ATM Trunks

ATM Trunks

Asynchronous Transfer Mode (ATM) technology has the inherent capability to transport voice, video, and data over the same infrastructure. And because ATM does not have any collision domain distance constraints like LAN technologies, ATM deployments can reach from the desktop to around the globe. With these attributes, ATM offers users the opportunity to deploy an infrastructure suitable for consolidating what are traditionally independent networks. For example, some companies have a private voice infrastructure between corporate and remote offices. The business leases T1 or E1 services to interconnect private branch exchanges (PBXs) between the offices.

The company can deploy or lease a separate network to transport data between the offices. And finally, to support video conferencing, an ISDN service can be installed. Each of these networks has its own equipment requirements, maintenance headaches, and in many cases recurring costs. By consolidating all of the services onto an ATM network, as in Figure 8-12, the infrastructure complexities significantly reduce. Even better, the recurring costs can diminish. Most importantly, this keeps your employer happy.

Figure 8-12. Service Consolidation over an ATM Network

For those installations where ATM provides a backbone service (either at the campus or WAN levels), users can take advantage of the ATM infrastructure to trunk between Catalysts. By inserting a Catalyst LANE module, the Catalyst can send and receive data frames over the ATM network. The Catalyst bridges the LAN traffic onto the ATM network to transport the frames (segmented into ATM cells by the LANE module) through the ATM system and received by another ATM-attached Catalyst or router.

Catalysts support two modes of transporting data over the ATM network: LANE and MPOA. Each of these are covered in detail in other chapters. LANE is discussed in Chapter 9, “Trunking with LAN Emulation,” and Chapter 10, “Trunking with Multiprotocol over ATM,” covers MPOA operations. The ATM Forum defined LANE and MPOA for data networks. If you plan to use ATM trunking, you are strongly encouraged to visit the ATM Forum Web site and obtain, for free, copies of the LANE and MPOA documents. The following sections on LANE and MPOA provide brief descriptions of these options for trunking over ATM.


LANE emulates Ethernet and Token Ring networks over ATM. Emulating an Ethernet or Token Ring over ATM defines an Emulated LAN (ELAN). A member of the ELAN is referred to as a LANE Client (LEC). Each ELAN is an independent broadcast domain. An LEC can belong to only one ELAN. Both Ethernet and Token Ring networks are described as broadcast networks; if a station generates a broadcast message, all components in the network receive a copy of the frame. ATM networks, on the other hand, create direct point-to-point connections between users.

This creates a problem when a client transmits a broadcast frame. How does the broadcast get distributed to all users in the broadcast domain? ATM does not inherently do this. A client could create a connection to all members of the ELAN and individually forward the broadcast to each client, but this is impractical due to the quantity of virtual connections that need to be established even in a small- to moderately-sized network. Besides, each client does not necessarily know about all other clients in the network. LANE provides a solution by defining a special server responsible for distributing broadcasts within an ELAN.

In Figure 8-13, three Catalysts and a router interconnect over an ATM network. On the LAN side, each Catalyst supports three VLANs. On the ATM side, each Catalyst has three clients to be a member of three ELANs.

Figure 8-13. Catalysts in a LANE Environment Attached to Three ELANs

Within the Catalyst configurations, each VLAN maps to one ELAN. This merges the broadcast domains so that the distributed VLANs can intercommunicate over the ATM network. Figure 8-14 shows a logical depiction of the VLAN to ELAN mapping that occurs inside a Catalyst.

Figure 8-14. A Catalyst with Three LECs Configured to Attach to Three ELANs

You need the router shown in Figure 8-13 if workstations in one VLAN desire to communicate with workstations in another VLAN. The router can reside on the LAN side of the Catalysts, but this example illustrates the router on the ATM side. When a station in VLAN 1 attempts to communicate with a station in VLAN 2, the Catalyst bridges the frame out LEC 1 to the router. The router, which also has three clients, routes the frame out the LEC which is a member of ELAN 2 to the destination Catalyst. The destination Catalyst receives the frame on LEC 2 and bridges the frame to the correct VLAN port.


In most networks, several routers interconnect subnetworks. Only in the smallest networks is a router a member of all subnetworks. In larger networks, therefore, a frame can cross multiple routers to get to the intended destination. When this happens in an ATM network, the same information travels through the ATM cloud as many times as there are inter-router hops. In Figure 8-15, a station in VLAN 1 attached to Cat-A desires to communicate with a station in VLAN 4 on Cat-B. Normally, the frame exits Cat-A toward Router 1, the default gateway. Router 1 forwards the frame to Router 2, which forwards the frame to Router 3. Router 3 transfers the frame to the destination Cat-B.

This is the default path and requires four transfers across the ATM network, a very inefficient use of bandwidth. This is particularly frustrating because the ATM network can build a virtual circuit directly between Cat-A and Cat-B. IP rules, however, insist that devices belonging to different subnetworks interconnect through routers.

Figure 8-15. Catalysts in an MPOA Environment

MPOA enables devices to circumvent the default path and establish a direct connection between the devices, even though they belong to different subnets. This shortcut path, illustrated in Figure 8-15, eliminates the multiple transits of the default path conserving ATM bandwidth and reducing the overall transit delay.

MPOA does not replace LANE, but supplements it. In fact, MPOA requires LANE as one of its components. Intra-broadcast domain (transfers within an ELAN) communications use LANE. MPOA kicks in only when devices on different ELANs try to communicate with each other. Even so, MPOA might not always get involved. One reason is that MPOA is protocol dependent. A vendor must provide MPOA capabilities for a protocol. Currently, IP is the dominant protocol supported. Another reason MPOA might not create a shortcut is that it might not be worth it. For MPOA to request a shortcut, the MPOA client must detect enough traffic between two hosts to merit any shortcut efforts.

This is determined by an administratively configurable threshold of packets per second between two specific devices. If the client detects a packets per second rate between an IP source and an IP destination greater than the configured threshold, the client attempts to create a shortcut to the IP destination. But if the packets per second rate never exceeds the threshold, frames continue to travel through the default path.

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