Abstract:
A system in which a single VLAN architecture spans multiple VLAN transport protocols and technologies, including a method and system in which multiple different VLANs may be combined in a single enterprise network. Each LAN-switch in the system identifies each frame with an identifier, and associates that identifier with particular VLAN identifiers for each type of VLAN technology. When a frame is bridged or routed from a first type of VLAN to a second type of VLAN, the first VLAN encapsulation is removed and the second VLAN encapsulation is added, with appropriate change in the VLAN identifier for the frame or packet. The identifier may also be implicit for the frame, such as when a particular set of sender&#39;s MAC addresses are identified with a particular VLAN. Individual VLANs, of whatever architecture, may be added, configured or reconfigured, modified, or deleted, using control tools associated with the multiple VLAN architecture system. Individual ports may be associated with particular VLANS, or may be designated “dynamic” so that frames or packets associated with those ports are associated with particular VLANs in response to source or destination addresses or other information.

Description:
This is a continuation of application Ser. No. 08/582,074 filed Jan. 2, 1996 now U.S. Pat. No. 6,035,105. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a multiple VLAN architecture system. 
     2. Description of Related Art 
     When interconnecting computers and other devices in a network, it has become desirable to create “virtual local area networks” (VLANs), in which all devices coupled to a VLAN receive all frames or packets which are universally addressed (whether by broadcast, multicast, or some other technique) on that VLAN, and in which all frames or packets which are universally addressed by a device on a VLAN are not distributed to devices outside the VLAN. However, there is more than one type of VLAN transport protocol technology which has been proposed and come to be accepted in the art. For example, VLAN technologies which are now common include LANE (for ATM LAN-Emulation), IEEE Standard 802.10, and various proprietary schemes such as ISL (for cisco Catalyst™ Inter-Switch Links). 
     One problem which has arisen in the art is that it is desirable to couple devices on a single VLAN even though those devices have been designed or configured for different VLAN transport protocols or technologies. Aspects of this problem are that it is desirable for devices to be on the same VLAN even though they are not physically proximal to each other or cannot be coupled to the same switching device (for example, due to limitations imposed by respective media speed), that it is desirable for devices (or networks comprising those devices) to be configurable so that a device may be moved from one VLAN to another VLAN with ease, and that it is desirable for a device to be coupled to different VLANs at different times or to multiple VLANs at one time. 
     Accordingly, it would be advantageous to provide a multiple VLAN architecture system, such as one which is capable of operating in a network environment with multiple different VLANs, and multiple different VLAN technologies. 
     The following U.S. patent(s) may be pertinent: 
     U.S. Pat. No. 5,394,402, issued Feb. 28, 1995, in the name of Floyd E. Ross, titled “Hub For Segmented Virtual Local Area Network With Shared Media Access”. 
     This patent discloses a hub for a segmented VLAN system. The hub receives packets from one of the devices, called “end stations” which are coupled thereto, and forwards them using a backbone network to other such hubs, for forwarding to other devices coupled to the same VLAN. Essentially, the hub serves to bridge packets among its ports such that packets are bridged only to those other devices which are on the same VLAN. 
     The pertinence of the related art will also be apparent to those skilled in the art after perusal of this application. 
     SUMMARY OF THE INVENTION 
     The invention provides a system in which a single VLAN architecture spans multiple VLAN transport protocols and technologies, including a method and system in which each VLAN may span multiple different VLAN technologies. Each LAN-switch in the system identifies each frame with an identifier, and associates that identifier with particular VLAN identifiers for each type of VLAN architecture. When a frame is bridged or routed from a first type of VLAN to a second type of VLAN, the first VLAN encapsulation is removed and the second VLAN encapsulation is added, with appropriate change in the VLAN identifier for the frame or packet. The identifier may also be implicit for the frame, such as when a particular set of sender&#39;s MAC addresses are identified with a particular VLAN. 
     In a preferred embodiment, individual VLANs, which may span the set of multiple VLAN technologies, may be added, configured or reconfigured, modified, or deleted, using control tools associated with the multiple VLAN architecture system. Individual ports may be associated with particular VLANs, or may be designated “dynamic” so that frames or packets associated with those ports are associated with particular VLANs in response to source or destination addresses or other information. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a network having multiple VLANs. 
     FIG. 2 is a diagram of a set of LAN-switches disposed for coupling messages between multiple VLANs. 
     FIG. 3 is a diagram showing a relationship between multiple VLANs, management domains, and network administration. 
