LAN Emulation and MPOA

This chapter provides an overview of LAN emulation and a related technology, Multiprotocol Over ATM (MPOA). The background and rationale for these protocols are discussed in "Layer 3 Protocols over ATM."

This chapter contains the following sections:

Note The information in this chapter is applicable to the Catalyst 8540 MSR, Catalyst 8510 MSR, and LightStream 1010 ATM switch routers. For detailed configuration information, refer to the ATM Switch Router Software Configuration Guide and the ATM Switch Router Command Reference publication.

LAN Emulation

LAN emulation (LANE) is a standard defined by the ATM Forum that provides ATM-attached stations the same capabilities they normally obtain from legacy LANs, such as Ethernet and Token Ring. As the name suggests, the function of the LANE protocol is to emulate a LAN on top of an ATM network. By making an ATM interface look like one or more separate Ethernet or Token Ring interfaces, LANE allows LAN users to take advantage of ATM's benefits without requiring modifications to end station hardware or software.

As Figure 6-1 illustrates, LANE uses ATM to replace the legacy LAN backbone. Multiple emulated LANs (ELANs), which are logically separated, can share the same physical ATM network and same physical ATM interface.

Figure 6-1: Physical and Emulated LANs

LANE Applications

LANE services provide connectivity between ATM-attached devices and LAN-attached devices. The following are two primary applications of LANE (see Figure 6-2):

Connectivity between LAN-attached stations across an ATM network, effectively extending LANs over a high-speed ATM transport backbone.
Connectivity between ATM-attached hosts and LAN-attached hosts. Centralized hosts with high-speed ATM interfaces can provide services, such as Domain Name System (DNS), to traditional LAN-attached devices.

Figure 6-2: LANE Applications

The following types of devices can be used to support LANE services:

Directly attached ATM hosts with ATM NICs
Layer 2 devices, such as switches with ATM interfaces or the ATM switch routers
Layer 3 devices, such as routers with ATM interfaces

How it Works

ATM is a connection-oriented service that uses point-to-point signaling or point-to-multipoint signaling to establish connections between source and destination devices. LAN-based protocols, on the other hand, are connectionless and use broadcasts so that source devices can find one or more destination devices. The primary purpose of LANE, then, is to provide the same services that a broadcast medium like Ethernet does.

The LANE protocol defines mechanisms for emulating either an IEEE 802.3 Ethernet or an 802.5 Token Ring LAN. Specifically, LAN broadcasts are emulated as ATM unicasts. The current LANE protocol does not define a separate encapsulation for Fiber Distributed Data Interface (FDDI). (FDDI packets must be mapped into either Ethernet or Token Ring emulated LANs by using existing translational bridging techniques.) Fast Ethernet (100BaseT) and IEEE 802.12 (100VG-AnyLAN) both can be mapped unchanged because they use the same packet formats.

LANE defines a service interface for network layer protocols that is identical to existing MAC layers. No changes are required to existing upper layer protocols and applications. However, LANE does not emulate every particular physical or data-link characteristic. For example, it does not support carrier sense multiple access collision detect (CSMA/CD) for either Ethernet or Token Ring. LANE clients on an ATM switch router only support the IP protocol.

The Function of ATM Network Devices

The basic function of the LANE protocol is to resolve MAC addresses to ATM addresses so that LANE end systems can set up direct connections between themselves and then forward data. The LANE protocol can be deployed in two types of ATM-attached equipment: ATM network interface cards (NICs) and LAN devices, such as switches and routers.

ATM NICs implement the LANE protocol and interface to the ATM network while presenting the current LAN service interface to the higher-level protocol drivers within the end system. The network-layer protocols on the end system continue to communicate as if they were on a known LAN, by using known procedures. However, they are able to take advantage of most of the advanced services of the ATM network.

The second class of network device that implements LANE consists of ATM-attached LAN switches and routers. These devices, together with directly attached ATM hosts equipped with ATM NICs, are used to provide a virtual LAN service in which ports are assigned to particular virtual LANs, independent of physical location. Figure 6-3 shows the LANE protocol stack used between these devices.

Figure 6-3: LANE Protocol Stack

Ethernet and Token Ring Emulated LANs

The LANE version 1 standard defines separate emulated LANs for Ethernet and Token Ring, but does not explicitly define how to connect the two types directly. An ATM equipped router, such as the Cisco 7000 with an ATM interface, acting as a LANE client on each emulated LAN, can provide this connectivity while allowing the administrator to construct firewalls or to filter traffic between emulated LANs.

