Alcatel Omni Switch/Router, Руководство пользователя

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Encapsulation
Page 18-8
IPX Encapsulation Transformation Rules
For IPX the encapsulation problems described above are compounded by the introduction of
a fourth encapsulation on Ethernet media. Novell introduced a frame format when the IEEE
802.3 standards committee produced its version of Ethernet which was incompatible with
Ethernet.
Novell places its network header and data within a raw IEEE 802.3 Ethernet frame with no
intervening IEEE 802.2 LLC header. This is in direct contravention of the standards but has
become a de facto standard encapsulation.
Novell refers to this encapsulation types as ETHERNET_802.3. It is also widely known as
Novell Proprietary, Novell Raw, Raw 802.3, etc. Such frames are identifiable only by the fact
that the Novell Network header starts with a two byte field called the
checksum , which is
never used and assumes the value 0xFFFF.
Routers, bridges and switches therefore check for the checksum after an 802.3 length field. In
effect, Novell has usurped the value OxFF for the Destination and Source SAP addresses
(DSAP/SSAP) of an LLC header.
Thus on Ethernet media there are four encapsulations for IPX
• Ethertype - value 0x8137
• Novell Proprietary
• LLC - SAP value 0xE0
• SNAP - Protocol Identifier 0x0000008137
On Token Ring and FDDI, the same LLC and SNAP encapsulations are found as on Ethernet
(without the length field.)
This leaves an aggregate of four encapsulations across all media with only two being univer-
sal (LLC and SNAP).
Unfortunately, the SNAP conversion rule isn’t applicable and there is no algorithmic determi-
nation for the use of particular encapsulations on any media - it’s purely the choice of the
network administrator. Worse, multiple encapsulations can be found on a single media to
create multiple logical networks over a single physical broadcast domain.
The Omni Switch/Router therefore allows configuration of the encapsulation transformations
of IPX frames. Before transmission of a frame occurs the switch determines first the current
encapsulation of the frame. Then, it consults configuration information to determine which of
the permitted encapsulations for the media the frame is to be transmitted on is required.
Thus, the administrator can choose not only a single output option but an option per possi-
ble received encapsulation.
For example, over FDDI media, LLC and SNAP are permissible so the administrator might
configure one of the following:
• LLC and SNAP encapsulations received from other FDDI, Token Ring or Ethernet media
are translated to SNAP.
• Ethertype and Proprietary encapsulations from Ethernet are translated to LLC.
Essentially, for each encapsulation, transformation to each of the other three encapsulations is
available, but may simply be left as is. This choice may be further constrained by the output
media type, for example, Ethertype is not a valid option on FDDI or Token Ring.