     FIG. 4 is a diagram showing message formats for use in a VLAN protocol. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, a preferred embodiment of the invention is described with regard to preferred process steps and data structures. However, those skilled in the art would recognize, after perusal of this application, that embodiments of the invention may be implemented using a set of general purpose computers operating under program control, and that modification of a set of general purpose computers to implement the process steps and data structures described herein would not require undue invention. 
     MULTIPLE VLAN ARCHITECTURE SYSTEM 
     FIG. 1 is a diagram of a network having multiple VLANs. 
     In an interconnected network  100 , a set of devices  101  may be coupled to a plurality of physical networks  102 . Each network  102  may comprise a local area network (LAN) such as an ethernet LAN, a token ring LAN, an FDDI network, or another LAN architecture. Each network  102  may transmit a set of frames  104  using one of a plurality of media access transmit protocols. Architectures for local area networks and for their media access transmit protocols are known in the art of networking. 
     The networks  102  are coupled using a set of LAN-switches  103 . The LAN-switches  103  forward frames (using a level two protocol) or packets (using a level three protocol) among the networks  102 . Each LAN-switch  103  is coupled to one or more networks  102 . For example, one of the LAN-switches  103  may be coupled to two or more networks  102 . LAN-switches are known in the art of networking. 
     The devices  101  are associated with a plurality of different VLAN technologies, and therefore may transmit frames using one of a plurality of VLAN transmit protocols. For example, each device  101  may be associated with a VLAN transmit protocol such as ATM LAN Emulation (LANE), IEEE 802.10, cisco Catalyst™ Inter-Switch Links (ISLs), or another VLAN transmit protocol. Each device  101  may be associated with just one VLAN technology, or with a plurality of VLAN technologies. 
     The devices  101  are assigned to a plurality of VLANs  106 , independent of their associated VLAN technologies. Each VLAN  106  need not comprise a single or homogeneous VLAN technology; moreover, each VLAN  106  is not subject to any requirement that all devices  101  on that VLAN  106  are proximate or even coupled using the same LAN or VLAN technology. 
     Those frames  104  to be transmitted on a particular VLAN segment  108  are identified with a tag  107  referencing that particular VLAN  106 . As the frame  104  is forwarded between differing VLAN technologies, the tag  107  uses a tagging technique particular to that VLAN technology. For example, the tag  107  for the same VLAN  106  may be the character string ‘GR” for LANE, the numeric value ‘1024’ for IEEE 802.10, or the numeric value ‘10’ for ISL. 
     The LAN-switches  103  are configured to (1) receive frames from a first VLAN associated with a first VLAN transmit protocol and encapsulated using a multiple-VLAN transmit protocol, (2) to remove the encapsulation, (3) to re-encapsulate the frames with a second VLAN transmit protocol, and (4) to transmit: the re-encapsulated frames onto a second VLAN. 
     In alternative embodiments, the frames  104  may have implicit tagging. In this case, those frames  104  which are addressed from a first particular set of MAC addresses (or are otherwise identifiable from their frame headers) are designated as being for a first VLAN segment  108 , while those frames  104  which are addressed from a second particular set of MAC addresses are designated as being for a second VLAN segment  108 . 
     As described herein, the LAN-switches  103  may also include the capability to perform layer-3 routing. LAN-switches  103  which include such layer-3 routing may couple a set of frames  104  between virtual LANs (VLANs) as well as physical networks  102 , it is possible for such a LAN-switch  103  to be coupled to only a single network  102 , and to route the frames  104  found on that network  102  between different VLANs. In that circumstance, the LAN-switch  103  receives the frame  104  from one of the VLANs implemented by the network  102 , and transmits the frame  104  onto another one of the VLANs implemented by the network  102 . An example of one such LAN-switch  103 , sometimes called a “lollypop router”  105 , is shown in the figure. 
     FORWARDING FRAMES AMONG MULTIPLE VLANS 
     FIG. 2 is a diagram of a set of LAN-switches disposed for coupling messages between multiple VLANs. 
     The set of LAN-switches  103  collectively comprises a subnet  200 , in which pairs of the LAN-switches  103  are coupled by a set of links  201 . Each link  201  comprises a physical network  102 , so that a set of frames  104  may be coupled between pairs of LAN-switches  103 . Each LAN-switch  103  comprises a set of ports  202  coupling the LAN-switch  103  to one of its links  201 , and therefore to another LAN-switch  103 . 
     Each LAN-switch  103  receives packets at each of its ports  202 , and recognizes each VLAN  106  which might be received at port  202 . When a frame  104  must be forwarded from a first VLAN segment  108  to a second VLAN segment  108 , the LAN-switch  103  removes tags  107  (shown in the figure as the numeric value ‘10’) for a first VLAN segment  108  and replaces them with tags  107  (shown in the figure as the character string value ‘GR’) for a second VLAN segment  108 . 
     In a preferred embodiment, the LAN-switch  103  identifies the second VLAN segment  108  required for the destination device  101  responsive to the output port  202  to which the frame  104  is routed. The port  202  may be one of three types shown in table 2-1. 
     