LANE Servers and Components

The LANE specification defines several components that enable the protocol to provide the broadcast and address resolution services required to emulate traditional LANs:

LANE client (LEC)---An entity in an end system such as a workstation, LAN switch, or router that performs data forwarding and receiving, address resolution, and other control functions for a single endpoint in a single emulated LAN. The LEC provides a standard LAN service to any higher layers that interface to it. A router or switch can have multiple resident LECs, each connecting with different emulated LANs. The LANE client registers its MAC and ATM address with the LES. In Token Ring environments, a LANE client configured for source-route bridging can register a route descriptor with the LES.
LANE server (LES)---A server that provides a registration facility for clients to join the emulated LAN. The LES handles LAN Emulation Address Resolution Protocol (LE_ARP) requests and maintains a list or look-up table of LAN destination MAC addresses. In Token Ring LANE environments, the LES maintains a list of route descriptors. Each emulated LAN must have an LES.
Broadcast-and-unknown server (BUS)---A server that floods unknown destination traffic and forwards multicast and broadcast traffic to clients within an emulated LAN. Each emulated LAN must have a BUS.

Note

LANE configuration server (LECS)---A server that assigns individual clients to particular emulated LANs by directing them to the LES that corresponds to the emulated LAN. The LECS maintains a database of LANE client ATM or MAC addresses and their emulated LANs. One LECS is required for each LANE cloud, but an LECS can serve multiple emulated LANs. The LECS can enforce security by restricting ELAN membership to certain LECs based on their MAC addresses.

These servers could be single points of failure in a LANE, but Cisco has developed a fault tolerance mechanism, known as Simple Server Redundancy Protocol (SSRP), which eliminates these single points of failure. Although this scheme is proprietary, no new protocol additions have been made to the LANE subsystems, which are described in the "SSRP for Fault-Tolerant Operation of LANE Server Components" section.

Comparing Virtual LANs and Emulated LANs

In the Catalyst family of switches, a virtual LAN (VLAN) is a logical group of end stations, independent of physical location, with a common set of requirements. Currently, the Catalyst switches support a port-centric VLAN configuration.

A VLAN is identified by a number, which is only significant to the Catalyst family of switches. On an ATM network, an emulated LAN is designated by a name. Therefore, the VLAN number must be mapped to the emulated LAN on the Catalyst switch. To create a VLAN that spans multiple Catalyst switches on an ATM network, you must assign the VLAN on each Catalyst switch to the same emulated LAN. Members of two or more different emulated LANs can communicate only through a router, whether they are on the same or different Catalyst switches.

LANE Virtual Connection Types

Communication among LANE components is ordinarily handled by several types of SVCCs. (In discussions of LANE, these SVCCs are commonly called virtual channel connections, or VCCs). Some VCCs are unidirectional; others are bidirectional. Some are point-to-point; others are point-to-multipoint. Figure 6-4 illustrates the various types of VCCs followed by a description of each.

Figure 6-4: LANE VCC Types

Control direct VCC---The LEC, as part of its initialization, sets up a bidirectional point-to-point VCC to the LES for sending or receiving control traffic. The LEC is required to accept control traffic from the LES through this VCC and must maintain the VCC while participating as a member of the emulated LAN.

Control distribute VCC---The LES can optionally set up a unidirectional VCC back to the LEC for distributing control traffic. Whenever an LES cannot resolve an LE_ARP request from a LEC, it forwards the request out the control distribute VCC to all of the clients in the emulated LAN. The control distribute VCC enables information from the LES to be received whenever a new MAC address joins the LAN or whenever the LES cannot resolve an LE_ARP request.

Data direct VCC---Once an ATM address has been resolved by a LEC, this bidirectional point-to-point VCC is set up between clients that want to exchange unicast data traffic. Most client traffic travels via these VCCs.

Multicast send VCC---The LEC sets up a unidirectional point-to-point VCC to the BUS. This VCC is used by the LEC to send multicast traffic to the BUS for forwarding out the multicast forward VCC. The LEC also sends unicast data on this VCC until it resolves the ATM address of a destination.

Multicast forward VCC---The BUS sets up a unidirectional VCC to the LECs for distributing data from the BUS. This can either be a unidirectional point-to-point or unidirectional point-to-multipoint VCC. Data sent by a LEC over the multicast send VCC is forwarded to all LECs over the multicast forward VCC.

Configure direct VCC---This is a transient VCC set up by the LEC to the LECS for the purpose of obtaining the ATM address of the LES that controls the particular LAN the LEC wishes to join.

