Abstract:
A method is implemented by which a Logical IP subnetwork attached (LIS) Next Hop Resolution Protocol (NHRP) client or an Emulated LAN (ELAN) subnetwork attached NHRP client in an Asynchronous Transfer Mode (ATM) network can send data to a destination LAN Emulation Client (LEC) on a different ISO layer-3 subnetwork via a data-direct Virtual Channel Connection. The method enables such a IP subnetwork attached NHRP clients to obtain the ATM address corresponding to the destination station by using the vendor-private extension within an NHRP packet. The NHRP client uses the ATM address to build a data-direct VCC over which it unicasts data bound for the destination station. The NHRP client appears to the destination LEC as any other LEC on its ELAN would appear.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application for patent is related to the following application for patent assigned to a common assignee: 
     ROUTE SWITCHING MECHANISMS FOR SOURCE-ROUTED ATM NETWORKS, Serial No. 081,977,459 (RA9-97-066). 
     This cross-referenced application is hereby incorporated by reference into this Application as though fully set forth herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to data processing networks, and in particular, to Asynchronous Transfer Mode networks. 
     BACKGROUND INFORMATION 
     Asynchronous Transfer Mode (ATM) is an emerging network technology that is designed to transport information between communicating stations in a point-to-point fashion. The interest in ATM is its promise of high bandwidths and quality of service. ATM is a connection oriented architecture, in contrast to network architectures that are structured to broadcast data from the source to the destination. In ATM, the source negotiates a connected path to the destination before it proceeds to transmit its information to the recipient. ATM protocols (or “rules,” usually implemented in software) define the communications necessary to establish the connection. An ATM attached device has an ATM address in addition to any other network addresses it might have, depending on the particular ATM configuration within which it is incorporated. Some possible configurations will be described subsequently. Once a connection is established, the source station transmits its data only to the destination (a “unicast”). 
     In contrast to connection oriented architectures are broadcast networks. In these, data is sent from a source station to a destination station by broadcasting it to all addresses where the recipient plucks it off the network while the other stations on the network ignore traffic not bound for them. Broadcast architectures provide one motivation for structuring a “network” as a set of interconnected subnetworks or “subnets.” 
     In a large network, the proliferation of broadcast packets would overwhelm the network. Another simply reflects the nature of the pattern of growth of network communication generally. Although a particular network may start out as a freestanding Local Area Network (LAN), eventually end-station users will probably want to avail themselves of the services available on other networks, and look to connect “their network” with other “networks.” When this occurs, it is intuitive, as well as more precise, to view the resulting network structure as a set of subnets within a larger network, for example, an “internetwork.” However, a station on one internetworking subnet that wishes to communicate with a destination on another subnet can only do so if there is connectivity between the subnet in which the source resides and the subnet in which the destination resides. 
     Communications methodologies between subnets are usually termed to as “layer-3” protocols. This refers to the layered architecture networking model of the International Standards Organization (ISO). This model is illustrated in FIG.  1 . Layer-3 may sometimes be referred to as the “network” layer, and is equivalent to the “internetworking” layer in the TCP/IP model. 
     Connectivity between layer-3 subnets is provided by a device termed a “router.” When a source station on one layer-3 subnet wishes to communicate with a destination station on another layer-3 subnet, it broadcasts the data in the usual way. However, now it is the router that plucks the data packets off the first subnet and forwards it to the destination station via the destination station&#39;s layer-3 subnet to which the router is also attached. 
     Numerous types of networks coexist in the data communications industry. In addition to ATM, there may be LANs, Wide Area Networks (WANs), and others. There is a need in the industry for interconnection between different network architectures and, in particular, users of preexisting LANs have a need to connect to emerging high speed network technologies, such as ATM. The need for incorporating or interfacing preexisting networks (more precisely subnetworks) into an ATM environment has led to the specification of several methodologies to support preexisting network architectures within ATM. 
     One such methodology is the Logical IP Subnetwork (LIS). In LIS, the ATM serves as the direct replacement for the “wires” and LAN segments connecting traditional “layer-3 ” protocol source and destination stations (collectively, end-stations) and routers. In the traditional internetworking architecture, one or more LANs may be grouped into internetworking, or layer-3 , subnetworks, hereafter referred to as “subnets.” Within a layer-3 subnetwork, end-stations communicate with each other by broadcasting traffic as described previously. 
