Alcatel Carrier Internetworking Solutions Switch/Router ユーザーズマニュアル

Page 24-4

Example 2 illustrates the use of AutoTracker 

VLAN

s in 

IPX

 networks – specifically, 

VLAN

s 

based on 

IPX

 network address rules. 

IPX

 networks have unique characteristics that must be 

considered when configuring 

VLAN

s based on network address rules.

The encapsulation type a 

MAC

 station uses is very important in 

IPX

 networks, because a close 

relationship exists between encapsulation type and 

IPX

 network number. In 

IPX

 networks, a 

network number and an encapsulation type are configured for each segment. When two 

IPX

 

servers share the same 

LAN

 segment, they must have the same network number and the same 

encapsulation type in order to communicate. In addition, only clients and servers that use the 
same encapsulation type can communicate. (The Omni Switch/Router removes this restriction 
somewhat through 

MAC

-layer translations, which will not be discussed at this time.) 

In summary, network number and encapsulation type define a broadcast domain in an 

IPX

 

network that is analogous to a 

LAN

 – or a 

VLAN

. (Remember that 

VLAN

s have the same charac-

teristics as 

LAN

s, with the exception that 

VLAN

s can span multiple segments as 

LAN

s cannot.)

An encapsulation type is configured within each 

IPX

 client

 prior to bootup on the 

network. An 

IPX

 client acquires its network number dynamically from an 

IPX

 server (or from an interven-

ing router) using a “Get_Nearest_Server” mechanism. Upon bootup, each client sends a query 
seeking the nearest server that uses the same encapsulation type as the client. Only those 
servers using the same encapsulation type respond to the query. (An intervening router can 
also respond to the query: routers traditionally interconnect 

LAN

 segments and can use differ-

ent encapsulation types for different networks.) This means that 

IPX

 clients do not know their 

network numbers at bootup, but rather acquire their network numbers after they have 
communicated with 

IPX

 servers or with an intervening router.

The close relationship between encapsulation type and network number in 

IPX

 networks is 

the main reason AutoTracker’s 

IPX

 network address policy requires you to specify both a 

network number and an encapsulation type. The Omni Switch/Router assigns devices to 

IPX

 

network address

 VLAN

s as follows:

•

. Frames from an 

IPX

 server always contain information on the server’s network 

number, so the Omni Switch/Router can always assign 

IPX

 servers to the correct 

VLAN

 

based on the server’s network number.

•

. As explained previously, 

IPX

 clients do not know their network number at 

bootup and so cannot, initially, be assigned to 

VLAN

s based on their network number. For 

this reason the Omni Switch/Router initially assigns clients to 

IPX

 network address 

VLAN

s 

based on their encapsulation type. An example of this is shown on the facing page. Once 
an 

IPX

 client communicates with a server or an intervening router, learns its network 

number and begins transmitting frames with that number, it is removed from all previously-
assigned 

IPX

 network address 

VLAN

s (but not from 

VLAN

s of other policy types) and placed 

into the correct 

IPX

 network address 

VLAN

 according to network number. 

As an example, 

IPX

 defines four different types of Ethernet encapsulation: Ethernet-

II

, 802.2, 

SNAP

, and 

IPX

 802.3 (also referred to as “raw”). So, what do you do to avoid conflicts when 

you have more than four servers and they use different encapsulation types? The solution is 
to put each server into a different 

VLAN

, as shown in the example on the facing page.