it to various endpoints.

•Multiple VCs on a single physical line are distinguished because each VC has its own DLCI.

Multiple Virtual Circuits

Chapter 310

Frame Relay Stack Layered Support

FlagFCSDataAddressFlag

Layer 3

Layer 2

Layer 1

IP Packet

Line Coding = 10101010100100101011111100101

Chapter 311

Frame Relay Header


•DLCI - Data Link Connection Identifier•DE – Discard Eligible•FECN – Forward Explicit Congestion Identifier•BECN- Backwards Explicit Congestion Identifier

Chapter 312

Star Topology

In a hub and spoke topology the location of the hub is chosen to give the lowest leased line cost

Chapter 313

Frame Relay Star Topology

Because Frame Relay tariffs are not distance related, the hub does not need to be in the geographical centre of the

network.

HUB

Chapter 314

Full-Mesh Topology

A full mesh topology is chosen when services to be accessed are geographically

dispersed and highly reliable access to them is required

Chapter 315

Frame Relay Partial Mesh

With partial mesh, there are more interconnections than required for a star arrangement, but not as many as for a full mesh

Chapter 316

Frame Relay bandwidth and

flow control

• Local access rate or port speed – This is the clock speed or port speed of the connection or local loop to the Frame Relay cloud. It is the rate at which data travels into or out of the network, regardless of other settings.

• Committed Information Rate (CIR) – This is the rate, in bits per second, at which the Frame Relay switch agrees to transfer data. The rate is usually averaged over a period of time, referred to as the Committed Time Interval (Tc).

= E1

E1

Chapter 317

Frame Relay bandwidth and flow control

• Committed Burst Information Rate (CBIR) is a negotiated rate above the CIR which the customer can use to transmit for short burst.

• It allows traffic to burst to higher speeds, as available network bandwidth permits.

• However, it cannot exceed the port speed of the link. A device can burst up to the CBIR and still expect the data to get through.

• The duration of a burst transmission should be short, less than three or four seconds. If long bursts persist, then a higher CIR should be purchased.

Chapter 318

Frame Relay Bursting

•Frames submitted above the CIR marked as Discard Eligible (DE) in the frame header, indicating that they may be dropped if there is congestion or there is not enough capacity in the network

Chapter 319

Frame Relay Header


•DE – Discard Eligible•FECN – Forward Explicit Congestion Identifier•BECN- Backwards Explicit Congestion Identifier

Chapter 320

• Forward Explicit Congestion Notification (FECN) – When a Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device, indicating 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, instructing the router to reduce the rate at which it is sending packets.


flow control = E1

E1

Chapter 321

• Discard eligibility (DE) bit is set on the traffic that was receivedafter the CIR was met – i.e. Burst Excess (BE).

• Data frames with the DE bit set are normally delivered. However, in periods of congestion, the Frame Relay switch will drop packets with the DE bit set first.

= E1

E1


flow control

Chapter 322

Frame Relay bandwidth• Several factors determine the rate at which a customer can send data on a Frame

Relay network. foremost in limiting the maximum transmission rate is the capacity of the local loop to the provider.

• If the local loop is an E1, no more than 2.048 Mbps can be sent. In Frame Relay terminology, the speed of the local loop is called the local access rate.

• Providers use the CIR parameter to provision network resources and regulate usage.

• For example, a company with an E1 connection to a packet-switched network (PSN) may agree to a CIR of 1024 Kbps. This means that the provider guarantees 1024 Kbps of bandwidth to the customer’s link at all times.

Chapter 323

• Typically, the higher the CIR, the higher the cost of service. • Customers can choose the CIR that is most appropriate to their

bandwidth needs, as long as the CIR is less than or equal to the local access rate.

• If the CIR of the customer is less than the local access rate, the customer and provider agree on whether bursting above the CIR is allowed.

• If the local access rate is E1 or 2.048 Mbps, and the CIR is 1024 Kbps, half of the potential bandwidth (as determined by the local access rate) remains available.

Frame Relay bandwidth

Chapter 324

• Many providers allow their customers to purchase a CIR of 0 (zero) - This means that the provider does not guarantee any throughput.

• In practice, customers usually find that their provider allows them to burst over the 0 (zero) CIR virtually all of the time.

• If a CIR of 0 (zero) is purchased, carefully monitor performance in order to determine whether or not it is acceptable.

• Frame Relay allows a customer and provider to agree that under certain circumstances, the customer can “burst” over the CIR.

• Since burst traffic is in excess of the CIR, the provider does not guarantee that it will deliver the frames.

Frame Relay bandwidth

Chapter 325

Network Address Mapping

• A mapping is needed in each FRAD or router between a data link layer Frame Relay address (DLCI) and a network layer address, such as an IP address.

