IEEE 802.1Q VLANs 145Tagging or untagging VLAN framesATTENTIONVLAN-tagged frames can pass through VLAN-aware or VLAN-unaware networkinterconnection devices, but the VLAN tags should be stripped off before passingthe frame on to any end-node host that does not support VLAN tagging.The following concepts apply to VLAN configuration:• VLAN classification—When the switch receives a frame, it classifies theframe in one of two ways. If the frame is untagged, the switch assignsthe frame to an associated VLAN (based on the default VLAN ID of thereceiving port). But if the frame is tagged, the switch uses the taggedVLAN ID to identify the port broadcast domain of the frame.• Port overlapping—Port overlapping can be used to allow access tocommonly shared network resources among different VLAN groups,such as file servers or printers. If you implement VLANs that do notoverlap, but still need to communicate, you can connect them by usingan external router.• Untagged VLANs—Untagged (or static) VLANs are typically used toreduce broadcast traffic and to increase security. A group of networkusers assigned to a VLAN form a broadcast domain that is separatefrom other VLANs configured on the switch. Packets are forwarded onlybetween ports that are designated for the same VLAN. Untagged VLANscan be used to manually isolate user groups or subnets. However,use IEEE 802.3 tagged VLANs with GVRP whenever possible to fullyautomate VLAN registration.SMBUsing the Nortel Business Ethernet Switch 50 SeriesNN47924-301 01.01 Standard1.00 October 2006Copyright © 2006, Nortel Networks Nortel Networks Confidential.
