Enterasys Networks 700 - Manual (Page 160)

FDDI Management
5-16 Configuring FDDI Frame Translation Settings
If the frame is exiting the FDDI ring through another FDDI/Ethernet bridge, the
FDDI frame must be converted back into an IEEE 802.3/Ethernet frame. As there
are four potential Ethernet frame types to which the two FDDI frame types can be
translated, you must determine which translation options you want in effect —
depending on which network protocols and applications are being run on the
destination network.
In addition, there are frame size differences between FDDI (which allows a
maximum frame size of 4500 bytes) and Ethernet frames (1518 byte maximum,
excluding preamble), so FDDI frames may need to be fragmented before being
bridged onto an Ethernet network.
The Frame Translation window lets you set the parameters for frame translation
and fragmentation when Ethernet traffic needs to traverse an FDDI ring. The
frame types that you select for translation will depend on which higher-layer
communications protocols and software you are running on the network
segments connected to your Ethernet-to-FDDI bridge. Each frame type and its
usage is described below.
Ethernet Frames
The DELHF-UA supports translation of the following four Ethernet frame types:
Ethernet II
Ethernet II is the Novell NetWare designation for the basic Ethernet frame type
(also commonly referred to as Ethernet or Ethernet DIX). This frame format has an
Ethernet II MAC header with a two byte Ethernet Type field. The Type field
contains a protocol ID which indicates the upper layer protocol (e.g., XNS,
DECnet, TCP/IP, etc.) used in the Data field of the packet. Most current
transmission protocols, including TCP/IP, use the Ethernet II frame format, as do
networks running Apple’s AppleTalk 1 protocol and Digital’s DECnet™ protocol.
Note that the Type field of an Ethernet II frame will always have a decimal value
greater than 1500, so that it can be differentiated from the Length field of Ethernet
802.2 frames (described below).
Ethernet 802.2
The Ethernet 802.2 frame format is the IEEE 802.3 formalization of the original
Ethernet frame format. This frame format is similar to the Ethernet II frame
format, except that the two byte Type field is eliminated and replaced with a two
byte Length field, and an 802.2 LLC header is encapsulated within the 802.3
frame. This LLC header contains the destination and source addressing
information for the LLC frame (DSAP and SSAP), and a one byte Control field
(the LSAP – or LLC Service Access Point value) which provides the frame’s
protocol ID. Ethernet 802.2 packets are differentiated from Ethernet II packets
because the Length field will always have a decimal value of 1500 or less (since
the data field in Ethernet frames cannot be greater than 1500 bytes), and the
Ethernet II Type field will always be greater than 1500 decimal.