     A LIS is simply a layer-3 subnet within an ATM network. When a station on one LIS wishes to communicate with another station in the same LIS it does so by using ATM protocols. In order to do this, it must learn the ATM address of the destination. It does this through the medium of an ATM Address Resolution Protocol Server which resolves the layer-3 address of the destination into its ATM address. The source station sends a request, an ATM Address Resolution Protocol request (ATMARP_Request), to a server in the LIS that provides the ATM address of the destination based on its layer-3 address. The layer-3 address is the address by which the destination is identified with respect to its internetworking subnet. By contrast, communication between a source on one LIS and a destination on a second LIS proceeds just as if the communicating devices were traditional LANs residing on different internetwork subnets. The traffic is sent to a router based on the layer-3 address of the destination. The router then forwards the data through the appropriate destination LIS, or to a second router if the router on the source LIS is not a member of the destination LIS. 
     Another methodology is the emulated LAN (ELAN) which simulates classical LAN protocols in an ATM environment. (Classical LAN protocols, for example Ethernet and Token Rings, are referred to as legacy LANs.) The protocols that provide the specification for ELANs are called LAN emulation (LANE). Layer-3 protocols run on top of ELANs just as they do in legacy LANs. Hosts attached to the ELAN include emulation software that allows them to simulate legacy LAN end stations. Such hosts are called LAN Emulation Clients (LEC). The LEC software hides the ATM from the LAN protocols within the LEC device, and a LEC can utilize those protocols as if it were a legacy LAN. A LEC can also provide a standard LAN service interface to a layer-3 entity in the same layer-3 subnet. Such a LEC is a LAN Switch that is usable to interface a legacy LAN with an ELAN. 
     Communication between LECs on an ELAN can be effected over the ATM. Each LEC has a physical, or Media Access Control (MAC) address associated with it, as well as an ATM address. For one LEC on a ELAN to communicate with another, it must obtain the ATM address of the destination LEC, given the destination MAC address. This address resolution is mediated through a LAN Emulation Server (LES). The source LEC issues a LANE Address Resolution Protocol Request (LE_ARP_Request) to the LES. Provided the destination station has previously registered its MAC address, ATM address pair with the LES serving the ELAN, the LES returns the ATM address of the destination to the requesting LEC in an ELAN Address Resolution Protocol Reply (LE_ARP_Reply). The source LEC can then use the ATM address to establish a connection to unicast data to the destination, a so-called data-direct Virtual Channel Connection (VCC), and transmit its data to the destination thereon. 
     Should the destination not have registered with the LES, the source communicates with the destination using conventional LAN methodology. This is mediated through a Broadcast and Unknown Server (BUS). The LEC sends its data to the BUS which then broadcasts it. Just as in a legacy LAN, the broadcast data is plucked from the network by the destination station, and is ignored by the other devices on the network. Exactly the same process is used if the destination is on a subnetwork, either a legacy LAN or an ELAN, in a different layer-3 subnet. In that case, the broadcast data is gathered by a router connected to the ELAN and forwarded via layer-3 protocols to the destination, as described hereinabove, just as if the ELAN were a legacy LAN. An ELAN forming a layer-3 subnetwork will be referred to as an ELAN IP subnetwork (or, equivalently, subnet). 
     Communication between a LIS attached station and an ELAN attached station must also proceed by router-connected paths. Without a mechanism for resolving the ATM address of a destination station, a source station attached to a LIS cannot exploit the ATM network to which it is attached, and data packets are sent to a router connecting the layer-3 subnets which forwards them via layer-3 protocols to the layer-3 subnet in which the destination station resides. The routed path through the network may involve forwarding the data from source to destination through one or more routers. A forwarding event will hereinafter be referred to as a “hop.” Having data proceed from the source to the destination in this way, via a series of hops, thus subverts the benefits of the ATM switched infrastructure because the source station is not exploiting the ATM connection-oriented infrastructure. 
     A method in the art to bypass a hop is provided by the Next Hop Resolution Protocol (NHRP). NHRP is a protocol that allows a source station, such as a host, connected to an ATM subnetwork to determine the ATM address of the next hop associated with a destination station. The next hop may be the destination station itself or a router “nearest” the destination station that provides egress from the ATM network to, an LAN attached destination host, for example. An NHRP client is a device that has NHRP functionality, or capability, included in the software it contains, and initiates requests to access NHRP service. NHRP service is provided by an NHRP server (NHS), an entity having NHRP functionality performing NHRP services. NHRP is presently part of the specification for Multiprotocol Over ATM, Version 1.0. NHRP is Annex C, ATM Forum, AF-MPOA-0087.000, July 1997, prior art hereby incorporated herein by reference. 