Joining an Emulated LAN

The following steps (see Figure 6-4) describe the normal process that occurs when a LEC requests to join an emulated LAN:

Step 1 The LEC requests to join an emulated LAN.

The LEC sets up a connection to the LECS (bidirectional, point-to-point configure direct VCC, link 3-11 in Figure 6-4) to find the ATM address of the LES for its emulated LAN.

The LEC finds the LECS by using the following interface and addresses in the listed order:

Statically configured ATM address
ILMI (from directly attached ATM switch router)
The well-known address (defined by the ATM Forum)

Step 2 The LECS identifies the LES.

Using the same VCC, the LECS returns the ATM address and the name of the LES for the LEC's emulated LAN.

Step 3 The LEC tears down the configure direct VCC.

Step 4 The LEC contacts the LES for its emulated LAN.

The LEC sets up a connection to the LES for its emulated LAN (bidirectional, point-to-point control direct VCC, link 1-7 in Figure 6-4) to exchange control traffic. When a control direct VCC is established between an LEC and an LES, it remains established.

Step 5 The LES verifies that the LEC is allowed to join the emulated LAN.

The LES for the emulated LAN sets up a connection to the LECS to verify that the LEC is allowed to join the emulated LAN (bidirectional, point-to-point server configure VCC, link 11-12 in Figure 6-4); this is a Cisco proprietary action. The LES configuration request contains the LEC MAC address, its ATM address, and the name of the emulated LAN. The LECS checks its database to determine whether the LEC can join that emulated LAN; then it uses the same VCC to inform the LES whether or not the LEC is allowed to join.

Step 6 The LES allows or does not allow the LEC to join the emulated LAN.

If allowed, the LES adds the LEC to the unidirectional, point-to-multipoint control distribute VCC (link 2-8 in Figure 6-4) and confirms the join over the bidirectional, point-to-point control direct VCC (link 1-7 in Figure 6-4).

If not allowed, the LES rejects the join over the bidirectional, point-to-point control direct VCC (link 1-7 in Figure 6-4).

Step 7 The LEC sends LE_ARP packets for the broadcast address, which is all 1s.

Sending LE_ARP packets for the broadcast address returns the ATM address of the BUS. Then the LEC sets up the multicast send VCC (link 4-9 in Figure 6-4), and the BUS adds the LEC to the multicast forward VCC (link 5-10 in Figure 6-4) to and from the BUS.

Resolving Emulated LAN Addressing

As communication occurs on the emulated LAN, each LEC dynamically builds an LE_ARP table. An LEC LE_ARP table can also have static, preconfigured entries. The LE_ARP table maps MAC addresses to ATM addresses.

When an LEC first joins an emulated LAN, its LE_ARP table has no dynamic entries, and the LEC has no information about destinations on or behind its emulated LAN. To learn about a destination when a packet is to be sent, the LEC begins the following process to find the ATM address corresponding to the known MAC address:

Step 1 The LEC sends an LE_ARP request to the LES for this emulated LAN (point-to-point control direct VCC, link 1-7 in Figure 6-4).

Step 2 If the MAC address is registered with the LES, it returns the corresponding ATM address. If not, the LES forwards the LE_ARP request to all LECs on the emulated LAN (point-to-multipoint control distribute VCC, link 2-8 in Figure 6-4).

Step 3 Any LEC that recognizes the MAC address responds with its ATM address (point-to-point control direct VCC, link 1-7 in Figure 6-4).

Step 4 The LES forwards the response back to the LEC (point-to-multipoint control distribute VCC, link 2-8 in Figure 6-4).

Step 5 The LEC adds the MAC address-ATM address pair to its LE_ARP cache.

Step 6 The LEC can establish a VCC to the desired destination and transmit packets to that ATM address (bidirectional, point-to-point data direct VCC, link 6-6 in Figure 6-4).

Broadcast, Multicast, and Traffic with Unknown Address

When an LEC sends broadcast, multicast, or unicast traffic with an unknown address, the following process occurs:

Step 1 The LEC sends the packet to the BUS (unidirectional, point-to-point multicast send VCC, link 4-9 in Figure 6-4).

Step 2 The BUS forwards (floods) the packet to all LECs (unidirectional, point-to-multipoint multicast forward VCC, link 5-10 in Figure 6-4).

: This VCC branches at each ATM switch router. The ATM switch router forwards such packets to multiple outputs. (The ATM switch router does not examine the MAC addresses; it simply forwards all packets it receives.)

Building a LANE Connection from a PC---Example

To learn about a destination when a Transmission Control Protocol/Internet Protocol (TCP/IP) file transfer is to be sent, the PC and the LEC in the Catalyst 5000 switch begin a process to associate a LAN destination MAC address with the ATM address of the ATM-attached file server. This process is illustrated in Figure 6-5.

