Chapter 3 – Frame-Relay CCNA pps

Introducing Frame RelayThe first Solution – Leased line • Using leased lines, each Span site is connected through a switch at the local telephone company's central office CO through the

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Chapter 3 – Frame-Relay

CCNA Exploration 4.0

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Introduction

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Basic Frame Relay Concepts

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Introducing Frame Relay

Frame Relay: An Efficient and Flexible WAN Technology

• Frame Relay has become the most widely used WAN technology in the world Large enterprises, governments, ISPs, and small businesses use Frame

Relay, primarily because of its price and flexibility

• Moreover, Frame Relay provides greater bandwidth, reliability, and resiliency

than private or leased lines

• Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation

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• In the example shown in the figure, Span Engineering has five

campuses across North America.

• The bandwidth requirement of each site:

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The first Solution – Leased line

• Using leased lines, each Span site is connected through a switch at the local telephone company's central office (CO) through the local loop, and then

across the entire network

• These lines are truly dedicated in that the network provider reserves that line for Span's own use There is no sharing, and Span is paying for the end-to-end circuit regardless of how much bandwidth it uses

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The second Solution – Frame Relay

• Frame Relay is a more cost-effective option for two reasons.

– First, with dedicated lines, customers pay for an end-to-end connection That

includes the local loop and the network link

• With Frame Relay, customers only pay for the local loop , and for the bandwidth

they purchase from the network provider.

– The second reason for Frame Relay's cost effectiveness is that it shares bandwidth across a larger base of customers.

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• The table shows a representative cost comparison for comparable

ISDN and Frame Relay connections.

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• The Flexibility of Frame Relay

– A virtual circuit provides considerable flexibility in network design

– In Frame Relay, the end of each connection has a number to identify it

called a Data Link Connection Identifier (DLCI)

– Any station can connect with any other simply by stating the address of

that station and DLCI number of the line it needs to use

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The Frame Relay WAN

• In the late 1970s and into the early 1990s, the WAN technology joining the end sites was typically using the X.25 protocol However, X.25 have much

overhead to the protocol

• Frame Relay has lower overhead than X.25 because it has fewer capabilities For example, Frame Relay does not provide error correction, modern WAN

facilities offer more reliable connection services and a higher degree of

reliability than older facilities

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Frame Relay Operation

• The connection between a DTE device and a DCE device consists of both a physical

layer component and a link layer component:

– The physical component defines the mechanical, electrical, functional, and

procedural specifications for the connection between the devices

– The link layer component defines the protocol that establishes the connection

between the DTE device, such as a router, and the DCE device, such as a switch

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Virtual Circuits

• The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC) The circuits are virtual because there is no direct electrical connection from end to end

• There are 2 ways to establish VCs:

– SVCs, switched virtual circuits, are established dynamically by sending

signaling messages to the network (CALL SETUP, DATA TRANSFER,

IDLE, CALL TERMINATION)

– PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes

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Local Significance

• VCs provide a bidirectional communication path from one device to another VCs are identified by DLCIs DLCI values typically are assigned by the Frame Relay service provider (for example, the telephone company)

• Frame Relay DLCIs have local significance, which means that the values

themselves are not unique in the Frame Relay WAN

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Idenfiying VCs

• Frame Relay labels each VC with a DLCI

• The DLCI is stored in the address field of every frame transmitted to tell the network how the frame should be routed

• The Frame Relay service provider assigns DLCI numbers Usually, DLCIs 0 to 15 and

1008 to 1023 are reserved for special purposes Therefore, service providers typically assign DLCIs in the range of 16 to 1007

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Multiple VCs

• Frame Relay is statistically multiplexed, meaning that it transmits only one

frame at a time, but that many logical connections can co-exist on a single

physical line

• 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

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• For example, Span Engineering has five locations, with its headquarters in

Chicago Chicago is connected to the network using five VCs and each VC is given a DLCI

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Frame Relay Encapsulation

• Frame Relay takes data packets from a network layer protocol, such as

IP or IPX, encapsulates them as the data portion of a Frame Relay

frame, and then passes the frame to the physical layer for delivery on the wire

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Frame Relay Topologies

• There are three topology types: star, full mesh, or partial mesh.

