3Com 4.2.2 manual Snooping

5-10CHAPTER 5: NETWORK MODIFICATION TASKS

Local VLANs and Local VLANs act as “holding” VLANs in the event that LAN Emulation VLAN Move connectivity is incomplete. Although local VLANs cannot be directly

established by the ATMVLAN manager, they can be activated during a VLAN Move operation involving ATM-based VLANs.

When Ethernet segments in a device are reassigned to a different VLAN they are detached from the LEC of the old VLAN and attached to the LEC of the new VLAN. When the new VLAN already exists in the device, then the new segments are simply added to the others.

A different situation occurs when the new VLAN does not yet exist in the device. A new LEC for the new VLAN needs to be established in the device and that LEC needs to be connected to its LES. This process can take time and in some cases may not succeed. Meanwhile, the Ethernet segments to be moved are placed in a local VLAN created to hold them temporarily. This is signified by a new VLAN icon with the label “Local VLAN” appearing on the VLAN window. The segments to be moved are inside the local VLAN icon and will appear on the destination VLAN display only when the connection to the LES is complete.

Policy-Based Moves Adding a segment or device to a policy results in the enforcement of the policy on that device. The policies may be configuration policies, security policies or other predefined policies. Devices or proxies that enforce the policy continuously follow the following three procedures:

1Snooping

2Resolution

3Configuration

Snooping

The device or proxy enforcing a policy will first snoop the frames going throughout the device. Snooping allows the device to recognize various frames or frame contents. This information is then used in the policy resolution process.

For example: If a segment is assigned to a MAC-based auto-VLAN configuration policy, the device containing that segment snoops on the frames originating from behind that segment. After snooping it tries to resolve the MAC address of the station residing behind that segment. This information (the source MAC address) is then used to resolve the VLAN configuration of that port.