Figure 6-5: Steps in Resolving Addresses and Building a LANE Connection

To build a LANE connection from a PC to an ATM attached LEC, the LANE components perform the following steps:

Step 1 PC---Before starting the file transfer the PC must locate the file server on the network. To find the file server's MAC address, the PC broadcasts an ARP request with the file server's IP address.

Step 2 LEC on Catalyst 5000 switch---Receives ARP requests and forwards to the BUS configured on the ATM switch router.

Step 3 BUS on ATM switch router---Broadcasts the ARP request to all members of the emulated LAN using a point-to-multipoint VCC.

Step 4 LEC on file server---Receives the ARP request, recognizes its own IP address and responds with an ARP reply back to the BUS in the ATM switch router.

Step 5 BUS on ATM switch router---Forwards the ARP reply to the Catalyst 5000 switch.

Step 6 LEC on Catalyst 5000 switch---Forwards the ARP reply to the originating PC.

Step 7 PC---Starts sending the packets of the file transfer using the multicast send VCC from the Catalyst 5000 to the BUS on the ATM switch router, which forwards the packets over the multicast forward VCC to the file server. This gets the data moving in the interim until the data direct VCC is set up.

Step 8 LEC on file server---Starts to set up the direct VCC to the Catalyst 5000 switch using an LE_ARP request to the LES. This request asks for the ATM address that corresponds to the PC's MAC address. (The PC's MAC address was obtained from the original ARP request in Step 4.)

Step 9 LES on ATM switch router---Looks up the PC's MAC address in its look-up table and multicasts the LE_ARP request to all LECs.

Step 10 LEC on Catalyst 5000 switch---Receives the LE_ARP request and finds the PC's MAC address in its look-up table. (It learned the PC's MAC address in Step 2.)

Step 11 LEC on Catalyst 5000 switch---Adds its own ATM address into the LE_ARP request and returns it to the LES in the ATM switch router.

Step 12 LES on ATM switch router---Multicasts the LE_ARP reply to all members of the emulated LAN, including the file server.

Step 13 LEC on File Server---Receives the LE_ARP as part of the emulated LAN and signals for a data direct VCC to the Catalyst 5000 using the ATM address.

Step 14 ATM switch router---Sets up a data direct VCC between the Catalyst 5000 and the file server.

Step 15 PC---The file transfers directly from the PC using the direct data VCC from the Catalyst 5000 to the ATM-attached file server.

Implementation Considerations

The following sections describe features and requirements you might want to keep in mind when you are considering implementing LANE. Also included are some key advantages and limitations of using LANE.

Network Support

The ATM switch router supports the following LANE features:

Ethernet-emulated LANs
Token Ring-emulated LANs

Note

Simple server redundancy for fault tolerant LESs and LECSs

Addressing

On a LAN, packets are addressed by the MAC-layer address of the destination and source stations. To provide similar functionality for LANE, MAC-layer addressing must be supported, and every LANE client must have a MAC address. In addition, every LANE component (LEC, LES/BUS, and LECS) must have a unique ATM address.

LANE uses NSAP-format ATM end system addresses, as described in the "Addressing" section in the chapter "ATM Signaling and Addressing."

Method of Automatically Assigning ATM Addresses for LANE

We provide the following standard method of constructing and assigning ATM and MAC addresses for use in an LECS's database. A pool of MAC addresses is assigned to each ATM interface on the router or switch. For constructing ATM addresses, the following assignments are made to the LANE components:

The prefix field is the same for all LANE components on the ATM switch router; the prefix indicates the identity of the directly attached ATM switch router. The prefix value must be configured, either manually or by autoconfiguration, on the ATM switch router. In most cases, the autoconfigured prefix is used.
The ESI field value assigned to every LEC on the interface is the first in the pool of MAC addresses assigned to the interface.
The ESI field value assigned to every LES on the interface is the second in the pool of MAC addresses.
The ESI field value assigned to the BUS on the interface is the third in the pool of MAC addresses.
The ESI field value assigned to the LECS is the fourth in the pool of MAC addresses.
The selector field value is set to the subinterface number of the LANE component---except for the LECS, which has a selector field value of 0.

