Abstract:
A method is implemented to enable networks employing source-route bridging to participate in route switched, Asynchronous Transfer Mode (ATM), networks. Such source-routed networks, for example, Token-Ring LANs, incorporate an end-to-end route description in the data packets transmitted by the source station. The end-to-end route description is contained in a Route Information Field (RIF). The method implemented herein associates an RIF with the ATM address corresponding to a destination station. This permits the source-route bridged network to exploit the efficiencies of layer- 2  connections and the high-speed switching characteristics of the ATM network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application for patent is related to the following application for patent assigned to a common assignee: 
     NEXT HOP RESOLUTION PROTOCOL CUT-THROUGH TO LANS, Ser. No. 08/968,354 filed Nov. 12, 1997 (RA9-97-068) 
     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 methods for source-route bridged networks to participate in route switched 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 traffic only to the destination. 
     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. 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 “intemetwork.” 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 “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 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 those protocols can utilize a LEC as if it were a legacy LAN. A LEC can also provide a standard LAN service interface in 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. 
     LANEs are also specified for emulation of source routed LANs, for example Token Rings, as well as nonsource routed LANs, such as Ethernets. 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. Operations performed on MAC address described hereinabove are correspondingly performed on the RIF in a source routed ELAN. 
     In the source-route bridged network, a source routed frame contains a RIF. The RIF includes an ordered list of ring and bridge numbers through which the frames are to pass from the source station to the destination station. Typically, the source station determines the route to the destination station by broadcasting an explorer frame. Bridges add the routing information to the RIF before forwarding the explorer frame. When the explorer frame reaches the destination, the destination station sends a response to the source station. The response contains the complete RIF that the source station then includes in subsequent frames addressed to that destination. Bridges make frame forwarding decisions based on the RIF. 
     Source routed LAN stations are connected to edge devices, for example LAN bridges, that bridge traffic between the legacy LAN ports and ELAN ports on the switched ATM network. However, traffic is still routed via the source routed path specified in the RIF, because the bridge does not have the information it needs in order to establish direct layer- 2  ATM connections. 
     In order for a network employing source route bridging to take advantage of the speed and efficiency associated with route switching, there is a need in the art for a mechanism to enable source-route bridged networks to participate in route switched networks. 
     SUMMARY OF THE INVENTION 
     The previously mentioned needs are addressed by the present invention, which enables a source-route bridged network to participate in a route switched network by incorporating a route resolution protocol in the network infrastructure. Route resolution client functionality is provided in the source station on a source-route bridged network. A route resolution request, issued by the source station, is used to determine the Media Access Control (MAC) address and the RIF associated with the layer- 3  protocol address of the destination station. 
     The source station then uses the supplied MAC address as the destination MAC address of frames bound for the associated layer- 3  protocol address. It also includes the supplied RIF in each of the frames sent to the destination. 
     The frames bound for the destination MAC address are then delivered using the normal layer- 2  procedures. This is accomplished by binding the layer- 2 , ATM, address associated with the destination MAC address to a virtual next-hop route descriptor that is embedded in the RIF included in each of the frames bound for the destination station. Consequently, the LEC associated with the ingress port of the source-route bridged network receives the ATM address of the edge device associated with the destination in response to its LE_ARP_Request. A data-direct virtual channel connection (VCC) is then established, and frames bound for the destination MAC address are delivered using normal layer- 2  procedures. 
     In a network having shortcut bridge connectivity between layer- 3  subnets, there is no need to introduce a virtual route descriptor bound to the ATM address associated with the destination MAC. The shortcut bridge functionality enables the ELAN segments in different layer- 3  subnets to appear as one collection or “super-ELAN.” The collection functions as a single ring and the associated ring number plays the role of the virtual route descriptor discussed above. Thus, the next-hop route descriptor in the merged RIF that is passed in the frame addressed to the destination MAC address is associated with the egress edge device, and that device&#39;s ATM address is provided in the LE_ARP_Response. The data frames are then passed using normal layer- 2  procedures as before. 
