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Application Example 3
Page 24-7
Application Example 3
IPX Network Address VLANs 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
VLANs, because trunked frames are not translated.
12345678
123456
Omni Switch/Router 2
12345678
123456
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.