The following example shows the autoconfigured ATM addresses for LANE components. The prefix is the default ILMI prefix:

Switch> show lane default-atm-addresses
interface ATM2/0/0:
LANE Client:        47.00918100000000E04FACB401.00400B0A2A82.**
LANE Server:        47.00918100000000E04FACB401.00400B0A2A83.**
LANE Bus:           47.00918100000000E04FACB401.00400B0A2A84.**
LANE Config Server: 47.00918100000000E04FACB401.00400B0A2A85.00
note: ** is the subinterface number byte in hex

Because the LANE components are defined on different subinterfaces of an ATM interface, the value of the selector field in an ATM address is different for each component. The result is a unique ATM address for each LANE component, even within the switch or router. For more information about assigning components to subinterfaces, see the "Rules for Assigning Components to Interfaces and Subinterfaces" section.

Using ATM Address Templates

You can use ATM address templates in many LANE commands that assign ATM addresses to LANE components (thus overriding automatically assigned ATM addresses) or that link client ATM addresses to emulated LANs. Using templates can greatly simplify the task of manual ATM address assignment.

Note E.164-format ATM addresses do not support the use of LANE ATM address templates.

The syntax of address templates, the use of address templates, and the use of wildcard characters within an address template for LANE are very similar to the address templates of International Organization for Standardization of Connectionless Network Service (ISO CLNS). Refer to the ATM Switch Router Software Configuration Guide for details on using ATM address templates.

Rules for Assigning Components to Interfaces and Subinterfaces

The following rules apply to assigning LANE components to the major ATM interface and its subinterfaces:

The LECS always runs on the major interface.

: The assignment of any other component to the major interface is identical to assigning that component to the 0 subinterface.

The LES and the LEC of the same emulated LAN can be configured on the same subinterface.
Clients of two different emulated LANs cannot be configured on the same subinterface.
Servers of two different emulated LANs cannot be configured on the same subinterface.

Note On the ATM switch router, LANE components can be configured only on terminating ATM interfaces (for example, the CPU port) or on one of its subinterfaces.

LANE Router and Switch Requirements

You must manually configure permanent virtual channel connections (PVCCs) for Signaling ATM Adaptation Layer (SAAL) and ILMI on routers and edge LAN switches to run LANE. However, these signaling PVCCs are automatically configured on the ATM switch router.

At least one ATM switch router is required to run LANE. For example, you cannot run LANE on routers connected back-to-back.

Advantages

Potential advantages of LANE include the following:

Supports both Ethernet and Token Ring legacy LANs without modification to upper layer protocols or applications.
Provides multicast and broadcast for support of LAN applications that require this capability.
Design allows for relatively easy scaling.

Limitations

Potential limitations of LANE include the following:

Compared to RFC 1577 and RFC 1483 protocols, LANE is relatively complex to configure.
Redundancy is problematic across vendors. Even with redundancy, there is a reaction time to switchover.
Can be difficult to troubleshoot.
Provides no QoS support.
The load on LANE services, such as the number of nodes in an emulated LAN and the total number of emulated LANs, should be monitored. Extremely heavy demand can degrade network performance.

LANE Configuration Overview

Before you begin to configure LANE, you must decide whether you want to set up one or multiple emulated LANs. If you set up multiple emulated LANs, you must also decide where the servers and clients will be located, and whether to restrict the clients that can belong to each emulated LAN.

You can create a LANE plan and worksheet, as described in the "Creating a LANE Plan and Worksheet" section to assist you in the configuration. Configuring LANE involves the following steps:

Step 1 Decide where you want to put the LECS and LES/BUS.

In Cisco's implementation, the LES and BUS must remain together. However, the LES/BUS for different emulated LANs could be on different devices; this arrangement will probably yield better performance, but it is much easier to manage if they are all left on the same device. The LECS also does not have to be on the same device as the LES/BUS.

Note

Step 2 Determine the LANE default addresses.

Display the LANE default addresses for each router or switch that is running any of the LANE services and write down the displayed addresses on your worksheet. On the ATM switch router, and other devices that run the Cisco IOS, use the
show lane default-atm-addresses command to display the default addresses.

Step 3 Enter the ATM address of the LECS.

You must enter the ATM address of the LECS into the ATM switch routers (and other LANE client devices in the LANE cloud) and save it permanently, so that the value is not lost when the device is reset or powered off. The LECS address can be specified for the entire ATM switch router, or per port.

Step 4 Set up the LECS database.

After you have determined all LESs, BUSs, and LECs on all ATM subinterfaces on all routers and switches that will participate in LANE, and have displayed their ATM addresses, you can use the information to populate the LECS database.

You can set up a default emulated LAN, whether or not you set up any other emulated LANs. You can also set up some emulated LANs with restricted membership and others with unrestricted membership.

Note

(a) Set up the database for the default emulated LAN only.