     These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to note the drawings are not intended to represent the only form 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 illustrates a data processing network according to the prior art. 
     FIG. 3 illustrates a data processing network incorporating an embodiment of the present invention. 
     FIG. 4 illustrates a flow diagram in accordance with a method of an embodiment of the present invention. 
     FIG. 5 illustrates a data processing network incorporating an alternative embodiment of the present invention. 
     FIG. 6 illustrates a data processing network incorporating another alternative embodiment of the present invention. 
     FIG. 7 illustrates a data processing system configured in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous descriptive 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. 
     Refer first to FIG. 2 in which is depicted an embodiment of a data processing network  200  according to the prior art. Data processing network  200  includes an ATM network  201  in which are embedded ELAN  202  and ELAN  203 . Data processing network  200  also includes source-routed networks, LAN  204  and LAN  205 . Communication between LAN hosts resident on LAN  204  and LAN hosts resident on LAN  205  must proceed through layer- 3  protocols. 
     LAN  204  is included in Layer- 3  Subnetwork  206  and LAN  205  is included in Layer- 3  Subnetwork  207 . For example, traffic bound from LAN Host  208 , residing on LAN  204 , and destined for LAN Host  209 , residing on LAN  205 , must be forwarded twice, or make two “hops,” first through Router  210  between Layer- 3   20  Subnetwork  206  and Layer- 3  Subnetwork  211 , and the second hop through Router  212  between Layer- 3  Subnetwork  211  and Layer- 3  Subnetwork  207 . Thus, all traffic from LAN Host  208  bound for LAN Host  209 , and conversely, must pass through Router  210  and Router  212 . Although LAN  204  has connectivity to ELAN  202  via LAN Switch  213  and LAN  205  has connectivity to ELAN  203  via LAN Switch  214 , LAN  204  and LAN  205  cannot exploit those layer- 2  connections because the required layer- 2  addresses are not contained in their source routed RIF. 
     The present invention alleviates the need for traffic flow through routers, such as Router  210  and Router  212  of FIG. 2, and permits source routed networks such as LAN  204  and LAN  205  in FIG. 2 to exploit the layer- 2  connections of a route switched network, such as ATM  201  in FIG.  2 . 
     Refer now to FIG. 3 in which an embodiment of the present invention is depicted. FIG. 3 illustrates a data processing network  300  including Layer- 3  Subnetwork  301 , Layer- 3  Subnetwork  302 , Layer- 3  Subnetwork  303 , and ATM  304 . ELAN  305 , in ATM  304 , is coupled to LAN  306  via LAN Switch  307 . ELAN  305  and LAN  306  are resident in Layer- 3  Subnetwork  301 . Similarly, ELAN  308 , in ATM  304 , is coupled to LAN  309  via LAN Switch  310 . ELAN  308  and LAN  309  reside in Layer- 3  Subnetwork  303 . LAN Host  311 , which is resident in LAN  306 , includes Protocol Stack  312 . Incorporated in Protocol Stack  312  is a route resolution protocol. Similarly, LAN Host  313  includes Protocol Stack  314  that also incorporates route resolution protocol client functionality. Incorporated in the routed connectivity between layer- 3  subnetworks is a Next Hop Server (NHS) that provides the functionality to service route resolution requests that are issued by a LAN host executing instructions implementing the route resolution protocol. Thus, Layer- 3  Subnetwork  301  and Layer- 3  Subnetwork  302  are connected by NHS/Router  315  and Layer- 3  Subnetwork  302  is connected to Layer- 3  Subnetwork  303  by NHS/Router  316 . 
     In the embodiment of the present invention of FIG. 3, route resolution protocol client functionality is incorporated in Protocol Stack  314  within LAN Host  313 . This enables that embodiment to operate symmetrically, as subsequently discussed. However, an alternative embodiment of the present invention need not operate symmetrically. 
