Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 718)

Application Example 4
Page 24-8
Application Example 4
Routing in IPX Networks
How Routing Works Generally
AutoTracker “activates” a VLAN – and its internal router interface – when the first port is
assigned to the VLAN . If a VLAN has a port policy, AutoTracker assigns the specified port(s)
and activates the VLAN immediately. If a VLAN has a logical policy, AutoTracker assigns the
first port to the VLAN when a frame is received from a source device that matches the VLAN ’s
policy. When such a frame is received, the source device – and the port to which that device
is connected – are assigned to the
VLAN and the VLAN is activated.
Until a port is assigned to a VLAN , that VLAN is maintained in an inactive state and its internal
router port is inactive – even if routing was enabled by the user. Use of a VLAN ’s routing
service is “on-demand” and AutoTracker does not enable routing until a port is present that
might require it. When AutoTracker assigns the first port to a particular
VLAN , it activates that
VLAN and its routing service (as long as routing was enabled by the user).
Once AutoTracker has established devices’ VLAN assignments and activated the appropriate
VLAN routing services, it does not participate in the routing process. Routing works correctly
as long as the policies of the IPX protocol were followed – with the exception below.
The Exception
There is one scenario in which AutoTracker affects the behavior of an IPX -routed network.
This situation occurs when an IPX server is a member of any VLAN with IPX network address
policies and IPX routing is enabled on the Group’s default VLAN #1. An exception condition
arises in this situation because all ports in a Group are always members of that Group’s default
VLAN #1 in addition to any other VLAN s of which they are members. As a result, default VLAN
#1 is always active.
The figure on the facing page illustrates this problem situation. In this figure, three VLAN s
within Group 2 – one of which is default VLAN #1 – have IPX routing enabled, as indicated by
the presence of the internal IPX router. VLAN s 2 and 3 both have IPX network address poli-
cies. When IPX Server A is connected to the Omni Switch/Router on port 1, the Server is
assigned to VLAN 2 (per the network address policy) and port 1 becomes a member of VLAN
2. When IPX Server B is connected to the Omni Switch/Router on port 2, the Server is
assigned to VLAN 3 (per the network address policy) and port 2 becomes a member of VLAN
3. However, ports 1 and 2 are also members of the Group’s default VLAN #1, so port 1 is now
a member of VLAN 1 and VLAN 2 and port 2 is now a member of VLAN 1 and VLAN 3.
When IPX Server A sends broadcasts, they are restricted to VLAN 2 because of the network
address policies. When IPX Server B sends broadcasts, they are restricted to VLAN 3, also
because of the network address policies. However, when the internal IPX router sends out
broadcasts on VLAN 1 the broadcasts are flooded out all ports in the Group, because all ports
in the Group are, by default, members of VLAN 1. IPX Server A responds to this with a router
configuration error because it is receiving broadcasts on VLAN 1 when it should only receive
them on VLAN 2. IPX Server B also responds with a router configuration error because it is
receiving broadcasts on VLAN 1 when it should only receive them on VLAN 3.
The Solution
The solution for this problem is to disable IPX routing on default VLAN #1. Because of this,
when your network includes IPX servers that are members of IPX network address VLAN s and
IPX routing is enabled, you should configure your network such that disabling IPX routing on
default VLAN #1 is not a problem.