VLAN frame format

Abstract

In a network device such as a network switch having a port coupled to a communications medium dedicated to a single virtual local area network and another port coupled to a communications medium shared among multiple virtual local area networks for transmitting data frames between the dedicated communications medium and the shared communications medium, a method of identifying the virtual network associated with each data frame received by the network switch when transmitting the data frames over the shared communications medium. The method comprises receiving data frames from the dedicated communications medium coupled to one port, and, with respect to each data frame so received, inserting a new type field and a virtual network identifier field. The contents of the new type field indicate the data frame comprises a virtual network identifier field. The method further includes placing a value in the virtual network identifier field identifying the virtual network associated with the data frame and transmitting the data frame over the shared communications medium. Upon receipt of the data frames from over the shared communications medium, another network device can discern from the virtual network identifier field in each data frame the virtual network from which the data frames were received and determine whether to forward the data frames accordingly.

We claim: 
     
       1. A method of identifying a virtual network associated with a data frame when transmitting said data frame between a communications medium and a shared communications medium, comprising the steps of:
 a) receiving said data frame from said communications medium, said data frame comprising a first type field and a data field;   b) inserting a second type field at a location within said data frame preceding said first type field, said second type field indicating said data frame comprises a virtual network identifier field;   c) inserting said virtual network identifier field at a location between said second type field and said first type field;   d) assigning a first value to said virtual network identifier field, said first value corresponding to said virtual network; and   e) transmitting said data frame over said shared communications medium.   
     
     
       2. The method of  claim 1 , further comprising the steps of:
 1) inserting between said second type field and said virtual network identifier field a virtual network identifier type field; and   2) assigning a second value to said virtual network identifier type field indicating a type of said first value in said virtual network identifier field.   
     
     
       3. The method of  claim 1 , further comprising the steps of:
 1) inserting between said second type field and said virtual network identifier field a virtual network identifier length field; and   2) assigning a second value to said virtual network identifier length field indicating a length of said first value in said virtual network identifier field.   
     
     
       4. The method of  claim 1  wherein said virtual network identifier field is 4 bytes. 
     
     
       5. The method of  claim 1  wherein said virtual network identifier field is a multiple of 4 bytes. 
     
     
       6. The method of  claim 1  wherein said first type field indicates a protocol type. 
     
     
       7. A method of identifying a virtual network associated with a data frame when transmitting said data frame between a communications medium and a shared communications medium, comprising the steps of:
 a) receiving said data frame from said communications medium, said data frame comprising a length field and a data field;   b) inserting a type field at a location within said data frame preceding said length field, said type field indicating said data frame comprises a virtual network identifier field;   c) inserting said virtual network identifier field at a location between said type field and said length field;   d) assigning a first value to said virtual network identifier field, said first value corresponding to said virtual network; and   e) transmitting said data frame over said shared communications medium.   
     
     
       8. The method of  claim 7 , further comprising the steps of:
 1) inserting between said type field and said virtual network identifier field a virtual network identifier type field; and   2) assigning a second value to said virtual network identifier type field indicating a type of said first value in said virtual network identifier field.   
     
     
       9. The method of  claim 7 , further comprising the steps of:
 1) inserting between said type field and said virtual network identifier field a virtual network identifier length field; and   2) assigning a second value to said virtual network identifier length field indicating a length of said first value in said virtual network identifier field.   
     
     
       10. The method of  claim 8  wherein said virtual network identifier field is a multiple of 4 bytes. 
     
     
       11. In a network device, a method of transmitting a virtual network identifier in a data frame transmitted on a shared communications medium coupled to said network device, comprising:
 a) transmitting a preamble field;   b) transmitting a destination and source media access control address field;   c) transmitting a first type field whose contents indicate said virtual network identifier is present in said data frame;   d) transmitting a virtual network identifier field containing said virtual network identifier;   e) transmitting a second type field whose contents indicate a protocol type associated with said data frame; and,   f) transmitting a data field.   
     
     
       12. The method of  claim 11  wherein said virtual network identifier field is 4 bytes. 
     
     
       13. In a network device having a first port coupled to a local area network (LAN) segment and a second port coupled to a shared communications medium, a method of associating a virtual network with a data frame received from said LAN segment and transmitted to said shared communications medium, comprising:
 a) receiving said data frame at said first port, said data frame comprising a type field and a data field;   b) replacing a first value in said type field representing a protocol type with a second value indicating said data frame comprises a virtual network identifier field;   c) inserting said virtual network identifier field in said data frame between said type field containing said second value and said data field;   d) assigning a value representing said virtual network to said virtual network identifier field; and   e) transmitting said data frame from said second port.   
     
     
       14. The method of  claim 13  further comprising:
 a) inserting a new type field between said virtual network identifier field and said data field; and 
 b) assigning said first value representing said protocol type to said new type field to preserve said protocol type. 
 
     
     
       15. The method of  claim 13  wherein said virtual network identifier field is 4 bytes. 
     
     
       16. The method of  claim 13  wherein said virtual network identifier field is a multiple of 4 bytes. 
     
     
       17. A network device, comprising:
 at least one port configured to be coupled to a shared communications medium; and   a processing unit configured to transmit, via the at least one port, on the shared communications medium:
 a destination address field containing a destination address for a data frame; 
 a source address field containing a source address associated with the data frame; 
 a virtual network type field having a value indicative that the data frame is associated with a virtual network; 
 a virtual network identifier field having a value indicative of the virtual network with which the data frame is associated, the value being in one of a plurality of formats as indicated by the value of the virtual network type field; 
 a length field having contents indicating a length of a data field; and 
 the data field. 
   
     
     
       18. The network device of claim 17, wherein the virtual network identifier field consists of an integer number of bytes. 
     
     
       19. The network device of claim 17, wherein the virtual network identifier field consists of an even number of bytes. 
     
     
       20. The network device of claim 17, wherein the virtual network identifier field is four bytes in length. 
     
     
       21. A network device, comprising:
 at least one port configured to be coupled to a shared communications medium; and   a processing unit configured to transmit, via the at least one port, on the shared communications medium:
 a destination address field containing a destination address for a data frame; 
 a source address field containing a source address associated with the data frame; 
 a virtual network type field having a value indicative that the data frame is associated with a virtual network; 
 a virtual network identifier field having a value indicative of the virtual network with which the data frame is associated, the value being in one of a plurality of formats as indicated by the value of the virtual network type field; 
 a second type field whose contents indicates a protocol type associated with the data frame; and 
 a data field. 
   
     
     
       22. The network device of claim 21, wherein the virtual network identifier field consists of an integer number of bytes. 
     
     
       23. The network device of claim 21, wherein the virtual network identifier field consists of an even number of bytes. 
     
     
       24. The network device of claim 21, wherein the virtual network identifier field is four bytes in length.