     A source that is an NHRP client within a LIS or an ELAN IP subnetwork can bypass a hop provided the destination meets three requirements. The destination must support NHRP, it must support a Virtual Channel Connection (VCC) as defined by Request for Comments  1483  (RFC- 1483 ), and the destination must support the NHRP registration function for NHRP clients. Unless the destination satisfies all of these, the NHRP will not bypass hops. RFC- 1483  is prior art, and is hereby incorporated by reference herein. 
     Thus, there is a need in the art for a method by which an NHRP client in a LIS or an ELAN IP subnetwork may bypass one or more hops when the destination fails to meet all of the aforesaid conditions. One such instance occurs when one of the communicating devices is a LAN Emulation (LANE) version- 1  client. In ATM networks, a set of services, functional units and protocols are provided that define a standard for LANE. LANE version- 1  does not support RFC- 1483  VCCs. However, the need is not limited to such a situation. The method of bypassing a hop should be able to exploit the high speed switching characteristics of the ATM infrastructure by establishing a direct layer-2 connection between the NHRP client and the LAN (legacy or emulated) destination. Moreover, in the case of an ELAN destination, the method should be independent of the implemented LANE version. 
     SUMMARY OF THE INVENTION 
     The previously mentioned needs are addressed by the present invention. The invention permits a LIS resident, or ELAN IP subnet resident, device that is an NHRP client to establish a layer-2 connection with either an ATM attached device of an ELAN or a non-ATM attached destination residing on a legacy LAN via a bridge. 
     The NHRP client effects the layer-2 connection by using the vendor-client extension provided in the NHRP packet structure. On receipt of an NHRP resolution request, an NHRP server gathers the same information that it would need in order to build a connection to the target destination. The NHRP server then returns this information to the NHRP client that issued the NHRP resolution request. The information is returned in the vendor-private extension that is included in the NHRP resolution reply. The NHRP client uses this information to establish a connection to the destination either directly or through a proxy in the event that the destination resides on a legacy LAN. 
     In an embodiment requiring multiple hops, only the ELAN attached router need recognize NHRP vendor-private extensions. NHRP requires all routers to forward all NHRP vendor-private extensions. Thus, from the perspective of the requesting NHRP client, there is no difference between a single hop route and one including multiple hops. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 schematically depicts the International Standards Organization (ISO) Reference Model of Open System Interconnection in accordance with the prior art. 
     FIG. 2 schematically illustrates an embodiment of a data processing network using a method of the present invention. 
     FIG. 3 schematically illustrates an alternative embodiment of a data processing network using a method of the present invention. 
     FIG. 4 illustrates a flow diagram in accordance with a method of an embodiment of the present invention. 
     FIG. 5 schematically illustrates a vendor-private extension for LANE shortcuts in accordance with an embodiment of the present invention. 
     FIG. 6 illustrates a data processing system configured in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     An invention that alleviates the necessity of layer-3 routing in an ATM network by a legacy LAN attached device that is an NHRP client will now be described in detail. Refer now to FIG. 2 in which is depicted an embodiment of a data processing network, ATM Network  200 , using the present invention. NHRP Client  201  resides on LIS subnet  202  within ATM Network  200 . As described hereinabove, an ATM attached device that is LIS attached is native to ATM. In an embodiment of the present invention, the NHRP functionality may be incorporated in software resident in a protocol stack, here, Protocol Stack  204  within ATM Host  205 . ATM Host  205  is ATM attached within LIS  202 . In an embodiment of the present invention, ATM Host  205  may be a workstation, for example. 
     In an embodiment using the prior art, traffic from NHRP Client  201  bound for a destination residing in a different layer-3 subnet, here ELAN  206 , must pass through Router  207  which provides layer-3 connectivity, that is, forwards packets based on layer-3 information, between ELAN  206  and LIS  202 . 