     The route resolution capability embedded in data processing network  300  enables the source routed infrastructure in Layer- 3  Subnetwork  301  and Layer- 3  Subnetwork  303  to communicate via the layer- 2  connections of ATM network  304 . If LAN Host  311  in Layer- 3  Subnetwork  301  wishes to send traffic to LAN Host  313  in Layer- 3  Subnetwork  303 , the route resolution client functionality in Protocol Stack  312  causes a route resolution request to be issued to determine the MAC address and RIF associated with LAN Host  313 . The route resolution request proceeds toward LAN Host  313  via layer- 3  protocols. 
     On receipt of the route resolution request, NHS/Router  316  acquires the MAC and ATM address associated with LAN Host  313 . NHS/Router  316  performs an ARP for the MAC address associated with LAN Host  313 , and a LE_ARP for the ATM address associated with LAN Host  313  in response to the route resolution request. These protocols cause, respectively, an ARP_Request and a LE_ARP_Request to be issued. In response to these requests, NHS/Router  316  receives the MAC address, ATM address, and RIF associated with the destination, LAN Host  313 . The MAC address and ATM address along with the RIF from NHS/Router  316  to the destination station, LAN Host  313 , are returned by NHS/Router  316  to NHS/Router  315 . NHS/Router  315  then incorporates this MAC address and ATM address in its response to the route resolution request. 
     NHS/Router  315  also builds a merged RIF. NHS/Router  315  merges the RIF it received from NHS/Router  316  with the RIF from the source station, LAN Host  311 , to NHS/Router  315 , by creating a “virtual” route descriptor to join the two route segments. The virtual route descriptor does not correspond to a physical ring or bridge, but provides a link between RIF segments which do. The merged RIF is returned by NHS/Router  315  in its route resolution reply along with the MAC and ATM addresses associated with LAN Host  313  that it received from NHS/Router  316 . 
     However, prior to returning its route resolution reply, NHS/Router  215  registers the virtual route descriptor that it created to merge the two route segments with LES/BUS  317  attached to ELAN  305 . This registration binds the ATM address associated with LAN Host  313  to the virtual route descriptor. LAN Host  311  receives the destination MAC address and the associated RIF returned by NHS/Router  315  through its route resolution client functionality embedded in Protocol Stack  312 . 
     Alternative embodiments of the present invention may be implemented in an ATM attached host running LAN emulation. Although the embodiment of the present invention depicted in FIG. 3 includes LAN Host  311  and LAN Host  313  having ATM connectivity via LAN Switch  307  and LAN Switch  310  respectively, the present invention operates in exactly the same fashion in an ATM attached host running LAN emulation. 
     The method of the invention may be further understood by referring to FIG. 4, where a flow chart depicting the steps of the method are illustrated. The method initiates with data bound for a destination residing on a layer- 3  subnetwork different than that of the source host. The process begins at step  400 . The source host, for example, LAN  311 , through its embedded route resolution protocol, issues a route resolution request in step  401 . The route resolution request is forwarded toward the destination via layer- 3  protocols. At initiation, only the layer- 3  address of the destination is known to the source host. 
     Route resolution service functionality is embedded in layer- 3  routers which span the layer- 3  route between source and destination. Such routers have the capability of servicing route resolution requests. On receipt of a route resolution request packet, a layer- 3  router incorporating route resolution service functionality, that is, an NHS, must determine whether the destination is on the layer- 3  network on which it resides (step  402 ). In other words, the NHS must determine if it is an egress server. If it is not an egress server, the current NHS sends the route resolution request to the next NHS along the layer- 3  route in step  403 . If the current NHS is an egress server, for example, NHS/Router  316 , it ascertains the layer- 2  addresses associated with the destination. It issues an ARP_Request in step  404 , and receives the MAC address and RIF associated with the destination in response. It then issues an LE_ARP_Request in step  405  to obtain the ATM address associated with the destination. The destination-associated ATM address, MAC address, and RIF are included in resolution reply route packets. The egress server returns the route resolution reply to the requesting host, for example LAN Host  311 , via layer- 3  routing in step  406 . The NHS receiving the route resolution reply determines in step  407  if the reply is a response to a request for which it was the ingress NHS. If not, it sends the route resolution reply on in step  408 . The first NHS to receive a route resolution request from a legacy LAN client is termed an ingress server. In the embodiment of the present invention illustrated in FIG. 3, NHS/Router  315  is an ingress server with respect to traffic from LAN Host  311 . In step  409  the ingress NHS generates a virtual route descriptor. It then registers the virtual route descriptor with an LES on the ingress ELAN, for example LES/BUS  317  on ELAN  305  (step  410 ). The registration binds the virtual route descriptor with the destination-associated ATM address. The ingress NHS then merges the ingress RIF with the RIF received in the route resolution reply request in step  411 . The merging of the RIF segments is done through the intermediation of the virtual route descriptor. 
