Alcatel Omni Switch/Router, User Manual

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Application Example 3
Page 24-7
Application Example 3
IPX
Network Address
VLAN
s and Translated Frames
Application Example 3 shows two
IPX
networks connected over a bridged
FDDI
ring span-
ning two Omni Switch/Routers.
VLAN
B exists in both switches and specifies an
IPX
network
address policy of network number 100 and Ethernet-
II
encapsulation.
The Problem
In the figure below, when the
IPX
client connected to Switch 1 boots up and sends a frame
seeking a server, Switch 1 assigns the frame to
VLAN
B (since
VLAN
B specifies Ethernet-
II
encapsulation) and then converts the frame to
SNAP
encapsulation so that the frame can
traverse the
FDDI
ring. When the frame arrives at Switch 2, the network number is not avail-
able (since, as previously explained,
IPX
clients do not know their network number at
bootup) and the frame’s encapsulation is no longer Ethernet-
II
– it is now
SNAP
. Because the
IPX
network address policy selects
VLAN
members according to network number and encap-
sulation, Switch 2 cannot assign the frame to
VLAN
B and send it to the
IPX
server.
The Solution
The solution for this problem is to specify a second encapsulation type for
VLAN
B in addi-
tion to Ethernet-
II
– for example,
SNAP
encapsulation. If
VLAN
B specifies Ethernet-
II
or
SNAP
encapsulation, the frame will match the network address policy for
VLAN
B when it arrives at
Switch 2 and can thus be assigned to
VLAN
B and sent to the server. Note that the second
encapsulation type must be specified for
VLAN
B in both Switches, to accommodate data
transmission in either direction.
Please Take Note
This situation can occur whenever translations occur,
such as with bridged
FDDI
rings or Token Rings. If you
are using
FDDI
trunking you do not need to specify a
second encapsulation policy for
IPX
network address
VLAN
s, because trunked frames are not translated.
12345678
123456
Omni Switch/Router 2
1 2 3 4 5 6 7 8
1 2 3 4 5 6
Omni Switch/Router 1
IPX
Workstation Client
IPX Server
VLAN B
IPX
Network 100
Ethernet- II encap
VLAN B
IPX
Network 100
Ethernet- II encap
Ethernet-
II
encap
IPX
Network 100
Ethernet- II encap
Bridged
FDDI
Ring
frame has been converted to
SNAP
encapsulation
With this configuration, a frame sent from the
workstation connected to
VLAN
B in Switch 1
and transmitted to Switch 2 over the FDDI ring
could not be assigned to
VLAN
B in Switch 2
and thus could not be sent to the Server.