     The destination may be a LAN Emulation Client (LEC), such as LEC  208  or a proxy LEC, such as LAN Switch  209 . It has an ATM address as well as a Media Access Control (MAC) address, that is, the physical network address of the device. Here, LEC  208  is attached to ELAN  206 . In the embodiment of FIG. 2, LAN Switch  209  bridges ELAN  206  and LAN  210 . In either case, the destination may not support NHRP, may not support inbound traffic over VCCs using the protocol defined in RFC  1483 , or may not have registered with an NHRP server. Then, NHRP Client  201  is unable to build an ATM connection to the destination or its proxy LEC if the destination is LAN resident. Thus, even though LIS  202  and ELAN  206  are subnets within the same ATM network, ATM  200 , devices on either subnet cannot exploit ATM to communicate with devices on the other subnet because devices on one are only aware of the layer-3 addresses of the devices on the other. 
     The present invention utilizes the vendor-private extension within an NHRP Resolution Request enable NHRP Client  201  to establish a LANE data-direct VCC. An NHRP Resolution Request is embodied in a data packet having a structure defined in the NHRP. An NHS issues an NHRP Resolution Reply in response to an NHRP Resolution Request. The NHRP Resolution Reply is embodied in a data packet also having a structure defined in NHRP. The vendor-private extension is one of a set of extensions within the NHRP that both an NHRP Resolution Request and a NHRP Resolution Reply may include. If a NHRP Resolution Request includes a vendor-private extension, the NHRP Resolution Reply generated in response must also contain one. Otherwise, extensions are optional portions of the NHRP request and reply packets. 
     NHRP Client  201  issues an NHRP Resolution Request that includes the vendor-private extensions for LANE Shortcuts. An NHS must forward any NHRP extensions that are included in an NHRP packet. The NHRP Resolution Request is transmitted toward the destination via layer-3 protocols, that is, using the internetwork address of the destination. In FIG. 2, the NHRP Resolution Request is routed through NHS/Router  207 . 
     On receiving the NHRP Resolution Request, NHS/Router  207  gathers the same information that it would need in order to establish a connection to the destination station and provides it to NHRP Client  201 . NHS/Router  207  issues requests for the addresses associated with the destination station: an ARP_Request for the MAC address, and an LE_ARP_Request for the ATM address. 
     NHS/Router  207  builds a reply packet including vendor-private extensions for LANE Shortcuts containing these addresses, and sends the reply packet to NHRP Client  201 . Vendor-private extensions for LANE Shortcuts contain the information NHRP Client  201  needs to establish to a VCC the destination and transfer data to it. The structure of a vendor-private extension for LANE shortcuts will be described subsequently. 
     NHRP Client  201  then transmits data directly to the ATM address associated with the destination station using layer-2 protocols. NHRP Client  201  uses the ATM address provided it in the vendor-private extension in the NHRP Resolution Reply sent by NHS/Router  207  to establish a data-direct VCC, or LANE Shortcut, to the ATM address associated with the destination station. A data-direct VCC is specified in LANE, and defines how data being sent is encapsulated, and how the VCC is established. The ATM address may be associated with an ATM attached station, such as LEC  208 , if the data is bound for an ATM attached device. However, if the data to be sent by NHRP Client  201  is destined for a station not a LEC, that is it is not attached to the ATM, the ATM address associated with the destination would be that of a bridge device, such as LAN Switch  209 , serving as a proxy LEC. Nevertheless, in either case, the NHRP client, such as NHRP Client  201 , appears to the destination LEC as any other LEC otherwise on ELAN  206  would appear to the destination LEC. 
     In an alternative embodiment of the present invention, LIS  202  may be an ELAN IP subnet. The method of the present invention is unchanged in such an embodiment. In the interest of clarity and simplicity, ATM network  200  has been schematically depicted in an embodiment in which the NHRP client is LIS attached. 
     It is an advantage of the present invention that the only devices that need support NHRP are the source station seeking to establish a LANE Shortcut and the NHS that responds. This can be better appreciated by referring to FIG. 3 in which an alternative embodiment of a data processing network, ATM  300 , using the present invention is depicted. As previously described, NHRP Client  301 , on LIS  302 , issues an NHRP Resolution Request for the ATM and MAC addresses associated with the destination station. The NHRP Resolution Request packets containing the vendor-private extensions are forwarded through LIS  303  and LIS  304  by Router  305  and Router  306  respectively. Router  305  provides layer-3 connectivity between LIS  303  and LIS  302  to which NHRP Client  301  is attached. 