     While constructing the merged RIF, the ingress NHS must perform two validity checks. The route descriptor contains a ring number and a bridge number. The end-to-end RIF consists of a ordered set of ring numbers and bridge numbers. The ingress NHS must check that the end-to-end RIF does not contain duplicate ring numbers (step  412 ). If this test fails, then the ingress NHS server returns a negative acknowledgment (NAK) in step  413 . If not, it then checks that the end-to-end RIF does not exceed a predetermined maximum RIF length in step  414 . If the end-to-end RIF is too large, the ingress NHS returns an NAK in step  413 . Otherwise, it returns the route resolution reply, containing the merged RIF, to the requesting host in step  416 . The requesting host then uses the layer- 2  addresses to send its data packets in step  417 . In the event of a failure, the source routed network, such as LAN  306  and LAN  309 , are unable to participate in the route switch network, such as ATM  304 , and the requesting host transmits its data to the destination host using layer- 3  protocols in step  415 . 
     An embodiment of the present invention may be implemented with the Next Hop Resolution Protocol (NHRP). NHRP is part of the specification for Multiprotocol Over ATM, version 1.0. NHRP is Annex C, ATM Forum, AF-MPOA-0087.000, July 1997, which is hereby incorporated herein by reference. 
     If the acquisition of an ATM path is successful, data frames bound for LAN Host  313  from LAN Host  311  are then delivered via normal layer- 2  procedures. Using its route resolution client functionality, LAN Host  311  transmits the data frames bound for LAN Host  313  using the MAC address of LAN Host  313  that it received by the previously described process, in accordance with step  417 . It also includes the RIF obtained at the same time. In response, LAN Switch  307 , acting as a source route bridge, issues an LE_ARP_Request for the next-hop route descriptor. This is the virtual route descriptor embedded in the merged RIF. LES/BUS  317  recognizes the virtual route descriptor by virtue of its prior registration and returns the ATM address associated with LAN Host  313  that is bound to the virtual route descriptor. LAN Switch  307  then sets up a data-direct VCC to the ATM associated with LAN Host  313 , namely, the ATM address of LAN Switch  310 . The data frames bound for LAN Host  313  are then delivered over this so-called shortcut VCC. 
     It would be understood by an artisan of ordinary skill that traffic initiated by LAN Host  313  bound for a destination on LAN  306  would proceed by the same process. In such a case NHS/Router  315  would then be the egress router and NHS/Router  316  the ingress router. The virtual route descriptor binding the ATM address associated with the destination with the RIF would be registered with the LES/BUS  318 . 
     The present invention is also adaptable to a data processing network having source-route bridge connectivity between the participating stations in different layer- 3  subnetworks. Refer now to FIG. 5 in which is depicted a data processing network  500  having source-route bridge connectivity between LAN Host  501  and LAN Host  502  residing in layer- 3  subnetwork  503  and layer- 3  subnetwork  504 , respectively. Such networks can forward data using the route descriptors in the RIF without reference to layer- 3  protocols. A source station, such as LAN Host  501  seeking to transmit data to a destination station such as LAN Host  502 , may obtain end-to-end routing by broadcasting an explorer frame to obtain a RIF. LAN Host  501  then transmits its data frames which contain the end-to-end RIF returned by explorer frame. In transporting each data packet from LAN Host  501  to LAN Host  502 , NHS/Router/Source-bridge  505  and NHS/Router/Source-bridge  506  make frame forwarding decisions based on the RIF. Although LAN Host  501  may obtain end-to-end routing in this way, it is unable to participate in the route switching infrastructure of ATM  507  without the ATM address associated with the destination, and therefore cannot take advantage of the efficiencies that such participation would offer. 