       
         
               
               
             
           
               
                 TABLE 2-1 
               
               
                   
               
               
                 Port Type 
                 Treatment 
               
               
                   
               
             
             
               
                 static 
                 Each frame from the port is for a particular VLAN 
               
               
                   
                 which is statically configured for that port, regard- 
               
               
                   
                 less of the address or content of that frame. 
               
               
                 dynamic 
                 The port may be assigned to one of a plurality of 
               
               
                   
                 VLANs, one at a time.  Each frame to or from the port 
               
               
                   
                 is examined and the port is dynamically reassigned to 
               
               
                   
                 a different VLAN in response to the address or con- 
               
               
                   
                 tent of that frame. 
               
               
                   
                 (For example, the port may be dynamically reassigned 
               
               
                   
                 in response to the MAC address of the sending device, 
               
               
                   
                 or in response to a layer three address.) 
               
               
                 trunk 
                 The port is assigned to a plurality of VLAN at once 
               
               
                   
                 Each frame to or from the port uses an encapsulation 
               
               
                   
                 or related technique to tag that frame as for a par- 
               
               
                   
                 ticular VLAN. 
               
               
                   
                 A VLAN trunk protocol causes each LAN-switch to 
               
               
                   
                 transmit advertisements regarding, and acquire infor- 
               
               
                   
                 mation about, VLANs for which that trunk port is con- 
               
               
                   
                 figured.  Thus, a new VLAN need only be configured 
               
               
                   
                 for one LAN-switch in a management domain, as the 
               
               
                   
                 VLAN trunk protocol will ultimately propagate that 
               
               
                   
                 information to all LAN-switches in the management do- 
               
               
                   
                 main. The VLAN trunk protocol is described in fur- 
               
               
                   
                 ther detail below. 
               