     The packets are received by NHS/Router  307  which gathers the ATM and MAC addresses associated with the destination station as described hereinabove. These addresses are included in vendor-private extensions for LANE Shortcuts contained in the NHRP Resolution Reply prepared by NHS/Router  307  and addressed back to NHRP Client  301 . The NHRP Resolution reply packets are routed back to NHRP Client  301  through LIS  303  via Router  306  and LIS  303  via Router  305 . Neither Router  305  nor Router  306  need support NHRP because NHRP requires all routers to forward all NHRP vendor-private extensions. In the embodiment of FIG. 3, NHS/Router  307  is an egress router. That is, it is the router that “delivers” data packets to the layer-3 subnet including the destination. Only the egress router, here NHS/Router  307  which is attached to ELAN  308 , must recognize and support vendor-private extensions. 
     Moreover, the ELAN attached destination, such as a destination attached to ELAN  308 , does not need to support NHRP. It only responds to an ARP_Request and an LE_ARP_Request. Thus, routers, such as Router  305  and Router  306 , implementing intermediate hops, those between layer-3 subnets in which neither the source nor destination reside, return the vendor-private extension in the NHRP Resolution Reply to the requesting NHRP client, such as NHRP Client  301 , and the NHRP client establishes the data-direct VCC to the ATM address associated with the destination station as previously described. Then, data bound for an ELAN attached device, such as LEC  309  or LAN  310  via LAN Switch  311  on ELAN  308 , is sent using layer-2 protocols as discussed hereinabove. 
     The invention may be better understood by referring now to FIG. 4 in which a flow chart depicting a method of bypassing router hops is shown. The method is initiated when an NHRP client, for example NHRP Client  301 , has data to be sent to a destination on a different layer-3 subnetwork, step  400 . The NHRP client issues an NHRP Resolution Request, step  401 . The NHRP Resolution Request is transmitted via layer-3 protocols toward the destination, and each receiving router must determine whether the NHRP Resolution Request packets are bound for a destination in its layer-3 subnetwork, step  402 . If the destination is not, the router forwards the packet to the next hop, step  403 . If the destination is in the router&#39;s layer-3 network, then the router, through its NHS functionality, ascertains the layer-2 addresses required by the requesting NHRP client. It does this by issuing an ARP_Request for the MAC address associated with the destination host, step  404 . The NHS/router then issues an LE_ARP_Request for the ATM address associated with the destination host, step  405 . These addresses may be associated with an ATM attached host, such as LEC  208  or LEC  309 , or a proxy LEC such as LAN Switch  209  or LAN Switch  311  if the destination is legacy LAN attached. These layer-2 addresses are then incorporated into the vendor private extensions of NHRP Resolution Reply packets that are built by the NHS/Router, step  406 . The packets forming the NHRP Resolution Reply are then sent to the NHRP client that issued the NHRP Resolution Request, step  407 . In an embodiment of a network having multiple hops between source and destination, such as ATM  300 , the NHRP Resolution Reply packets are forwarded to the source station by intermediate routers, step  408 . The NHRP client receives the NHRP Resolution Reply, and establishes a data-direct connection, a LANE “shortcut,” to the ATM address received in the NHRP Resolution Reply, step  409 , and transmits its data via layer-2 protocols using the MAC address included in the NHRP Resolution Reply, step  410 . After all the data bound for the destination host has been transmitted, the process ends, step  411 . 
     In the context of the present invention, the destination host need not be the ultimate destination of the data packets. The destination host may be a LEC that forwards the data to an ultimate destination that is not otherwise reachable by a LANE shortcut. An embodiment of such a LEC may be an ELAN attached router “closest” to the ultimate destination which then forwards the data via layer-3 protocols to the ultimate destination. 
     In an embodiment of a data processing network, having a single hop between layer-3 subnetworks, for example, ATM  200 , the steps of determining whether the destination is in a current layer-3 subnetwork and forwarding to the next hop, steps  402  and  403  respectively, may be eliminated. 