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• A fully meshed topology means that each node on the periphery of a

given packet-switching network has a direct path to every other node

on the cloud.

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• A partially meshed topology reduces the number of routers within a

region that have direct connections to all other nodes in the region

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• A data-link connection identifier (DLCI) identifies the logical VC between the CPE and the Frame Relay switch

• The Frame Relay switch maps the DLCIs between each pair of routers to create a PVC

• DLCIs have local significance , although there some implementations that use global

DLCIs.

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

• Service providers assign DLCIs in the range of 16 to 1007.

– DLCI 1019 - 1022: Multicasts

– DLCI 1023: Cisco LMI

– DLCI 0: ANSI LMI

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Frame Relay Address Mapping

• Cisco routers support all network layer protocols over Frame Relay, such as

IP, IPX, and AppleTalk This address-to-DLCI mapping can be accomplished

either by static or dynamic mapping:

– Manual

• Manual: Administrators use a frame relay map statement.

– Dynamic

• Inverse Address Resolution Protocol (I-ARP) provides a given DLCI

and requests next-hop protocol addresses for a specific connection

• The router then updates its mapping table and uses the information in the table to forward packets on the correct route

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Inverse ARP

• Once the router learns from the switch about available PVCs and their

corresponding DLCIs, the router can send an Inverse ARP request to the

other end of the PVC (unless statically mapped)

• For each supported and configured protocol on the interface, the router sends

an Inverse ARP request for each DLCI (unless statically mapped)

• In effect, the Inverse ARP request asks the remote station for its Layer 3

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• However, with ARP, the device knows the Layer 3 IP address and

needs to know the remote data link MAC address

• With Inverse ARP, the router knows the Layer 2 address which is the DLCI, but needs to know the remote Layer 3 IP address.

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Inverse ARP

• On a Cisco router, Inverse ARP is on by default when an interface is configured to use Frame Relay encapsulation

• If static mapping for a specific DLCI is configured, Inverse ARP is

automatically disabled for the specified protocol on the specified

DLCI

• Use static mapping if the router at the other end either does not

support Inverse ARP or does not support Inverse ARP for a specific protocol being used over Frame Relay.

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Inverse ARP Limitations

• Inverse ARP only resolves network addresses of remote Frame-Relay

connections that are directly connected

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

• When using dynamic address mapping, Inverse ARP requests a next-hop

protocol address for each active PVC

• Once the requesting router receives an Inverse ARP response, it updates its DLCI-to-Layer 3 address mapping table

• 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

• Therefore, no static address mapping is required

Frame Relay Network Headquarters

Hub City

Satellite Office 1 Spokane

172.16.1.1 172.16.1.2

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Local Management Interface (LMI)

• LMI is a signaling standard between the DTE and the Frame Relay switch.

• LMI is responsible for managing the connection and maintaining the status

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• LMI virtual circuit status messages provide communication and synchronization between Frame Relay DTE and DCE devices These messages are used to periodically report

on the status of PVCs

• The LMI global addressing extension gives Frame Relay data-link connection identifier (DLCI) values global rather than local significance.

• The LMI multicasting extension allows multicast groups to

be assigned Multicasting saves bandwidth by allowing

routing updates and address-resolution messages to be

sent only to specific groups of routers The extension also transmits reports on the status of multicast groups in

update messages.

