∙Variable bit rate−real time (VBR−RT)—Typically used for connections that carry VBR traffic in which a fixed timing relationship exists between either VBR video or voice compression.

∙Variable bit rate−non real time (VBR−NRT)—Used to carry VBR traffic in which no timing relationship exists for data traffic where a guarantee of bandwidth or latency is needed. This type of connection is used in Frame Relay where the committed information rate (CIR) of the Frame Relay connection is mapped into a bandwidth guarantee within the ATM network.

∙Unspecified bit rate−real time (UBR−RT)—Does not offer any service guarantees whatsoever. This type of connection is typically for the bursty or unpredictable traffic patterns from LAN protocols served by ATM routers.

ATM Addressing

ATM devices must have unique ATM addresses in order to connect to other ATM devices. The device at the other end of your circuit must know your address. ATM uses both private and public types of addresses. Because the ATM standard has adopted the subnetwork model of addressing, the ATM layer is responsible for mapping Network layer addresses to the ATM addresses.

Currently, two types of ATM addressing plans are used. The ATM UNI address format defined by ITU−T uses telephone−type E.164 addresses. This format is used to connect an endpoint to a telephone carrier’s network. One drawback to this type of address is that E.164 addresses are available only from large telephone carriers, which prevents the addresses from being assigned to competitors and private businesses.

The ISO has defined a second address type that uses a Network Service Access Point (NSAP) format. This format is used to connect an ATM endpoint to a private network. The ATM Forum has now used this method to incorporate the E.164 address of the public networks into the address of customers using NSAP addresses. The ATM Forum is also working on a method for the phone carriers to use NSAP−based addressing on their networks. Let’s take a look at the components of an NSAP address, as shown in Figure 8.5:

Figure 8.5: The format of an ATM NSAP address.

∙Authority and format identifier (AFI)—Used to indicate which standard is being used for the ATM address. An AFI of 47 indicates a British Standards Institute address (used by Cisco on all its ATM devices); an AFI of 39 indicates an ISO address and an E.164 address.

∙Initial domain identifier (IDI)—Indicates the address allocation and administrative authority.

∙Domain specific part (DSP)—Contains the actual routing information.

∙End−system identifier (ESI)—Places the end system’s MAC address in the frame.

∙NSAP selector field (SEL)—Identifies the LANE components.

Local Area Network Emulation (LANE)

In a LAN environment, broadcast support is an inherent part of the networking technology. Legacy networks have native broadcast support to perform address mapping resolution. In contrast, ATM networks are Non−Broadcast Multiple Access (NBMA) networks with no such support. The LANE standard was created by the ATM Forum in 1994 to provide connectivity for ATM networks to legacy Ethernet and Token−Ring networks.

LANE provides these broadcast services by making an ATM interface look like an Ethernet or Token Ring interface. LANE gives ATM devices MAC addresses, just like Ethernet or Token Ring devices. Because the ATM interfaces can use the same frame format as legacy devices, LAN−based applications can run without

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changing the application itself or Layer 3 drivers.

This approach allows backward compatibility with existing LANs, broadcast support, and connectionless delivery. LANE has some drawbacks, however: It prevents the use of ATM−specific benefits such as QoS and doesn’t have the ability to provide flexible bandwidth allocations.

LANE is the primary component that provides connectivity between ATM devices and the devices residing on the Layer 2 LAN. This connectivity extends to devices attached to ATM stations and devices attached to LAN devices spanning the ATM network. This connectivity between ATM devices and other LAN devices is done through ELANs.

What Are ELANs?

ELANs are just like VLANs—one of their functions is to create independent broadcast domains in ATM, the same way that VLANs do in Ethernet and Token Ring networks. ELAN workstations are independent of the physical location, and like VLANs, ELANs must be connected to a Layer 3 device in order to communicate with members of another ELAN.

The Data Link layer’s MAC sublayer allows ELANs to use the Microsoft or Novell upper−level NDIS/ODI driver interfaces. This method allows ELANs to transmit Layer 3 protocols such as TCP/IP, IPX, and AppleTalk.