               
                   
               
             
          
         
       
     
     When the LAN-switch  103  receives a frame  104  on a static port  202 , it knows that frame  104  must be for the VLAN  106  statically configured for that port  202 . When the LAN-switch  103  transmits that frame  104  on another port  202  which is a trunk port  202 , it must encapsulate the frame  104  with the appropriate outgoing tag  107  to indicate the VLAN  106 . 
     When the LAN-switch  103  receives a frame  104  on a dynamic port  202 , it knows that the frame  104  must be configured according to one of the plurality of VLANs  106  configured for that port  202 , responsive to the address or contents (preferably the MAC address) of that frame  104 . The LAN-switch  103  identifies the incoming VLAN  106  responsive to the MAC address of the frame  104 . When the LAN-switch  103  transmits that frame  104  on another port  202  which is a trunk port  202 , it must encapsulate the frame  104  with the appropriate outgoing tag  107  to indicate the VLAN  106 . 
     When the LAN-switch  103  receives a frame  104  on a trunk port  202 , it knows that the frame  104  must be encapsulated and that the encapsulated frame may be for one of a plurality of VLANs  106 . The LAN-switch  103  identifies the VLAN  106  for the frame  104  responsive to the encapsulation header, and removes the encapsulation. When the LAN-switch  103  transmits that frame  104  on another trunk port  202 , it must re-encapsulate that frame  104 . 
     An network administrative workstation  203  is coupled to one LAN-switch  103  or elsewhere, and comprises a processor, program and data memory and mass storage, for executing application programs and for recording information, at least one input device (such as a keyboard or a pointing device such as a mouse) for receiving information from an operator  204 , and at least one output device (such as a monitor or printer) for presenting information to the operator  204 . 
     To identify the outgoing tag  107  which corresponds to the incoming tag  107 , the LAN-switch  103  maintains a database  205  which is preferably also available at the network administrative workstation  203 . The database  205  comprises a table  206 ; the table  206  comprises a set of rows  207  and a set of columns  208 , with an entry  209  at the intersection of each row  207  and each column  208 . The table  206  is indexed by a column  208  for a VLAN management ID; there is one row  207  for each VLAN  106  and one column  208  for each VLAN transmit protocol. 
     In a preferred embodiment, the VLAN management ID comprises a character string, such as “red” or “green”. One VLAN management ID, “default”, is reserved for an initial VLAN  106 . LAN-switches  103  are configured for the “default” VLAN  106  when they are shipped from the factory. In a preferred embodiment, the VLAN management ID comprises an ASCII character string of eight characters or less. 
     In a preferred embodiment, the table  206  comprises is one column  208  for LANE, one column  208  for IEEE 802.10, and one column  208  for ISL. Each entry  209  comprises a tag  107  for the VLAN corresponding to its row  207  and the VLAN transmit protocol corresponding to its column  208 . The data for each entry  209  has a data type which depends on the particular VLAN technology, such as numeric data, ASCII character data, or other data. 
     In a preferred embodiment, the ISL tag  107  for the “default” VLAN  106  is ‘1’. For IEEE 802.10, frames  104  for the “default” VLAN  106  may be transmitted as native frames without IEEE 802.10 encapsulation. 
     In a preferred embodiment, the database  205  also comprises information regarding properties associated with each VLAN  106  (such as, for example, whether the VLAN  106  has had its operation suspended). 
     In alternative embodiments, the database  205  may be made available at other locations. For example, the database  205  may be recorded and updated separately at each LAN-switch  103 , at a particular LAN-switch  103 , or at a particular non-routing device  101 . 
     MANAGEMENT DOMAINS AND NETWORK ADMINISTRATION 
     FIG. 3 is a diagram showing a relationship between multiple VLANs, management domains, and network administration. 
     The network administrative workstation  203  controls a set of management domains  300 , each of which comprises one or more VLANs  106 . Each VLAN  106  comprises one or more devices  101  on one or more networks  102  in the interconnected network  100 . 
     Each VLAN  106  must have a unique name within its management domain  300 . 
     When two management domains  300  are coupled via a trunk port  202 , the default behavior (prior to any additional configuration) is that no frames  104  are forwarded between the two management domains  300 . However, the respective ports  202  may he configured using the network administrative workstation  203  to forward frames  104  for specific VLANs  106 . 
     When two management domains  300  are coupled via a non-trunk port  202  (thus, via a static port  202  or a dynamic port  202 ), the respective ports  202  may be configured using the network administrative workstation  203  to forward frames  104  for specific VLANs  106 . 
     When frames  104  are transmitted across a trunk port  202  between a first management domain  300  and a second management domain  300 , the LAN-switch  103  must have a mapping between the source VLAN  106  in the first management domain  300  and the destination VLAN  106  in the second management domain  300 . 
     Each LAN-switch  103  maintains a list of VLANs  106  which are valid in its management domain  300 . This list is associated at each LAN-switch  103  with a configuration revision number; the configuration revision number is updated whenever a change is made to the configuration for that management domain  300 . Thus, a LAN-switch  103  can compare its configuration revision number with the new configuration to determine which is more recent. 
     When they are initially shipped from the factory, LAN-switches  103  are configured in a “no-management-domain” state. In this state the LAN-switch  103  does not belong to any particular management domain  300  and will update its database to learn about new VLANs  106  from all VLAN trunk protocol advertisements it receives. When a LAN-switch  103  is configured for a particular management domain  300 , it will ignore advertisements from different management domains  300  and it will check advertisements from the same management domain  300  for consistency. 
     When a LAN-switch  103  learns about a VLAN  106 , it will have received frames  104  from that VLAN  106  on any trunk port  202 , and will forward those frames  104  to all of its other trunk ports  202  (if any). This behavior may be altered by reconfiguring the LAN-switch  103  using the network administration workstation  203  to disable one or more VLANs  106  for a particular trunk port  202 . 
     MULTIPLE VLAN NETWORK ADMINISTRATION 
     The operator  204  at the network administrative workstation  203  may alter the database  205  and cause those alterations to be propagated to the LAN-switches  103 . Table 3-1 shows the changes which the operator  204  at the network administrative workstation  203  may make. 
     