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• Starting with Cisco IOS software release 11.2, the default LMI

autosense feature detects the LMI type supported by the directly

connected Frame Relay switch Based on the LMI status messages it receives from the Frame Relay switch, the router automatically

configures its interface with the supported LMI type acknowledged by the Frame Relay switch

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Using LMI and Inverse ARP to Map Addresses

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• Activity 3.1.5.5

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Configuring Frame Relay

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Frame Relay Configuration Tasks

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Enable Frame Relay Encapsulation

• Step 1 Setting the IP Address on the Interface

• Step 2 Configuring Encapsulation

R(config-if)# encapsulation frame-relay [cisco | ietf]

• Step 3 Setting the Bandwidth

• Step 4 Setting the LMI Type (optional)

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Configuring a Static Frame Relay Map

• Static mapping is manually configured on a router Establishing static mapping depends on your network needs

Router(config-if)# frame-relay map protocol protocol-address dlci

[broadcast] [ietf | cisco]

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Remote IP Address

Local DLCI

Uses cisco encapsulation for this DLCI (not

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• If the Cisco encapsulation is configured on a serial interface, then by default, that encapsulation applies to all VCs on that serial interface

• If the equipment at the destination is Cisco and non-Cisco, configure the Cisco encapsulation on the interface and selectively configure IETF encapsulation per DLCI, or vice versa

Applies to all DLCIs unless configured otherwise

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Case study: Hub and Spoke Topology

Frame Relay Network

Headquarters Hub City

Satellite Office 1 Satellite Office 2

172.16.1.2 DLCI 101

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Satellite Office 2 Spokomo

172.16.1.1 172.16.1.3

172.16.1.2 DLCI 101

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HubCity# show frame-relay map

Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast,

status defined, active

Spokane# show frame-relay map

Spokomo# show frame-relay map

172.16.1.2 DLCI 101

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• Inverse ARP resolved the ip addresses for HubCity for both

Spokane and Spokomo

• Inverse ARP resolved the ip addresses for Spokane for HubCity

• Inverse ARP resolved the ip addresses for Spokomo for HubCity

• What about between Spokane and Spokomo?

Configuration using Inverse ARP

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Inverse ARP Limitations

• Can HubCity ping both Spokane and Spokomo? Yes!

• Can Spokane and Spokomo ping HubCity? Yes!

• Can Spokane and Spokomo ping each other? No! The Spoke

routers’ serial interfaces (Spokane and Spokomo) drop the ICMP

packets because there is no DLCI-to-IP address mapping for the

destination address.

• Solutions to the limitations of Inverse ARP

1 Add an additional PVC between Spokane and Spokomo (Full Mesh)

2 Configure Frame-Relay Map Statements

3 Configure Point-to-Point Subinterfaces.

172.16.1.2 DLCI 101

DLCI 102

DLCI 112

DLCI 211

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172.16.1.2 DLCI 101

Frame-Relay Map Statements

Notice that the routers are configured to use either IARP or Frame Relay maps Using both on the same interface will cause problems.

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172.16.1.2 DLCI 101

• What if we were to use I-ARP between the spoke routers and the hub,

• There would be a problem!

Inverse ARP

Mixing Inverse ARP and Frame Relay

Map Statements

Frame Relay maps

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172.16.1.2 DLCI 101

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Serial0 (up): ip 172.16.1.1 dlci 101, dynamic ,

broadcast, status defined, active

Serial0 (up): ip 172.16.1.3 dlci 102, static , CISCO, status defined, active

Serial0 (up): ip 172.16.1.1 dlci 211, static , CISCO, status defined, active

Mixing Inverse ARP and Frame Relay Map Statements

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Good News:

• Everything looks fine!

• Now all routers can ping each other!

Bad News:

• Problem when using Frame-Relay map statements AND Inverse

ARP

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Frame-Relay Map Statement Rule:

• When a Frame-Relay map statement is configured for a particular

protocol (IP, IPX, …) Inverse-ARP will be disabled for that specific

protocol, only for the DLCI referenced in the Frame-Relay map

statement.

Tiêu đề	Frame-Relay CCNA pps
Trường học	Học viện mạng Bach Khoa
Chuyên ngành	Networking
Thể loại	lecture notes
Năm xuất bản	not specified
Thành phố	Hanoi

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Số trang	107
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