When you configure an LECS for one default emulated LAN, you provide the following information:

A name for the database
The ATM address of the server for the emulated LAN
The ring number of the emulated LAN (for Token Ring)
A default name for the emulated LAN

Because you are setting up the LECS database for a single default emulated LAN, you do not have to provide any entries that link the ATM addresses of any clients with the ELAN name.

(b) Set up the database for unrestricted-membership emulated LANs.

When you set up a database for unrestricted emulated LANs, you create database entries that link the name of each emulated LAN to the ATM address of its LES. It is not necessary to specify the clients that can participate in the emulated LAN. That is, when you set up the LECS database, you do not have to provide any database entries that link a LEC with an ELAN name.

When you set up the database for restricted-membership emulated LANs, you create database entries that link the name of each emulated LAN to the ATM address of its LES. However, you also must specify where the LANE clients are located. That is, for each restricted-membership emulated LAN, you provide a database entry that explicitly links the ATM address or MAC address of each LEC of that emulated LAN with the name of that emulated LAN.

Those client database entries specify the clients that are allowed to join the emulated LAN. When a client requests that the LECS indicate which emulated LAN it is to join, the LECS consults its database and then responds as configured.

When clients for the same restricted-membership emulated LAN are located in multiple routers, each client's ATM address or MAC address must be linked explicitly with the name of the emulated LAN. As a result, you must configure as many client entries as you have LECS for emulated LANs in all the routers. Each client must have a different ATM address in the database entries.

Step 5 Enable the LECS.

After you create the database entries appropriate to the type and to the membership conditions of the emulated LANs, you enable the configuration server on the selected ATM interface, router, or switch, and specify that the LECS ATM address is to be computed automatically.

Note

Step 6 Set up the LES/BUS.

For one default emulated LAN, you must set up one set of servers: one as a primary server and the rest as backup servers for the same emulated LAN. For multiple emulated LANs, you can set up servers for another emulated LAN on a different subinterface on the same interface of this router or switch, or you can place the servers on a different device.

Note

Each emulated LAN is a separate subnetwork. Make sure that the clients of the same emulated LAN are assigned protocol addresses on the same subnetwork, and that clients of different emulated LANs are assigned protocol addresses on different subnetworks.

Step 7 Set up the LECs on subinterfaces.

Where you put the clients is important, because any router with clients for multiple emulated LANs can route frames between those emulated LANs.

On any given router or switch, you can set up one client for one emulated LAN or multiple clients for multiple emulated LANs. You can set up a client for a given emulated LAN on any routers you select to participate in that emulated LAN. Any router with clients for multiple emulated LANs can route packets among those emulated LANs.

Note

Creating a LANE Plan and Worksheet

A paper plan and LANE worksheet can be helpful in configuring LANE. Record the following information, leaving spaces for the ATM address of each LANE component on each subinterface of each participating router or switch:

The component and interface where the LECS will be located.
The component, interface, and subinterface where the LES/BUS for each emulated LAN will be located. Each emulated LAN can have multiple servers for fault-tolerant operation.
The component, interfaces, and subinterfaces where the clients for each emulated LAN will be located.
The component and database name of the default database.
The name of the default emulated LAN (optional).
The names of the emulated LANs that have unrestricted membership.
The names of the emulated LANs that have restricted membership.

The last three items in this list are very important; they determine how you set up each emulated LAN in the LECS database.

Example LANE Plan and Worksheet

Figure 6-6 shows a single emulated LAN example network.

Figure 6-6: LANE Plan Example Network

The following sample worksheet describes the LANE plan in Figure 6-6:

LECS:
---Location: ATM_Switch
---Interface: atm0
---ATM address: 47.00918100000000E04FACB401.00E04FACB405.00
LES:
---Location: Switch_1
---Interface/Subinterface: atm0.1
---Type: Ethernet
---ATM address: 47.00918100000000E04FACB401.00E04FACB403.01
BUS:
---Location: Switch_1
---Interface/Subinterface: atm0.1
---Type: Ethernet
---ATM address: "use default"
Database:
---Location: ATM_Switch
---Name: eng_dbase
---ELAN name: eng_elan
---Default ELAN name: eng_elan
---ATM address: 47.00918100000000E04FACB401.00E04FACB403.01
LANE Client:
---Location: ATM _Switch
---Interface/Subinterface: atm0.1
---Server/BUS name: eng_elan
---IP Address/Subnet mask: 172.16.0.4 255.255.0.0
---Type: Ethernet
LANE Client:
---Location: Switch_1
---Interface/Subinterface: atm 0.1
---Server/BUS name: eng_elan
---Type: Ethernet
LANE Client:
---Location: Router_1
---Interface/Subinterface: atm 3/0.1
---Server/BUS name: eng_elan
---IP Address/Subnet mask: 172.16.0.1 255.255.0.0
---Type: Ethernet

Note VLANs need to be configured on the LAN edge switches. These VLANs must be mapped to the appropriate emulated LANs.