       
         
               
               
             
           
               
                 TABLE 3-1 
               
               
                   
               
               
                 Change 
                 Effect 
               
               
                   
               
             
             
               
                 create a 
                 A new VLAN is created. A record is created for the 
               
               
                 VLAN 
                 new VLAN; a new VLAN management ID is created; en- 
               
               
                   
                 tries are made for the new VLAN for each VLAN tech- 
               
               
                   
                 nology 
               
               
                 delete a 
                 An existing VLAN is removed from the configuration 
               
               
                 VLAN 
                 for the management domain. The deleted VLAN&#39;s entries 
               
               
                   
                 in the database at the network administration work- 
               
               
                   
                 station are purged and any ports configured for the 
               
               
                   
                 deleted VLAN are automatically disabled. 
               
               
                   
                 In an alternative embodiment, the deleted VLAN is 
               
               
                   
                 only purged from the database after all ports config- 
               
               
                   
                 ured for the deleted VLAN are reconfigured for an- 
               
               
                   
                 other VLAN. 
               
               
                   
                 The “default” VLAN cannot be deleted. 
               
               
                 suspend a 
                 An existing VLAN has its operation suspended. All 
               
               
                 VLAN 
                 traffic for the suspended VLAN is turned off for the 
               
               
                   
                 duration of the suspension. (This function might be 
               
               
                   
                 useful, for example, if traffic for the suspended 
               
               
                   
                 VLAN was disrupting the network.) All ports config- 
               
               
                   
                 ured for the suspended VLAN are disabled for the du- 
               
               
                   
                 ration of the suspension. 
               
               
                   
                 The “default” VLAN cannot be suspended. 
               
               
                 resume a 
                 A suspended VLAN has its suspension terminated. All 
               
               
                 VLAN 
                 ports for the resumed VLAN are re-enabled and traffic 
               
               
                   
                 is allowed to flow for the resumed VLAN. 
               
