Enterasys Networks 2H252, 2E253 Receiving Frames from Vlan Ports, Untagged Frames, Tagged Frames

VLAN Switch Operation

12.7.1 Receiving Frames from VLAN Ports

When a switch is placed in 802.1Q Operational Mode, every frame received by the switch must belong, or be assigned, to a VLAN.

Untagged Frames

The switch receives a frame from Port 1 and examines the frame. The switch notices that this frame does not currently have a VLAN tag. The switch recognizes that Port 1 is a member of VLAN A and classifies the frame as such. In this fashion, all untagged frames entering a VLAN switch assume membership in a VLAN.

NOTE: A VLAN ID is always assigned to a port. By default, it is the Default VLAN (VLAN ID = 1).

The switch will now make a forwarding decision on the frame, as described in Section 12.7.2.

Tagged Frames

In this example, the switch receives a tagged frame from Port 4. The switch examines the frame and notices the frame is tagged for VLAN C. This frame may have already been through a VLAN-aware switch, or originated from a station capable of specifying a VLAN membership. If a switch receives a frame containing a tag, the switch will classify the frame in regard to its tag rather than the PVID for its port.

The switch will now make a forwarding decision on the frame, as described in Section 12.7.2.

12.7.2 Forwarding Decisions

The type of frame under consideration and the filter setting of a VLAN switch determines how the switch forwards VLAN frames.

12.7.2.1 Broadcasts, Multicasts, and Unknown Unicasts

If a frame with a broadcast, multicast, or other unknown address is received by an 802.1Q VLAN-aware switch, the switch checks the VLAN classification of the frame. The switch then forwards the frame out all ports that are identified in the Egress List for that VLAN. For example, if Port 3, shown in Figure 12-2, received the frame, the frame would then be sent to all ports that had VLAN C in their Port VLAN List.

12-10VLAN Operation and Network Applications