Alcatel Carrier Internetworking Solutions Switch/Router 用户手册

Page 18-11

The frame size requirement for the different media cause two problem areas which have to 
be addressed by the any to any switching transformation function. 

• Ethernet has a minimum frame size requirement. This requires that padding is inserted on 

frames switched to it which are below the minimum size and stripped from frames 
switched from it.

• All media have different maximum frame size requirements. This gives rise to the prob-

lems of fragmenting large frames and/or negotiating maximum frame sizes.

Ethernet has a minimum frame size of 64 bytes. For frames smaller than 64 bytes it is a simple 
task for the Omni Switch/Router to perform padding. Stripping such padding from Ethernet 
frames when switching to FDDI or Token Ring is not so easy.

In most implementations of IP that we have tested the presence of padding on FDDI or 
Token Ring frames appears not to cause any problems. However, IPX implementations are 
adversely affected by its presence. Therefore the Omni Switch/Router takes a conservative 
approach for all frames, regardless of protocol type, and strips padding where it can be 
detected.

Ethernet frames in IEEE 802.3 format can be stripped of padding because of the presence of 
the length field. This includes all LLC and hence SNAP encapsulated protocols as well as 
Novell Proprietary format.

Padding can only be detected for Ethertype encapsulated frames if the protocol is known and 
the protocol has some length information which can allow the valid data size to be inferred. 
This is protocol specific and is currently only performed for IPX frames. Thus, the Omni 
Switch/Router does not strip padding from non-IPX Ethertype encapsulated frames including 
IP.

For IPX the Omni Switch/Router performs pad stripping for all frame types including Ether-
type. This is possible because all IPX frames have a common header that includes the data 
length, allowing the frame size to be inferred.

In fact, for IPX, the length in the IPX header is used to strip padding in all frame encapsula-
tions including the 802.3 based formats. This is because many IPX Ethernet implementations 
also pad frames to an even byte length. This single byte pad when performed on 802.3 based 
frames is included in the 802.3 length field. Thus the generic 802.3 based stripping technique 
is not sufficient to strip this odd-byte padding. When performing any to any switching FDDI 
implementations of IPX were found to be tolerant of this extra byte whereas Token Ring 
implementations would not work with it present. By adopting the single IPX stripping strat-
egy of using the IPX header length these problems are avoided thus the Omni Switch/Router 
unconditionally strips all padding from IPX frames. 

Also, it does not  support odd-byte pad insertion when switching to Ethernet. This was a 
feature added to overcome limitations of some NIC cards which is now of only historical 
importance and in fact, Netware 4.1 servers provide this insertion as a port configuration 
option.