LANE is a standardized conversion process that allows a connectionless environment in a LAN to connect to a connection−oriented ATM environment. LANE fragments an incoming Layer 3 into a 48−byte payload and places a 5−byte ATM−specific identification header on the front of the packet, yielding a 53−byte cell. It then removes the checksum from the cell and forwards the cell through the ATM network. When the cell has traveled the ATM network, the ATM information is removed and the cell fragments are reassembled and returned to the LAN environment as a packet.

The LANE 1.0 standard can be summed up as a software interface for the Layer 3 protocol environment that encapsulates user data for either Ethernet or Token Ring packets. LANE isn’t actually the media access method for this conversion process—LANE uses three servers, which clients access over the ATM connections. The LANE servers provide address registration and resolution functions, including collecting address and route descriptor types based on the LANE standard. Let’s take a look at the LANE components.

Note FDDI can be used with LANE 1.0; however, it is not accurately defined like Ethernet and Token Ring protocols. ATM uses translational bridging techniques to map FDDI packets into either Ethernet or Token Ring.

LANE Components

LANE uses several components to provide LAN−based network connectivity. The interaction of these components allows address registration, address caching, and searchable databases. LANE uses the following components:

∙LAN Emulation Client (LEC)—Emulates a LAN interface to higher−layer protocols and applications of the OSI Reference Model.

∙LAN Emulation Server (LES)—Provides a database of LANE services, resolves addresses, manages stations that make up an ELAN, and provides registration services to LANE clients for the emulated LAN.

∙LAN Emulation Configuration Server (LECS)—Uses a database to track device memberships in each ELAN.

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∙Broadcast and Unknown Server (BUS)—Sends broadcasts, sequences cells, controls unicast flooding, and distributes multicast packets.

Warning Notice that although LEC and LECS sound the same, they are completely different terms and components in LANE.

LAN Emulation Client (LEC)

The LEC resides in every ATM end system. It provides services to emulate the Data Link layer interface that allows communication of all higher−level protocols and applications to occur. It provides both ATM−attached devices and ATM−capable Token Ring, Ethernet, and legacy LAN topologies the ability to coexist within an ATM emulated LAN and WAN environment.

The LEC is the component responsible for passing traffic between separate VLANs on the Catalyst switches and between ELANs on the ATM switch. You can configure multiple LECs for one or more ELANs on the ATM modules. Prior to configuring a LEC on an ATM module, a VLAN must be configured on the switch, and the LES/BUS or an ELAN must be configured on one or more ATM module subinterfaces.

The LEC forwards data to other LANE components in the ELAN and performs control functions. Each LEC is a member of only one ELAN. In many instances, an Ethernet switch may have multiple LECs for each ELAN. Examples of LEC implementations include servers, routers, switches, or other network hosts. The LEC has the following functions:

∙Resolves MAC addresses

∙Transfers data

∙Performs address caching

∙Interfaces with other LANE components

∙Provides interface driver support

LAN Emulation Server (LES)

The LES for an ELAN is the central piece of LANE. It gives the LECs the information they need to establish ATM connections to other LECs in their ELAN. A single LES is responsible for address registry and resolution for an ELAN. When a LEC joins an ELAN, it forms a connection with the LES. The LEC registers its MAC and ATM addresses with the LES. The LES has the following functions:

∙Supports LECs

∙Registers addresses from LECs

∙Resolves addresses from LECs

∙Interfaces to the LEC, LECS, and BUS

The LES performs traffic control for all LECs connecting to an ELAN. This component provides the address resolution, registration, broadcast, and unknown server information that guides communication among LECs. When configuring each LEC, the LEC must request a connection from the LES. The request information contains the ATM address of the LEC, a LAN identifier, and an optional MAC address. This component also performs verification of each LEC during the initial connection with the server, checking to make sure that each LEC has permission to join the requested ELAN.

Address registration is also a function of the LES. It must maintain a database to aid in resolving addresses. This registration occurs after the LEC joins an ELAN. Each LEC provides the LES with one registered address with a join request, and no separate registrations are required.

The LES with the ATM address database responds to all address resolution queries and attempts to locate partnering LECs. The LES responds with the ATM addresses for the targeted ELANs. If no address can be found, the LES attempts to forward the request to other LECs on other ELANs.

