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

Page 24-7

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.

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 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. 

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

1

2

3

4

5

6

7

8

1

2

3

4

5

6

IPX

 Network 100

Ethernet-

II

 encap

IPX

 Network 100

Ethernet-

II

 encap

Ethernet-

II

 encap

IPX

 Network 100

Ethernet-

II

 encap

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.