Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 667)

AutoTracker VLANs
Page 22-7
How Devices are Assigned to AutoTracker VLAN s (continued)
Router Traffic in IP and IPX Network Address VLAN s
Prior to release 2.1, AutoTracker handled VLAN assignments for router traffic in IP and IPX
network address VLAN s in the same manner as normal traffic. In release 2.1 and later,
AutoTracker differentiates router traffic from normal traffic and can distinguish traffic that is
routed through a router from traffic that is generated by a router.
AutoTracker now determines VLAN assignments for router interfaces (that is, the MAC
addresses of router interface ports) in IP and IPX network address VLANs based on router
update messages generated by the router itself. This minimizes VLAN leakage and avoids the
problem situation described on the facing page.
The Problem with Router Traffic
AutoTracker functions on the assumption that data in a frame can be associated with the
frame’s source
MAC address. For example, if a frame has an IPX network number of 300,
AutoTracker assumes that it has received the frame directly and that the source device is a
member of
IPX network 300. This is not true in the case of routed frames. Routers route
frames from one network to another by changing the frame’s MAC header but keeping the
layer 3 content intact. This can lead to the problem situation described on the facing page.
In the network on the facing page, Device A gets correctly assigned to VLAN 2 and Device B
gets correctly assigned to VLAN 3 without problem. The two router interfaces will be assigned
to the correct VLAN s if AutoTracker learns the router interface MAC addresses from their RIP
updates. However, this may not happen. The problem situation on the facing page shows
what can occur if AutoTracker learns the router interface
MAC addresses from traffic routed
through the router rather than from traffic generated by the router (such as a RIP update).
How AutoTracker Handles Router Traffic
To avoid the problem situation on the facing page, AutoTracker now determines if any IP or
IPX device it has learned is a router. If it is, AutoTracker marks the device as a router,
unlearns all previous VLAN assignments for that device, and reassigns the device based on a
router-generated update packet (such as a RIP packet).
AutoTracker determines if a learned device is a router by searching further within the frame.
For example, if AutoTracker receives an
IP frame, it searches beyond the source IP address
and also checks if the IP frame is a RIP , OSPF , BGP , DVMRP , or IGRP update. If it is, as
explained, AutoTracker marks the device as a router, unlearns its previous VLAN assignments,
and reassigns it using the router-generated update packet.
AutoTracker recognizes the following types of router-generated frames:
• IP protocol: RIP frames, OSPF frames, BGP4 frames, DVRMP frames, and IGRP frames
• IPX protocol: IPX RIP frames and SAP frames
AutoTracker maintains a record of the devices it has learned are routers. Each time a router-
generated frame is received from a device marked as a router, AutoTracker updates that
device’s membership in
IP or IPX network address VLAN s. If a frame received from a device
marked as a router is not IP or IPX , VLAN membership is updated normally.
Please Take Note
This special handling of router traffic occurs in IP
and IPX network address VLAN s only. Note that it
does not alter normal VLAN assignment processes
such as checking for VLAN policy matches other
than IP or IPX network address.