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.
Summary of Contents for Omni Switch/Router
Page 1: ...Part No 060166 10 Rev C March 2005 Omni Switch Router User Manual Release 4 5 www alcatel com ...
Page 4: ...page iv ...
Page 110: ...WAN Modules Page 3 40 ...
Page 156: ...UI Table Filtering Using Search and Filter Commands Page 4 46 ...
Page 164: ...Using ZMODEM Page 5 8 ...
Page 186: ...Displaying and Setting the Swap State Page 6 22 ...
Page 202: ...Creating a New File System Page 7 16 ...
Page 270: ...Displaying Secure Access Entries in the MPM Log Page 10 14 ...
Page 430: ...OmniChannel Page 15 16 ...
Page 496: ...Configuring Source Route to Transparent Bridging Page 17 48 ...
Page 542: ...Dissimilar LAN Switching Capabilities Page 18 46 ...
Page 646: ...Application Example DHCP Policies Page 20 30 ...
Page 660: ...GMAP Page 21 14 ...
Page 710: ...Viewing the Virtual Interface of Multicast VLANs Page 23 16 ...
Page 722: ...Application Example 5 Page 24 12 ...
Page 788: ...Viewing UDP Relay Statistics Page 26 24 ...
Page 872: ...The WAN Port Software Menu Page 28 46 ...
Page 960: ...Deleting a PPP Entity Page 30 22 ...
Page 978: ...Displaying Link Status Page 31 18 ...
Page 988: ...Displaying ISDN Configuration Entry Status Page 32 10 ...
Page 1024: ...Backup Services Commands Page 34 14 ...
Page 1062: ...Diagnostic Test Cable Schematics Page 36 24 ...
Page 1072: ...Configuring a Switch with an MPX Page A 10 ...
Page 1086: ...Page B 14 ...
Page 1100: ...Page I 14 Index ...