• The DLCI for each VC must be associated with the network address of its remote router. This information can be configured statically by using map commands.

• The DLCI can also be configured automatically using Inverse ARP.

Chapter 326

LMI – Local Management Interface

• LMI is a signaling standard between the DTE and the Frame Relay switch. • LMI is responsible for managing the connection and maintaining the status between devices.• Cisco supports 3 LMI standards – ANSI, Q933a, Cisco

LMI includes:

• A keepalive mechanism, which verifies that data is flowing. • A multicast mechanism, which provides the network server (router) with its local DLCI.• Multicast addressing, which can give DLCIs global rather than local significance in Frame Relay

networks (not common).• A status mechanism, which provides an ongoing status on the DLCIs known to the switch.

Chapter 327

• There are three types of LMI, none of which is compatible with the others.

• Cisco, StrataCom, Northern Telecom, and Digital Equipment Corporation (Gang of Four) released one type of LMI, while the ANSI and the ITU-T each released their own versions.

• The LMI type must match between the provider Frame Relay switch and the customer DTE device.

LMI

Frame RelayNetwork


Chapter 328

• In Cisco IOS releases prior to 11.2, the Frame Relay interface must be manually configured to use the correct LMI type.

• If using Cisco IOS Release 11.2 or later, the router attempts to automatically detect the type of LMI used by the provider switch.

• This automatic detection process is called LMI autosensing.


LMI

Frame RelayNetwork

Chapter 329

LMI – Local Management Interface• The 10-bit DLCI field allows VC identifiers 0 through 1023. The

LMI extensions reserve some of these identifiers. This reduces the number of permitted VCs.

• LMI messages are exchanged between the DTE and DCE using these reserved DLCIs.

VC IDent VC Types

0 LMI (ANSI, ITU)

1-15 Reserved

992-1007 CLLM

1008-1022 Reserved (ANSI, ITU)

1019-1022 Cisco Multicasting

1023 LMI (Cisco)

Chapter 330

LMI ExtensionsIn addition to the Frame Relay protocol functions for transferring data, the FrameRelay specification includes optional LMI extensions that are extremely useful in an Internetworking environment. Some of the extensions include:

• VC status messages - Provide information about PVC integrity by communicating and synchronizing between devices, periodically reporting the existence of new PVCs and the deletion of already existing PVCs. VC status messages prevent data from being sent into black holes (PVCs that no longer exist).

• Multicasting - Allows a sender to transmit a single frame that is delivered to multiple recipients. Multicasting supports the efficient delivery of routing protocol messages and address resolution procedures that are typically sent to many destinations simultaneously.

• Global addressing - Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network. Global addressing makes the Frame Relay network resemble a LAN in terms of addressing, and ARPs perform exactly as they do over a LAN.

• Simple flow control - Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface. It is intended for those devices whose higher layers cannot use the congestion notification bits and need some level of flow control.

Chapter 331

Frame Relay Address Mapping

DLCI 101

DLCI 102

R1

1. R1 connects to the Frame Relay network, it sends an LMI status inquiry message (75) to the network.

2. Network replies with an LMI status message (7D) containing details of every VC configured on the access link.

3. If the router needs to map the VCs to network layer addresses, it sends an Inverse ARP message on each VC.

4. The Inverse ARP message includes the network layer address of the router, so the remote DTE, or router, can also perform the mapping.

Chapter 332

Configuring Frame Relay maps

• If the environment does not support LMI autosensing and Inverse ARP, a Frame Relay map must be manually configured.

• Once a static map for a given DLCI is configured, Inverse ARP is disabled on that DLCI.

Hub City Spokane

DLCI 101 DLCI 102

172.16.1.1172.16.1.2

HubCity(config)# interface serial 0

HubCity(config-if)# frame-relay map ip 172.16.1.1 101 broadcast

Spokane(config)# interface serial 0

Spokane(config-if)# frame-relay map ip 172.16.1.2 102 broadcast

Frame Relay Network

Chapter 333

HubCity# show frame-relay map

Serial0 (up): ip 172.16.1.1 dlci 101,

dynamic, broadcast, status defined, active

Inverse ARP

• dynamic refers to the router learning the IP address via Inverse ARP

• DLCI 101 is configured on the Frame Relay Switch by the provider.

Hub City Spokane

DLCI 101 DLCI 102

172.16.1.1172.16.1.2Frame Relay

Network

Chapter 334

Inverse ARP Limitations

• Inverse ARP only resolves network addresses of remote Frame-Relay connections that are directly connected via frame-relay nodes.

• Inverse ARP does not work with Hub-and-Spoke connections.

• Dynamic address mapping is enabled by default for all protocols enabled on a physical interface.

