Fiber Distributed/Copper Distributed Data Interface (FDDI/CDDI)

Document ID: 17055

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Encapsulation
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Introduction

This document describes the transformations that occur with particular Ethernet encapsulations when bridging over an IEEE 802.10 Virtual LAN.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Encapsulation

The scenario is as shown.

                     --------                    --------
                     |      |                    |      |
                     |Switch|                    |Switch|
Originating Side ----|  A   |==   IEEE 802.10  ==|  B   |----Destination Side
                     |      |    VLAN Backbone   |      |
                     |      |                    |      |
                     --------                    --------

Note: This is the case as of the 10.3 Release of IOS (March 1995).

1. IEEE 802.3 Format on originating side:

   
       -------------------------------------------------------------
       |               |          |          |             |       |
       |  Destination  |  Source  |  Length  |  802.2 LLC  |  Data |
       |               |          |          |             |       |
       -------------------------------------------------------------

   appears on the 802.10 VLAN backbone as:

    
   
      --------------------------------------------------------------
      |    |      |Length|        |    |Station|Frag|Original |    |
      |Dest|Source|      |0A 0A 03|SAID|       |    |         |Data|
      |    |      | + 16 |        |    |  ID   |Flag|802.2 LLC|    |
      --------------------------------------------------------------
                         <-------- 16 bytes -------->

   Note: The 802.2 LLC Type 1 field now carries the 802.10 reserved LSAP value of 0x0A 0x0A 0x03.

   The SAID is the 802.10 defined Security Association ID (4-bytes).

   The Station ID comprises 8 bytes: the first six of which are the canonical Source Address of the original packet, and the last two are flags that are undefined and thus set to NULL.

   The Fragmentation Flag is a boolean field and denotes the frame to be a fragment if true. Cisco does not support fragmentation of 802.10 packets and therefore this indicator is always set to NULL.

   Switch B will then translate this frame back to its original 802.3/ 802.2 form.

2. Ethernet Version 2 (a.k.a. ARPA) format on originating side:

   
      -------------------------------------
      |               |          |        |
      |  Destination  |  Source  |  Type  |
      |               |          |        |
      -------------------------------------

   appears on the 802.10 VLAN backbone as:

    
   
   -----------------------------------------------------------------------------
   |    |      |      |      |    |          |Frag|RFC 1042 SNAP Header |Original|
   |Dest|Source|Length|0A0A03|SAID|Station ID|    |                     |        |
   |    |      |      |      |    |          |Flag|AA AA 03 OUI 00 00 00|  Type  |
   -----------------------------------------------------------------------------
                      <-------------------- 22 Bytes ------------------->

   Translation sequence as the original frame passes through both switches is:

   Ethernet V2 ---> SNAP ---> 802.10 ---> SNAP ---> Ethenet V2.

   Switch B knows that this frame originated as an Ethernet V2 from the NULL Organizationally Unique Identifier contained within the SNAP Header and therefore translates it back to the original Ethernet V2 format.

3. SNAP encapsulation on Ethernet:

   
      ---------------------------------------------------------------------------
     |               |          |          | RFC 1042 SNAP Header|      |      |
     |  Destination  |  Source  |  Length  |                     | Type | Data |
     |               |          |          |   AA AA 03   OUI    |      |      |
      ---------------------------------------------------------------------------

   appears on the 802.10 backbone as:

    
   
      ---------------------------------------------------------------------------
     |    |      |      |      |    |          |Frag|SNAP Header |Original|    |
     |Dest|Source|Length|0A0A03|SAID|Station ID|    |            |        |Data|
     |    |      |      |      |    |          |Flag|AA AA 03 OUI| Type   |    |
      ---------------------------------------------------------------------------
                         <--------- 16 bytes -------->

   Note: Switch B will translate this back to SNAP format provided that the OUI is non-NULL in agreement with the standard. However if the originating OUI is 0x00 0x00 0x00 then the frame will be converted to Ethernet V2 format. (This is the well known problem with AppleTalk ARP which uses SNAP with a NULL OUI on Ethernet, and thus causes bridges and switches problems).

4. 802.2 on FDDI:

    
   
   
     -------------------------------------------------------
     |    |               |          |             |       |
     | FC |  Destination  |  Source  |  802.2 LLC  |  Data |
     |    |               |          |             |       |
     -------------------------------------------------------

   appears on the 802.10 VLAN backbone as:

    
   
   --------------------------------------------------------------
   |  |    |      |      |    |          |Frag|                 |
   |FC|Dest|Source|0A0A03|SAID|Station ID|    |DSAP SSAP Control|
   |  |    |      |      |    |          |Flag|                 |
   --------------------------------------------------------------
                  <------- 16 Bytes ---------->

5. SNAP originating on FDDI, translated to 802.10 on FDDI:

    
   
     ---------------------------------------------------------------------
     |    |               |          | RFC 1042 SNAP Header|      |      |
     | FC |  Destination  |  Source  |                     | Type | Data |
     |    |               |          |   AA AA 03   OUI    |      |      |
     ---------------------------------------------------------------------
   
   ---------------------------------------------------------------------------
   |  |    |      |      |    |          |Frag|RFC 1042 SNAP Header |Original|
   |FC|Dest|Source|0A0A03|SAID|Station ID|    |                     |        |
   |  |    |      |      |    |          |Flag|AA AA 03 OUI 00 00 00|  Type  |
   ---------------------------------------------------------------------------
   <-- 13 Bytes --><-------- 16 Bytes --------><--------- 8 Bytes ----------->

6. Novell's "Raw FDDI" on FDDI

   Note: This is a non-standard bastardization and is treated as an 802.2 LSAP. For example:

    
   
   
     -------------------------------------------------------
     |    |               |          |             |       |
     | FC |  Destination  |  Source  |  0xFF 0xFF  |  Data |
     |    |               |          |             |       |
     -------------------------------------------------------

   where the Data field commences with the IPX header length etc ...

   This appears on the 802.10 VLAN backbone as:

    
   
   
   ---------------------------------------------------------------
   |  |    |      |      |    |          |Frag|           |      |
   |FC|Dest|Source|0A0A03|SAID|Station ID|    | 0xFF 0xFF | Data |
   |  |    |      |      |    |          |Flag|           |      |
   ---------------------------------------------------------------
                  <------- 16 Bytes ---------->

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