Alcatel Carrier Internetworking Solutions Switch/Router 用户手册

Page 18-6

Encapsulation is the biggest problem for implementing a transformation algorithm in support 
of any to any switching. All of the media provide a choice of more than one encapsulation 
and not all encapsulations are available on all media. Additionally, the methodology of these 
encapsulations vary from protocol to protocol. 

An ideal protocol would dictate a single encapsulation which would be the same on all 
media. 

Most protocols make use of more than one encapsulation. For example, IP uses Ethertype 
most of the time on Ethernet and SNAP (an instance of an 802.2 LLC) on FDDI and Token 
Ring. In this case, there may be clearly established rules for transforming from one encapsula-
tion to another as media are traversed. 

Some protocols may allow more than one encapsulation even on a single media type. Some 
might use the encapsulation to separate functional parts of the protocols, for example, rout-
ing table updating protocols from user data forwarding protocols. Others, like IPX may simply 
allow the user to arbitrarily choose them. 

Some, most notably IPX, may entangle the notion of encapsulation with the notion of the 
network level broadcast domain to create multiple logical networks over a single physical 
broadcast domain.

Clearly, then there is no single algorithmic rule by which the any to any transformation func-
tion can switch arbitrary protocols. There are two choices available to address this situation. 

1. The switch must be configurable, per device, per protocol, per media to select the trans-

formation of encapsulations.

2. The switch performs a single transformation and the user must configure all end-stations 

and routers to use this single choice made by the switch.

The Omni Switch/Router uses the first approach for IP and IPX as the dominant protocols in 
the market. It uses the second approach for all other protocols.

For protocols other than IP or IPX three encapsulations are possible on Ethernet media:

• Ethertype

• IEEE 802.2 LLC 

• IEEE 802.2 SNAP (This is an instance of an LLC encapsulation defined by the 802.2 

committee to support the transformation of Ethertype Ethernet frames to media which 
don’t support that encapsulation.)

On Token Ring and FDDI, two encapsulations are permitted by the standards:

• IEEE 802.2 LLC 

• IEEE 802.2 SNAP.