• If the Frame Relay environment supports LMI autosensing and Inverse ARP, dynamic address mapping takes place automatically.

Hub City Spokane

DLCI 101 DLCI 102

172.16.1.1172.16.1.2Frame Relay

Network

Chapter 335

Frame Relay Configuration

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I 102

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I 201

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DLCI 203 DLCI 103 DLCI 301

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•Default Frame Relay encapsulation enabled on supported interfaces is the Cisco encapsulation.

•Use Cisco encapsulation if connecting to another Cisco router, use IETF if connecting to non-Cisco devices.

Chapter 336

Frame Relay Configuration

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•If the environment does not support LMI autosensing and Inverse ARP, a Frame Relay map must be manually configured.

•Once a static map for a given DLCI is configured, Inverse ARP is disabled on that DLCI.

Static Map

.0

Chapter 337

Reachability issues with

routing updates

• An None Broadcast Multiple Access (NBMA) network is a multi-access network, which means more than two nodes can connect to the network.

• Frame Relay is a NBMA, and its nodes cannot see broadcasts of other nodes unless they are directly connected by a virtual circuit.

• This means that Branch A cannot directly see the broadcasts from Branch B, because they are connected using a hub and spoke topology.

Chapter 338

• The Central router must receive the routing update broadcast from Branch A and then send its own update broadcast to Branch B & C.

• In this example, there are problems with routing protocols because of the split horizon rule.

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

Reachability issues with

routing updates

Chapter 339

• Use of multiple point-to-point serial interfaces solves the problem of split-horizon, as routing updates are not being sent from the same physical interface.

• Expensive to implement, as multiple serial interfaces and point-to-point connections are required.

172.30.1.1/24S0

DLCI 300

172.30.3.1/24S3

DLCI 100

172.30.2.1/24S2

DLCI 200

Frame Relay Network

172.30.1.2/24S0

DLCI 301

172.30.2.2/24S0

DLCI 201

172.30.3.2/24S0

DLCI 101

Chapter 340

Point-to-Point Sub-interface

• A single point-to-point sub-interface is used to establish one PVC connection to another physical interface or sub-interface on a remote router.

• In this case, each pair of the point-to-point routers is on its own subnet and each point-to-point sub-interface would have a single DLCI.

• In a point-to-point environment, each sub-interface is acting like a point-to-point interface. Therefore, routing update traffic is not subject to the split-horizon rule.

Chapter 341

• Point-to-point sub-interfaces are equivalent to using multiple physical “point to point” interfaces.

172.30.1.1/24DLCI 300

172.30.3.1/24DLCI 100

172.30.2.1/24DLCI 200

Frame Relay

Network

172.30.1.2/24S0

DLCI 301

172.30.2.2/24S0

DLCI 201

172.30.3.2/24S0

DLCI 101

S0.100

S0.200

S0.300

Interface S0

Chapter 342

Router(config)# interface Serial 0Router(config-if)# encapsulation frame-relay ietfRouter(config-if)#frame-relay lmi-type ansiRouter(config-if)# interface Serial0.100 point-to-pointRouter(config-if)# ip address 172.30.3.1 255.255.255.0Router(config-subif)# frame-relay interface-dlci 100Router(config-subif)# exitRouter(config-if)# interface Serial0.200 point-to-pointRouter(config-if)# ip address 172.30.2.1 255.255.255.0Router(config-subif)# frame-relay interface-dlci 200

Point-to-point Sub-interfaces

172.30.3.1/24DLCI 100

172.30.3.2/24S0

DLCI 101

S0.100

S0.200

S0.300

Interface S0

172.30.2.1/24DLCI 200

172.30.2.2/24S0

DLCI 201

172.30.1.1/24DLCI 300

172.30.1.2/24S0

DLCI 301

Chapter 343

Frame-Relay service supplies multiple PVCs over a single physical interface and point-to-point sub-interfaces subdivide each PVC as if it were a physical point-to-point interface.

Point-to-point sub-interfaces are like conventional point-to-point interfaces and do not need or allow:

• Inverse-ARP• Frame-relay map statements• Multiple DLCIs for a sub-interface

Point-to-point sub-interfaces completely bypass the local DLCI to remote network address mapping issue.

Point-to-point Sub-interfaces

Chapter 344

Multipoint Sub-interface

• A single multipoint sub-interface is used to establish multiple PVC connections to multiple physical interfaces or sub-interfaces on remote routers.

• All the participating interfaces would be in the same subnet.

• The sub-interface acts like an NBMA Frame Relay interface so routing update traffic is subject to the split-horizon rule.

