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Stackable NWay Ethernet Switch User’s Guide
38 Switch Management Concepts
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 5-5. 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.
Broadcast Management
Broadcast transmissions, packets sent to every device on the LAN, are a vital part of any network. However,
they can often cause problems on the network and even network failure. For this reason the Switch has a
number of tools for managing broadcast packets on your network.