Application Example 2
Page 24-4

Application Example 2

VLANs in IPX Networks

Example 2 illustrates the use of AutoTracker VLANs in IPX networks – specifically, VLANs
based on IPX network address rules. IPX networks have unique characteristics that must be
considered when configuring VLANs based on network address rules.
Encapsulation Type in IPX Networks
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 VLANs have the same charac-
teristics as LANs, with the exception that VLANs can span multiple segments as LANs 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.
VLAN Assignment in IPX Networks
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 VLANs as follows:
•IPX servers. 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.
•IPX clients. As explained previously, IPX clients do not know their network number at
bootup and so cannot, initially, be assigned to VLANs based on their network number. For
this reason the Omni Switch/Router initially assigns clients to IPX network address VLANs
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 VLANs (but not from VLANs of other policy types) and placed
into the correct IPX network address VLAN according to network number.
So How Do I Avoid Conflicts?
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.
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