Allied Telesis Rapier Switch - Manual (Page 42)

42 Rapier Switch Software Reference
Software Release 2.5.1
C613-02025-00 REV B
VLAN Membership of Untagged Packets
A VLAN that does not send any VLAN-tagged frames is a logical grouping of
ports. All untagged traffic arriving at those ports belongs to that VLAN.
VLANs based on untagged ports are limited, because each port can only
belong to one VLAN as an untagged port. Limitations include:
■ It is difficult to share network resources, such as servers and printers,
across several VLANs. The routing functions in the switch must be
configured to interconnect using untagged ports only.
■ A VLAN that spans several switches requires a port on each switch for the
interconnection of the various parts of the VLAN. If there are several
VLANs in the switch that span more than one switch, then many ports are
occupied with connecting the VLANs, and so are unavailable for other
devices.
If the network includes VLANs that do not need to share network resources or
span several switches, VLAN membership can usefully be based on untagged
ports. Otherwise, VLAN membership should be determined by tagging (see
“VLAN Tagging” on page 39).
Figure 1-3 on page 43 shows two port-based VLANs with untagged ports
belonging to them. Ports 1-3 belong to the marketing VLAN, and ports 14-16
belong to the training VLAN. The switch acts as two separate bridges: one that
forwards traffic between the ports belonging to the marketing VLAN, and a
second one that forwards traffic between the ports belonging to the training
VLAN. Devices in the marketing VLAN can only communicate with devices in
the training VLAN by using the switch’s routing functions.
Table 11: VLAN membership of example of a network using tagged ports.
VLAN Member ports
Training 3, 26 on Switch A
21, 22, 25 on Switch B
Marketing 2, 4, 26 on Switch A
23, 25 on Switch B
Admin 1, 2 on Switch A