Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 503)

Encapsulation
Page 18-7
The SNAP Conversion
The intent of the 802.2 committee is that Ethertype frames are transformed to SNAP on cross-
ing from Ethernet media to 802 media and restored to Ethertype in the reverse direction.
The Omni Switch/Router could follow this rule for all protocols including IP; however, this
would prevent AppleTalk interworking between Ethernet and FDDI. The Omni Switch/Router
explicitly checks for the AppleTalk protocol. If found, the rule is not applied. In addition, the
Omni Switch/Router checks for the Banyan Vines protocol and translates according to the
media type (see Banyan Vines on page 18-13).
As there may be other protocols with this problem, the SNAP-to-Ethertype transformation is
configurable for all protocols other than AppleTalk.
Other Conversions
There are no equivalent algorithmic approaches which the transformation function can adopt
for dealing with protocols which require Ethertype on Ethernet and some form of LLC encap-
sulation on FDDI and/or Token Ring. The mapping between Ethertype values and LLC values
is arbitrary requiring tables indexed by protocol.
The approach followed in the Omni Switch/Router is therefore to simply pass LLC encodings
between Ethernet, FDDI and Token Ring with no changes other than to insert/strip the length
field required by IEEE 802.3 on Ethernet.
This leaves protocols which require transformations between Ethertype and LLC encapsula-
tions as unswitchable unless the clients and servers can be configured to use SNAP.
Summary of Non-IPX Encapsulation Transformation Rules
To summarize:
• Ethertype/SNAP transformations are configurable for all protocols except AppleTalk and
Banyan Vines. Ethertype frames going to FDDI or Token Ring are translated to SNAP
unconditionally. SNAP frames going to Ethernet are translated to Ethertype or left as SNAP
as per configuration, unless the protocol is AppleTalk in which case they are left as SNAP.
• LLC frames are passed unchanged in value but with the length field required on Ethernet
media stripped/inserted.