               
                   
               
             
          
         
       
     
     Those skilled in the art will recognize, after perusal of this application, that other and further management functions would not require undue experimentation, and are within the scope and spirit of the invention. 
     VLAN PROTOCOLS AND MESSAGE FORMATS 
     FIG. 4 is a diagram showing message formats for use in VLAN protocols. 
     VLAN Trunk Protocol 
     Each LAN-switch  103  transmits advertisements regarding all VLANs  106  about which it knows, and possibly other information global to the management domain  300 . Advertisements are transmitted via the “default” VLAN  106 ; thus, only one advertisement is transmitted for each trunk port  202 . Advertisements are transmitted as multicast frames  104  but not forwarded using normal bridging techniques. 
     Each LAN-switch  103  maintains a configuration revision number for each managment domain  300  for which it is configured. The configuration revision number is an unsigned 32 bit value, which is initially set to zero and is incremented by one for each modification or reconfiguration at the LAN-switch  103  until the maximum value of 4,294,967,295 (hexadecimal ‘FFFF FFFF’) is reached, at which point the configuration revision number is wrapped around back to zero. 
     When a LAN-switch  103  receives an advertisement and it is not configured for any management domain  300 , it updates its database  205  from that advertisement. When a LAN-switch  103  receives an advertisement and it is configured for one or more particular management domains  300 , it authenticates that advertisement using its current configuration revision number for the appropriate management domain  300 . If the advertisement is authentic and its configuration revision number exceeds the LAN-switch&#39;s current configuration revision number, the LAN-switch  103  updates its database  205  from that advertisement; otherwise the LAN-switch  103  ignores the advertisement. 
     The configuration revision number A is deemed to be less than the configuration revision number B if and only if 
     ((A&lt;B and (B−A)&lt;2,147,483,648) or (A&gt;B and (A−B)&gt;2,147,483,648)) 
     The types of advertisement messages are shown in table 4-1. 
     
       
         
               
               
             
           
               
                 TABLE 4-1 
               
               
                   
               
               
                 Port Type 
                 Treatment 
               
               
                   
               
             
             
               
                 Advert- 
                 This message requests that an advertisement be sent. 
               
               
                 Request 
               
               
                 Summary- 
                 This message provides the management domain, configu- 
               
               
                 Advert 
                 ration revision number, and checksum for the adver- 
               
               
                   
                 tisement. The Summary-Advert message is followed by 
               
               
                   
                 zero or more Subset-Advert messages,  as appropriate. 
               
               
                 Subset- 
                 This message comprises all advertised information for 
               
               
                 Advert 
                 one or more VLANs. Each Subset-Advert message is la- 
               
               
                   
                 belled with a sequence number in case more than one 
               
               
                   
                 Subset-Advert message is sent. 
               
               
                   
               
             
          
         
       
     
