D-Link DES-6300 User Manual page 33

Modular L3 Ethernet Switch User's Guide
VLANs Over 802.1Q-compliant Switches
When switches maintaining the same VLANs are 802.1Q-compliant, it is possible to use tagging.
Tagging puts 802.1Q VLAN information into each packet header, enabling other 802.1Q-compliant
switches that receive the packet to know how to treat it. Upon receiving a tagged packet, an 802.1Q-
compliant switch can use the information in the packet header to maintain the integrity of VLANs, carry
out priority forwarding, etc.
Data transmissions between 802.1Q-compliant switches take place as shown below.
Figure 5- 4. Data transmissions between 802.1Q-compliant switches
In the above example, step 4 is the key element. Because the packet has 802.1Q VLAN data encoded in
its header, the ingress port can make VLAN-based decisions about its delivery: whether server #2 is
attached to a port that is a member of VLAN 2 and thus, should the packet be delivered; the queuing
priority to give to the packet, etc. It can also perform these functions for VLAN 1 packets as well, and, in
fact, for any tagged packet it receives regardless of the VLAN number.
If the ingress port in step 4 were connected to a non-802.1Q-compliant device and was thus receiving
untagged packets, it would tag its own PVID onto the packet and use this information to make
forwarding decisions. As a result, the packets coming from the non-compliant device would
automatically be placed on the ingress ports VLAN and could only communicate with other ports that
are members of this VLAN.
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