     Providing route resolution protocol client functionality in data processing network  500  enables a data-direct VCC to be established on behalf of a source station. The present invention operates within network  500  having source-route bridging in exactly the same way as it operates in a network having routed connectivity between participating stations, such as network  300  of FIG.  3 . Route resolution client functionality embedded in protocol stack  508  of LAN Host  501  causes a route resolution request to be issued. This route resolution request is transmitted toward the destination via NHS/Router/Source-route Bridge  505  and NHS/Router/Source-route Bridge  506 . Egress NHS/Router/Source-route Bridge  506  determines a MAC address and an ATM address associated with destination station, LAN Host  502 . It also ascertains an egress segment RIF. NHS/Router/Source-route Bridge  506  provides this information to NHS/Router/Source-route Bridge  505  which merges the egress segment RIF with the ingress segment RIF through the intermediation of a virtual route descriptor, as previously described. NHS/Router/Source-route Bridge  505  registers the virtual route descriptor that it created with LES/BUS  509  on ELAN  510  and transmits the route resolution reply containing the MAC address associated with a destination station and merged RIF to the source station, LAN Host  501 . LAN Host  501  then uses this MAC address and RIF when transmitting data frames to the destination station, LAN Host  502 . LAN Switch  511 , acting as a bridge between ELAN  510  and LAN  512 , then sets up a data-direct VCC to the ATM address associated with LAN Host  502  which it receives in response to an LE_ARP_Request as described hereinabove. The frames destined for LAN Host  502  are then delivered over this data-direct VCC. 
     In an embodiment of the present invention, the route resolution protocol may be NHRP. 
     Similarly, data to be transmitted from LAN Host  502  destined for LAN Host  501  may be delivered using normal layer- 2  procedures via ATM  507  using the process hereinabove described. The route client finctionality contained in data processing network  500  may operate in symmetric fashion, although it need not necessarily do so. In such an embodiment, LAN Host  502  contains route client functionality in Protocol Stack  513 . LAN Host  502  resides on LAN  514  having connectivity to ELAN  515  through LAN Switch  516 . NHS/Router/Source-route Bridge  506  provides connectivity between ELAN  515  and ELAN  517  which is in Layer- 3  Subnetwork  518 . ELAN  517  is also connected to NHS/Router/Source-route Bridge  505 . With respect to data being transmitted from LAN Host  502  and destined for LAN Host  501 , NHS/router  505  is the egress route resolution server, NHS/router  506  is the ingress route resolution server, and the virtual route descriptor is registered with LES/BUS  519  on ELAN  515 . The data is transmitted using normal layer- 2  procedures in a fashion symmetric to that described hereinabove with respect to data transmitted from LAN Host  501  to LAN Host  502 . 
     In a network having shortcut bridged connectivity between participating stations, the method of the present invention operates without the creation of a virtual route descriptor. Refer now to FIG. 6 in which an embodiment of such a network is depicted. The capabilities of a shortcut bridge enable it to use the destination MAC address rather than the RIF to route frames bound for the destination host. This enables each of the ELAN segments, ELAN  601 , ELAN  602 , and ELAN  603  to appear as one collection, or “super-ELAN.” In the context of a source-routed frame, ELAN  601 , ELAN  602 , and ELAN  603  function as a single ring, and their associated route descriptor in the RIF is associated with a single ring number. Consequently, although ingress segment RIFs and egress segment RIFs must still be merged in the present invention, there is no need to introduce virtual route descriptors. Consequently, no step of registration of route descriptors is needed, either. 