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The ultimate goal of the LES is to arrange and control connections with a LEC. This connection is commonly known as a control direct ATM virtual channel connection (VCC). After this connection is established, it will handle address resolution and registration responses.

The LANE servers provide the address registration and resolution functions. These functions include collecting address and route descriptor types based on the LANE standard. Let’s take a look at the address resolution process, which is shown in Figure 8.6 and outlined as follows:

Figure 8.6: The LES address resolution process.

1.A workstation connects to a router or ATM switch and performs a physical outbound packet transmission. This example uses the Address Resolution Protocol (ARP) query to try to locate a device on a remote segment.

Note The local router is typically the ATM LEC and provides the circuit for the initial ATM address mapping.

2.The LEC takes an Ethernet frame and assigns an immediate LEC link, which is used to obtain the ATM address identifier needed to establish an ATM connection. If this process is not successful, the LEC must locate a LES.

3.The LES circuit holds the main ATM network address table and returns with the VCI assignment.

LAN Emulation Configuration Server (LECS)

The LECS provides key services such as registration for Integrated Local Management Interface (ILMI) and configuration support for the LES addresses for the corresponding emulated LAN identifiers.

The LECS contains a database of ATM addresses for the LES and BUS pairs for known ELANs. The LEC consults the LECS to determine the LES’s ATM address when it first joins an ELAN.

Note At least one LECS is required per ATM LANE switch cloud.

The LECS has the following functions:

∙Registers the LECS ATM addresses for known ELANs

∙Supplies LECs with LESs’ ATM addresses

∙Provides interfaces to the LEC and the LES

The registration process of the LECS ATM address uses the ILMI functions to connect to the ATM network; this situation usually includes an ATM switch. Support for configurations from the LECS ensures that the correct LES address is supplied to the LEC.

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Broadcast and Unknown Server (BUS)

The BUS provides broadcasting support for an ELAN. The BUS distributes multicast data, sends or distributes unicast data, and connects the other LANE components. When the destination address of an Ethernet or Token Ring frame contains a local broadcast or a multicast address, the LEC forwards the traffic to the BUS, which forwards it to all the other LECs in the ELAN. At least one combined LES and BUS is required per ELAN. The BUS has the following functions:

∙Distributes multicast data

∙Sends or distributes unicast data

∙Interfaces to LEC and LES

The LES is the component responsible for resolving MAC addresses to ATM addresses, and the BUS is the component responsible for servicing multicast, Ethernet, and Token Ring broadcasts. The Cisco LANE implementation calls for the LES and the BUS to be configured in the same end−station.

ATM Module Subinterfaces

In order to configure ATM components on an ATM LANE module for the Catalyst 5000 or 6000 family of switches, you need to use subinterfaces. Depending on the level of traffic you expect on your network, you may want to place different LES/BUS components throughout the framework of your network. To configure a LES or BUS on the ATM module, you need to complete the following tasks:

1.Enter Interface Configuration mode.

2.Specify the subinterface for the ELAN.

3.Specify the type of link: Ethernet or Token Ring.

4.Enable the LES and BUS on the ELAN.

5.Repeat this process for each LES/BUS.

The BUS must be used to sequence and distribute broadcast data to all the LECs. However, sending a large volume of broadcast data to all the LECs can severely impact the overall performance of the network. For this reason, it may be necessary for the BUS to place restrictions on the LANE components to control the maximum throughput rate for each device. The BUS’s primary function is to provide broadcast management support for LANs. The BUS must supply the following services:

∙Distribute unicast and multicast data to all the LECs in the network

∙Connect interfaces to the ELAN

Distribution of unicast and multicast data includes the transmission of data to the LECs in the network. Whenever possible, the LEC will establish a direct connection to another LEC. If this isn’t possible, then data the BUS receives is broadcast to each LEC on the ELAN. This option can be enabled and disabled, and you should carefully consider whether you need this option, because it can eat up costly bandwidth.

Note When interfacing to ELANs, the BUS establishes a bi−directional connection that allows forwarding of multicast and unicast frames with unknown destinations.

LEC Queries

LECs send queries for configuration information to receive the LES address. The LECS then assigns the correct LES address for each LEC. The LES also has the ability to establish a connection with the LECS.

A reply to a query can be as simple as providing a single LES address or it can provide more information, such as:

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