SSRP for Fault-Tolerant Operation of LANE Server Components

Cisco's LANE implementation includes the Simple Server Redundancy Protocol (SSRP), a feature that provides fault tolerance using standard LANE protocols and mechanisms. If a failure occurs on the LECS or on the LES/BUS, the emulated LAN can continue to operate using the services of a backup server.

Note SSRP is a Cisco proprietary protocol; the redundancy feature works only with Cisco LECSs and LES/BUS combinations. Third-party LANE components continue to interoperate with the LECS and LES/BUS function of Cisco routers, but cannot take advantage of the redundancy features.

How It Works

SSRP provides redundancy through multiple LECS and LES/BUS components in the LANE cloud, as follows:

LECS redundancy---uses a master-backup scheme for a given set of emulated LANS.
- There is one master LECS; there can be multiple backup LECS.
- The databases of all LECS must be identical; that is, they must include the same LES addresses and corresponding ELAN names. The LECS turns on server redundancy by adjusting its database to accommodate multiple LES addresses for a particular emulated LAN. The additional servers provide backup for that emulated LAN.
- LECs maintain multiple LECS addresses via ILMI; if the master LECS fails, a backup responds. When a LECS switches over, no previously joined clients are affected.
LES/BUS redundancy---uses a master-backup scheme for a given emulated LAN.
- The LECS always keeps an open VCC with each LES/BUS. In the case of an LES/BUS failure, the LECS establishes a connection with the next LES/BUS serving that emulated LAN.
- When a LES/BUS switches over, momentary loss of clients occurs if any of the control VCCs go down. They then reinitialize and are all transferred to the new LES/BUS.

Configuration Overview

Configuring SSRP for LANE requires the following steps:

Step 1 Configure LES/BUS pairs on the switches and routers where you want to place these servers. There is no limit on the number of LES/BUS pairs you can configure per emulated LAN.

Step 2 Configure the LECS database on one system, making sure you include all the LES server addresses and corresponding ELAN names. Enter them in the order of priority, so that the first one is your master LES, while the others serve as backups.

Step 3 Configure backup LECSs; you can have up to 16. To ensure that the database contents are the same, copy the entries from the master, configured in Step 2, to each of the backup LECSs.

Step 4 Enter the addresses of the LECSs on the client devices in the identical order of priority on each system.

SSRP is supported in Cisco IOS Release 11.2 software and later, and is enabled automatically when you configure multiple LES/BUS and LECS components. Older LANE configuration files continue to work with this new software. LANE configurations that network with non-Cisco ATM equipment continue to work, but the non-Cisco ATM equipment cannot participate in the LANE simple server redundancy.

Other Considerations

You should be aware of the following operational details of SSRP when configuring redundancy:

Up to 16 LECS addresses can be handled by the LANE subsystem.
There is no limit to the number of LESs that can be defined per emulated LAN.
When a LECS switches over, no previously joined clients are affected.
When a LES/BUS switches over, clients are momentarily lost until they are transferred to the new LES/BUS.
LECSs come up automatically as masters until a higher-level LECS tells them otherwise.
By default, when a higher-priority LES comes online, it does not preempt the current LES on the same emulated LAN. However, a higher-priority LES configured as preemptable does bump the current LES on the same emulated LAN when the LES comes online. In that case, there might be some changing of clients from one LES to another after a powerup, depending on the order of the LESs coming up. Changing should settle after the last highest priority LES comes up.
If none of the specified LESs is up or connected to the master LECS, and more than one LES is defined for an emulated LAN, a configuration request for that specific emulated LAN is rejected by the LECS.
Changes made to the list of LECS addresses on ATM switch routers can take up to a minute to propagate through the network. Changes made to the configuration database regarding LES addresses take effect almost immediately.
Overriding any of the LECS addresses can cause SSRP to become nonoperational. To avoid affecting the fault-tolerant operation, do not override any LECS, LES, or BUS addresses.
If an underlying ATM network failure occurs, there might be multiple master LECSs and multiple active LESs for the same emulated LAN. This situation creates a "partitioned" network. The clients continue to operate normally, but transmission between different partitions of the network is not possible. When the network break is repaired, the system recovers. This, however, is not a problem particular to LANE but would occur whenever a breakage occurs in the ATM network.
Server redundancy guards against the failure of the hardware on which server components are running. This includes all the ATM interface cards in our routers and Catalyst switches. Fault tolerance is not effective for ATM network as a whole or for other switch or LAN failures.