     Embodiments of the present invention may be used with source routed ELANs. LANEs are specified both for emulation of source routed LANs, for example Token Rings, and nonsource routed LANs, such as Ethemets. The embodiments of the present invention heretofore discussed have focused on nonsource routed LAN architectures. In source routed LANs, packets being sent to a destination station contain a Routing Information Field (RIF) that includes a path from source to destination that is an ordered set of route descriptors, ring and bridge numbers, forming the route between source and destination station. An embodiment in which an ELAN, such as ELAN  206  or ELAN  308 , is source routed includes a set of route descriptors within the vendor-private extension for LANE shortcuts. Routers servicing source routed layer-3 subnetworks always build an RIF. Thus, in the present invention, a router/server building the vendor-private extension for LANE shortcuts will build an RIF and then incorporate it in the vendor-private extension for LANE shortcuts. This may be better appreciated by referring now to FIG. 5 in which a vendor-private extension for LANE shortcuts is schematically illustrated. 
     The structure of the vendor-private extension for LANE Shortcuts is depicted in FIG.  5 . In an embodiment of the present invention, vendor-private extension for LANE Shortcuts  500  includes seventeen fields having lengths as depicted in therein. Field  501  is the holding time associated with the LANE shortcut. If the connection is inactive longer than the holding time, it will be cleared. The next three fields contain data with respect to the ATM address associated with the destination station. These fields are defined in NHRP, incorporated herein by reference. The first, field  502  contains the ATM address type and length. Field  503  holds the ATM address and field  504  the ATM subaddress. The fifth and sixth fields contain the MAC address associated with the destination. Field  505  includes the first four bytes of the MAC address of the device to which the shortcut is to be established with the last two bytes contained in field  506 . The LAN type field,  507 , contains a designator denoting the LAN encapsulation type of the LAN in which the destination resides. The value “ 1 ” denotes a Token Ring, “ 2 ” signifies an Ethernet V 2  and “ 3 ” signifies an Ethernet DIX. The next two fields relate to the layer-3 protocol address, to which the data packets should be sent. The first of these fields,  508  contains the protocol address length, in bytes, and the second,  509 , contains the protocol address itself Field  510  holds the maximum frame size as defined for destination ELAN. The next field,  511 , contains the Maximum Transmission Unit (MTU) which is the maximum protocol packet size that can be sent over the shortcut VCC. The twelfth and thirteenth fields contain the MAC address of the source of the vendor-private extension for LANE shortcuts; the first four bytes are contained in field  512  and the last two bytes in field  513 . This is usually the MAC address of the egress NHS. The next fields are used if the ELAN emulates a source routed LAN, such as a Token Ring. The first of these,  514 , contains the control field used in a Routing Information Field of a source routed LAN. The remaining fields contain route designators, usually an ordered pair of ring and bridge numbers, the bridges providing the interface between rings. The embodiment illustrated in FIG. 5, vendor-private extension for LANE Shortcuts  500  includes three route designator fields, route designator  1 , field  515 , route designator  2 , field  516 , and route designator  3 , field  517 . Alternative embodiments may include up to fourteen route designators. 
     A representative hardware environment for practicing the present invention in such an embodiment is depicted in FIG. 6, which illustrates a typical hardware configuration of workstation  613  in accordance with the subject invention having central processing unit (CPU)  610 , such as a conventional microprocessor, and a number of other units interconnected via system bus  612 . Workstation  613  includes random access memory (RAM)  614 , read only memory (ROM)  516 , and input/output (I/O) adapter  618  for connecting peripheral devices such as disk units  620  and tape drives  640  to bus  612 , user interface adapter  622  for connecting keyboard  624 , mouse  626 , speaker  628 , microphone  632 , and/or other user interface devices such as a touch screen device (not shown) to bus  612 , communication adapter  632  for connecting workstation  613  to a data processing network, and display adapter  636  for connecting bus  612  to display device  638 . CPU  610  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  610  may also reside on a single integrated circuit. In one embodiment, a protocol stack, for example, Protocol Stack  204  of FIG. 2 may be stored in ROM  616 . In an alternative embodiment it may be located in a mass storage device, such as disk units  620 . If any application being executed on workstation  613  seeks to transmit data to a LEC on an ELAN in a different layer-3 subnetwork, for example, LEC  208  of FIG. 2, an instruction set representing the NHRP within the protocol stack would be loaded into RAM  614  from ROM  616  or disk units  620 , as appropriate. CPU  610  would then execute the instruction set in order to perform those steps of the NHRP that are within its scope. These steps would include step  402  of FIG.  4 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     While there have been described herein the principles of the invention, it is to be clearly understood to those skilled in the art that this description is made by way of example only and not as a limitation to the scope of the invention. Accordingly, it is intended, by the appended claims, to cover all modifications of the invention which fall within the true spirit and scope of the invention.