     The Advert-Request message  400  comprises a VLAN trunk protocol version number  401  (1 byte, which is always ‘1’), a type of message code  402  (1 byte, which is preferably ‘3’ for the Advert-Request message  400 ), a reserved byte  403 , a management domain name  404  (variable length, but preferably a multiple of 4 bytes), a length value  405  (1 byte) for the management domain name  404 , and a start value  406  (2 bytes). 
     The Summary-Advert message  410  comprises the VLAN trunk protocol version number  401 , the type of message code  402  (which is preferably ‘1’ for the Summary-Advert message  410 ), the management domain name  404  (variable length, but preferably a multiple of 4 bytes), a length value  405  (1 byte) for the management domain name  404 , a configuration revision number  411  (4 bytes), an MD5 digest value  412  (4 bytes), and a followers value  413  (1 byte). 
     The Subset-Advert message  420  comprises the VLAN trunk protocol version number  401 , the type of message code  402  (which is preferably ‘2’ for the Subset-Advert message  420 ), a reserved byte  403 , the configuration revision number  411 , a sequence number  421  (1 byte), and a sequence of VLAN blocks  430 . 
     Each VLAN block  430  comprises a status value  431  (1 byte), a VLAN type value  432  (1 byte), a VLAN name  433  (variable length, but preferably a multiple of 4 bytes), a length value  434  (1 byte) for the VLAN name  433 , an ISL VLAN identifier  435  (2 bytes), an IEEE 802.10 index value  438 , and a maximum frame size value  439 . 
     The start value  406  is used in the event that the LAN-switch  103  does not desire all VLANs  106  to be advertised to it. In the actual advertisement, the Subset-Advert messages  420  are ordered by ISL VLAN identifier  435 . The start value  406  indicates from which ISL VLAN identifier  435  to start; all VLANs  106  which precede the start value  406  are not advertised. If the start value  406  is zero, all VLANs  106  are advertised. 
     Advertisements are authenticated; learning only occurs from authentic advertisements. Each advertisement comprises a checksum, preferably the MD5 digest value  412 , which is computed using a one-way cryptographic hash function (the MD5 digest function) of the concatenation of (1) the Summary-Advert message  410  with the followers value  413  replaced with a zero value, (2) the VLAN blocks  430  ordered by ISL VLAN identifier  435 , and (3) a “secret value”. The default secret value is all zeros, thus providing non-secure but immediately compatible operation. The secret value may be configured for each LAN-switch  103  using the network administrative workstation  203 , thus providing secure operation. Since each advertisement comprises a new configuration revision number  411 , the MD5 digest value  412  cannot be repeated until the configuration revision number  411  is itself repeated. 
     The followers value  413  indicates how many Subset-Advert messages  420  follow the Summary-Advert message  410 . The number of Subset-Advert messages  420  which follow the Summary-Advert message  410  are shown in table 4-2. 
     
       
         
               
               
             
           
               
                 TABLE 4-2 
               
               
                   
               
               
                 Reason for Sending Advertisement 
                 Number of Followers 
               
               
                   
               
             
             
               
                 Neither this LAN-switch or any 
                 zero 
               
               
                 other LAN-switch has recently 
               
               
                 (within the timeout period) sent 
               
               
                 an advertisement. 
               
               
                 A configuration change has been 
                 the minimun number required to 
               
               
                 made. 
                 contain all information on ex- 
               
               
                   
                 actly those VLANs which have 
               
               
                   
                 changed, ordered by ISL VLAN 
               
               
                   
                 identifier 
               
               
                 An Advert-Request message for 
                 the minimum number required to 
               
               
                 information for all VLANs was 
                 contain all information on all 
               
               
                 received. 
                 VLANs, ordered by ISL VLAN iden- 
               
               
                   
                 tifier 
               
               
                 An Advert-Request message for 
                 the minimum number required to 
               
               
                 information about a subset of 
                 contain all information on all 
               
               
                 all VLANs was received. 
                 VLANs except those which were 
               
               
                   
                 not requested, ordered by ISL 
               
               
                   
                 VLAN identifier 
               
               
                   
               
             
          
         
       
     
