Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 716)

Application Example 2
Page 24-6
In this example one Group was created – Group 3 – that includes all ports to which IPX serv-
ers and clients are connected. Within this Group five VLAN s were created, one for each server:
When the Omni Switch/Router receives frames from the five servers, each server is assigned
to the appropriate
VLAN and no conflict occurs. IPX routing is enabled for each VLAN – with
appropriate framing specified – so that traffic can route between the VLAN s.
When a client workstation boots up and queries for a server, the Omni Switch/Router assigns
the client to the appropriate
VLAN (s) based on encapsulation type. If the client uses 802.2
encapsulation, SNAP encapsulation, or IPX 802.3 encapsulation, VLAN assignment is simple:
the client is assigned to VLAN D (802.2 encapsulation), VLAN E ( SNAP encapsulation), or VLAN
F ( IPX 802.3 encapsulation), respectively.
However, when a client workstation using Ethernet- II encapsulation boots up and queries for
a server, the Omni Switch/Router initially assigns the client to both VLAN B and VLAN C, since
both of these VLAN s specify Ethernet- II encapsulation. However, the Omni Switch/Router
recognizes that the client’s frame is a “Get_Nearest_Server” query and remembers that the
client is in search of its network number. While the client remains in this transitional state, it
remains assigned to all
VLAN s that specify Ethernet- II encapsulation. Once the client has
received response from a server or servers or from an intervening router, the client selects its
network number and begins transmitting frames with the network number embedded. The
Omni Switch/Router detects these frames, removes the client from all previously-assigned
IPX
network address VLAN s (but not from VLAN s of other policy types) and assigns it to the
proper IPX network address VLAN according to network number.
Please Take Note
IPX clients often are not particular about the server to which they
attach. However, clients can select a preferred server if the / PS
(preferred server name) option is included in their start-up script.
Why is this Solution Recommended?
As as been explained, isolating each IPX server in its own IPX network address VLAN is the
recommended way to avoid conflicts. No problems occur if a client receives broadcast and
multicast traffic from multiple servers, especially for the brief period that the client remains in
a transitional state in search of a server.
Problems do occur if two servers with different network numbers and the same encapsula-
tion type are members of the same
VLAN , because each server will detect the other’s frames,
notice conflicting network numbers for the same VLAN , and respond with a router configura-
tion error. For this reason it is not advisable to create four VLAN s based on IPX network
address policies within the same Group, each configured for one of the four encapsulation
types. It is important to isolate the servers, but it is not important to isolate the clients – at
least immediately.
While it is not important to isolate IPX clients immediately at bootup, it is desirable to isolate
them as soon as possible. Isolating clients – rather than letting them remain in multiple VLAN s
that specify the same encapsulation type – increases efficiency and reduces broadcast and
multicast traffic in the network. If a client remains in multiple
VLAN s that specify the same
encapsulation type, the client receives all broadcast and multicast traffic from each server
using that encapsulation type, even though the client only communicates with the server that
shares its network number. In addition, when a
VLAN is extended across a WAN backbone, it
is wasteful and inefficient to transmit unnecessary frames across the WAN . For these reasons,
as soon as a client learns its network number and begins transmitting frames with that
number, the Omni Switch/Router removes the client from all previously-assigned
IPX network
address VLAN s and assigns it to a single IPX VLAN according to network number.
VLAN E
IPX Network 40
SNAP encap
VLAN F
IPX Network 50
IPX 802.3 encap
VLAN D
IPX Network 30
802.2 encap
VLAN B
IPX Network 10
Ethernet-II encap
VLAN C
IPX Network 20
Ethernet-II encap