Multiprotocol over ATM

With LANE, connectivity between hosts in different emulated LANs is possible only by traversing a router. With heavy inter-ELAN traffic, this can lead to congestion at the router and increased latency.

Multiprotocol over ATM (MPOA) relieves the router bottleneck for inter-ELAN traffic by adding "cut-through" routing to existing LANE capability. (Intra-ELAN traffic continues to be serviced by LANE alone.) With cut-through routing, based on the Next Hop Resolution Protocol (NHRP), inter-ELAN traffic with significant flow (described later in this section) can avoid going through the router, a normal requirement of LANE, and can be switched via a direct connection through the ATM network.

In addition to the performance enhancement MPOA provides, there is the additional benefit of QoS support for features such as packetized video. IP's Resource Reservation Protocol (RSVP) parameters can be mapped to ATM's QoS parameters to take advantage of ATM's traffic contract.

An MPOA-enabled network uses the following components:

Routers---run their conventional routing and discovery protocols, while also providing multicast forwarding between VLANs and forwarding on behalf of LANE-only clients. Routers can also forward short-lived "flows," such as DNS or SMTP queries from MPOA clients, also called MPCs.
Edge devices---forward packets between an ATM backbone and LANs. Edge devices can serve as an MPOA client, as can a LEC. ATM-attached hosts or servers can also contain an MPOA client. The edge device is usually a LAN switch with a LANE interface.
MPOA server (MPS)---is responsible for responding to queries from the MPOA client to resolve IP-to-ATM addresses.

How it Works

Figure 6-7 illustrates an ATM network with four emulated LANS and attached routers. Using LANE only, a packet sent from the LEC on ELAN 1 to the LEC on ELAN 4 has to go through four routers.

Figure 6-7: Multiple Emulated LANs with Router Congestion

The following steps describe the stages of an MPOA connection between ELAN 1 and ELAN 4:

Step 1 The first time traffic needs to be forwarded from the ingress MPOA client to the egress MPOA client, it is forwarded over the routers. This method ensures that both classical bridging and inter-VLAN routing operations are preserved and are always available.

Step 2 The MPOA client determines where there is a "significant flow." Significant flow means that a certain number of packets (ATM Forum default is 10) are sent to the same destination in a given time (ATM Forum default is 1 second).

Step 3 If a significant flow is detected, an MPOA query is initiated. To set up a direct "cut-through" connection, the edge devices (or MPOA clients) must obtain the ATM address of the exit point that corresponds to the respective Layer 3 destination address. To obtain this information, the MPOA client sends an MPOA query to the MPOA server at each hop. Meanwhile, the MPOA client continues sending data traffic to the default forwarder (the router) while it waits for a reply. Query between the MPOA servers is NHRP-based.

Step 4 Before the MPOA server at the egress router replies, it performs a cache imposition information exchange with the edge device where the destination is attached. A cache imposition helps to ensure reliable operation, validates forwarding information, and, optimally, provides information used to increase forwarding performance in the MPOA clients.

Step 5 The MPOA server can then respond to the MPOA query with the ATM address of the exit point or ATM-attached host used to reach the destination Layer 3 address.

Step 6 When the reply arrives at the source MPOA client, it sets up a direct inter-ELAN cut-through ATM connection.

Advantages

MPOA offers the following key advantages:

Like LANE, on which it is based, requires no modification to upper layer applications.
Reduces latency caused by multiple router hops for inter-ELAN traffic.
Provides for QoS support via RSVP.
Can use Cisco SSRP for LANE redundancy with added redundancy at the router level using the Hot Standby Router Protocol (HSRP).
Can be implemented incrementally, adding MPOA in areas where it is needed. The entire network does not have to be upgraded at the same time.

Limitations

The following might be limitations to MPOA, depending upon your needs:

Like LANE, is appropriate only for LAN, not WAN.
Supports only IP unicast.

Configuration Overview

MPOA actually builds upon the LANE infrastructure. The LECS on your ATM switch router supports the MPOA client. Beyond LANE configuration, no specific configuration of MPOA on the ATM switch router is required. The ATM router module does not support MPOA.

Table of Contents

LAN Emulation and MPOA

Using ATM Address Templates

Example LANE Plan and Worksheet

Configuration Overview

Other Considerations