     The VLAN type value  432  indicates what type the VLAN  106  is: ethernet or IEEE 802.3, token ring or IEEE 802.5, or FDDI. 
     The maximum frame size value  439  indicates the maximum frame size for that particular VLAN  106 . 
     An Advert-Request message  400  is sent in the following cases: 
     when the LAN-switch  103  is rebooted. 
     when the LAN-switch  103  receives a Subset-Advert message  420  having a configuration revision number  411  higher than the LAN-switch&#39;s own configuration revision number  411 . 
     when the LAN-switch  103  receives a Summary-Advert message  410  having a configuration revision number  411  higher than the LAN-switch&#39;s own configuration revision number  411 , and followed by zero Subset-Advert messages  420 . 
     when the LAN-switch  103  does not receive the expected number of Subset-Advert messages  420  within a short period after receiving a Summary-Advert message  410  having a configuration revision number  411  higher than the LAN-switch&#39;s own configuration revision number  411 . In this case, the Advert-Request message  400  is set to request only the missing Subset-Advert messages  420 , by setting the start value  406  to one more than the highest ISL VLAN identifier  435  received. 
     when the LAN-switch  103  receives a Summary-Advert message  410  having a configuration revision number  411  more than one value higher than the LAN-switch&#39;s own configuration revision number  411 . 
     An advertisement, comprising a Summary-Advert message  410  and zero or more Subset-Advert messages  420 , is sent in the following cases: 
     immediately after its configuration revision number is modified (thus, immediately after any configuration change); 
     periodically on any trunk port  202  for which it has not sent an advertisement or received an advertisement matching its own, for a configurable timeout period, preferably about five minutes. The actual time for sending advertisements is jittred (modified by a small random or pseudorandom value) to avoid syncrhonization effects. Periodic advertisements can be disabled using the network administrative workstation  203 . 
     when a request for an advertisement is received. In this case, the timeout period is truncated to a small random or pseudorandom value. 
     In a preferred embodiment, the timeout for sending an advertisement is between about 2 minutes and about 10 minutes. Whenever this timeout is started, a pseudorandom value of less than about 1 second is added to it. When a consistent advertisement is received, the timeout is restarted without sending any advertisement. When an Advert-Request message  400  is received, the timeout is truncated to the value of the most recent pseudorandom value. 
     Those skilled in the art will recognize, after perusal of this application, that the VLAN trunk protocol may be used to distribute other and further types of information, that such activity would not require undue experimentation, and that such activity is within the scope and spirit of the invention. For example, such other and further types of information could include the following: 
     port configuration information—whether a particular port  202  is a static port  202 , dynamic port  202 , or a trunk port  202 ; or 
     dynamic assignment configuration information—either (1) which VLAN  106  a dynamic port  202  is associated with, or (2) a mapping between a layer three protocol address space, or a subspace thereof, and which VLAN  106  a dynamic port  202  should be associated with. 
     VCS Protocol 
     A VLAN configuration server comprises local information about VLANs  106 , including port configuration information and dynamic assignment configuration information. In a preferred embodiment, the VLAN configuration server is available at the network work administrative workstation  203 , but in alternative embodiments, may be a separate device  101  or may be distributed over several LAN-switches  103  or other devices  101 . 
     To configure its ports  202 , each LAN-switch  103  sends a message to the VCS to request configuration information. If the LAN-switch  103  is coupled to a ATM network  102 , it also attempts, for each VLAN  106  it knows about, to join the LANE emulated-LAN (ELAN) having the same name. 
     For static ports  202 , the LAN-switch  103  receives configuration information specifying with which VLAN  106  the port  202  is associated. 
     For dynamic ports  202 , the LAN-switch  103  receives configuration information specifying a mapping to VLANs  106  for MAC addresses for the sending devices  101  for frames  104 . 
     The message requesting configuration information is sent directly to the VCS if the LAN-switch  103  is not coupled to a ATM network  102 . Otherwise, the message is encapsulated using the LANE protocol and sent to a LANE configuration server (LECS). 
     If the VCS (or LECS) responds for a port  202  with a VLAN name (or ELAN name) which is known to the LAN-switch  103 , the port  202  is assigned to the VLAN  106  with that name. 
     If the VCS (or LECS) responds for a port  202  with a refusal, that port  202  is disabled. 
     If the VCS (or LECS) responds for a port  202  with a VLAN name (or ELAN name) which is not known to the LAN-switch  103 , or if the VCS (or LECS) does not respond after a number of retries, or if the VCS (or LECS) cannot be reached, the LAN-switch  103  retries the request at periodic intervals. 
     However, if the LAN-switch  103  has local configuration information which maps a source MAC addresses to VLANs  106  for a dynamic port  202 , it uses that local configuration information to reassign the dynamic port  202  in response to source MAC addresses. 
     Alternative Embodiments 
     Although preferred embodiments are disclosed herein, many variations are possible which remain within the concept:, scope, and spirit of the invention, and these variations would become clear to those skilled in the art after perusal of this application.