     In a network having shortcut bridged connectivity between participating stations, such as network  600  of FIG. 6, ingress RIF segments and egress RIF segments are linked at the common ring number associated with each of the ELAN segments, ELAN  601 , ELAN  602 , and ELAN  603 . This common ring number appears as the last ring number in the ordered set of route descriptors in the ingress RIF and the first ring number in the ordered set of route descriptors in the egress RIF. The merged RIF may be viewed as the set-theoretic union of the ingress RIF and the egress RIF, with the common ring number thereby appearing in the ordered set of route descriptors only once. As described previously, the egress RIF is provided by NHS/Router/Shortcut Bridge  604  to NHS/Router/Shortcut Bridge  603  in a route resolution reply issued in response to a route resolution request initiated by LAN Host  605 . NHS/Router/Shortcut Bridge  603  merges the ingress and egress RIFs and returns them to LAN Host  605  in a route resolution reply as described hereinabove. The MAC address associated with a destination, here LAN Host  606 , is also returned in the route resolution reply as previously discussed. In an embodiment of the present invention, the route resolution protocol may be the NHRP. 
     When LAN Host  605  seeks to transmit data to LAN Host  606 , it uses the merged RIF and the MAC address associated with LAN Host  606  when transmitting frames to LAN Host  606 . However, now when LAN Switch  607 , bridging LAN  608 , issues an LE_ARP_Request for the next-hop route descriptor, the normal shortcut bridge mechanisms return the ATM address associated with the destination host, LAN Host  606 , the ATM address of LAN Switch  609 , bridging LAN  610 . The normal LE ARP mechanisms also bind the common ring number associated with ELAN  601 , ELAN  602  and ELAN  603  with that ATM address. LAN Switch  607  then uses the ATM address of LAN Switch  609  to establish a data-direct VCC over ATM  612 , and the data packets destined for LAN Host  606  are transmitted over this VCC. 
     In symmetric fashion, LAN Host  606  can participate in the route-switched network, ATM  612 , to transmit data bound for LAN Host  605 . In this instance, LAN Switch  609  uses the ATM address of LAN Switch  607  to establish the VCC. The merged RIF associated with the establishment of this data path is the “mirror” image of that related to data flow from LAN Host  605  to LAN Host  606 , previously described. 
     In an embodiment of the present invention, a LAN Host, such as LAN Host  311  or LAN Host  313  of FIG. 3, may be a workstation. A representative hardware environment for practicing the present invention in such an embodiment is depicted in FIG. 7, which illustrates a typical hardware configuration of workstation  713  in accordance with the subject invention having central processing unit (CPU)  710 , such as a conventional microprocessor, and a number of other units interconnected via system bus  712 . Workstation  713  includes random access memory (RAM)  714 , read only memory (ROM)  716 , and input/output (I/O) adapter  718  for connecting peripheral devices such as disk units  720  and tape drives  740  to bus  712 , user interface adapter  722  for connecting keyboard  724 , mouse  726 , speaker  728 , microphone  732 , and/or other user interface devices such as a touch screen device (not shown) to bus  712 , communication adapter  732  for connecting workstation  713  to a data processing network, and display adapter  736  for connecting bus  712  to display device  738 . CPU  710  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  710  may also reside on a single integrated circuit. 
     In one embodiment, a protocol stack, for example, Protocol Stack  312  of FIG. 3 may be stored in ROM  716 . In an alternative embodiment it may be located in a mass storage device, such as disk units  720 . If an application being executed on workstation  713  seeks to transmit data to another LAN Host residing on a different layer- 3  subnetwork, for example, LAN Host  313  of FIG. 3, an instruction set implementing the route resolution protocol within the protocol stack would be loaded into RAM  714  from ROM  716  or disk units  720 , as appropriate. CPU  710  would then execute the instruction set in order to perform those steps of the route resolution protocol that are within its scope. These steps would include steps  401 and  417  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.