Chapter 345

• Multipoint sub-interfaces are equivalent to using multiple physical “hub to spoke” interfaces

DLCI 300

DLCI 200

Frame Relay

Network

172.30.1.4/24S0

DLCI 301

172.30.1.3/24S0

DLCI 201

172.30.1.2/24S0

DLCI 101

S0.1172.30.1.1/24

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DLCI 100

Multipoint

Point-to-Point

Point-to-Point

Point-to-Point

Chapter 346

DLCI 300

DLCI 200

Frame Relay

Network

172.30.1.4/24S0

DLCI 301

172.30.1.3/24S0

DLCI 201

172.30.1.2/24S0

DLCI 101

S0.1172.30.1.1/24

Interface S0

DLCI 100

Multipoint

Point-to-Point

Point-to-Point

Point-to-Point

• Router(config)# interface Serial 0• Router(config-if)# encapsulation frame-relay ietf• Router(config-if)# interface Serial0.123 multipoint• Router(config-if)# ip address 172.30.1.1 255.255.255.0• Router(config-subif)# frame-relay interface-dlci 100• Router(config-subif)# frame-relay interface-dlci 200• Router(config-subif)# frame-relay interface-dlci 300

Chapter 347

Share many of the same characteristics as a physical Frame-Relay interface.

With multipoint sub-interface you can have:

• Multiple DLCIs assigned to it.• Frame-relay map & interface dlci statements• Inverse-ARP

Which is the opposite of point-to-point interfaces.

Multipoint Sub-interfaces

Chapter 348

Frame-Relay Map Statement Rule:

When a Frame-Relay map statement is configured for a particular protocol (e.g. IP or IPX) Inverse-ARP will be disabled for that specific protocol, not only for the DLCI referenced in the Frame-Relay map statement.

Mixing Inverse ARP and Frame Relay Map Statements

Chapter 349

Solution: Do not mix IARP with Frame Relay maps statements. If need be use Frame-Relay map statements instead of IARP.

DLCI 300

DLCI 200

Frame Relay

Network

172.30.1.4/24S0

DLCI 301

172.30.1.3/24S0

DLCI 201

172.30.1.2/24S0

DLCI 101

S0.1172.30.1.1/24

Interface S0

DLCI 100

Multipoint

Point-to-Point

Point-to-Point

Point-to-Point

Chapter 350

Frame Relay Sub-interfacesSummary

Point-to-Point

•Sub-interfaces act as a leased line•Each point-to-point sub-interface requires its own subnet•Applicable to hub and spoke topologies

Multipoint

•Sub-interfaces act as NBMA network, so they do not resolve the split horizon issue•Can save address space because it uses a single subnet•Applicable to partial-mesh and full-mesh topology

Chapter 351

Verify Frame Relay

Chapter 352

Verify Frame Relay

•Shows the number of status messages exchanged between the local router and the local Frame Relay switch.

Chapter 353

Verify Frame Relay

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

PVC status can be active, inactive, or deleted.

Chapter 354

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

Verify Frame Relay

Chapter 355

Debug frame-relay lmi Command

•0x0 - Added/inactive – Not usable•0x2 - Added/active - Operational•0x4 - Deleted means that the Frame Relay switch does not have this DLCI programmed for the router

Chapter 356

Chap 3 – Frame Relay Learning Objectives

• Describe the fundamental concepts of Frame Relay technology in terms of Enterprise WAN services including Frame Relay operation, Frame Relay implementation requirements, Frame Relay maps, and LMI operation.

• Configure a basic Frame Relay PVC including configuring and troubleshooting Frame Relay on a router serial interface and configuring a static Frame Relay map.

• Describe advanced concepts of Frame Relay technology in terms of Enterprise WAN services including Frame Relay sub-interfaces, Frame Relay bandwidth and flow control.

• Configure an advanced Frame Relay PVC including solving reachability issues, configuring Frame Relay sub-interfaces, verifying and troubleshooting Frame Relay configuration.

Chapter 357

AnyQuestions?

Chapter 358

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DLC

I 201

DLC

I 102 D

LCI 301

DLCI 103

R1

R2

R3

Chapter 3.1 – Frame RelayMap Config Lab Topology

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Chapter 359

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DLC

I 201

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R2

R3

Chapter 3.2 – Frame RelayPoint-to-Point Config Lab Topology

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Chapter 360

DLC

I 201

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I 102 D

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Chapter 3.3 – Frame RelayMultipoint Config Lab Topology

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Chapter 361

Port 1 / 1(Adtran)

Serial 0 (PT4.1)

Port 1 / 2(Adtran)

Serial 1 (PT4.1)

Port 2 / 1(Adtran)

Serial 2 (PT4.1)

Port 2 / 2(Adtran)

Serial 3 (PT4.1)

DLCI102

DLCI201

DLCI103

DLCI301

DLCI104

DLCI203

DLCI302

DLCI401

chap 3 – frame relay learning objectives

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