Bull 1GB Intel Ethernet Switch Module, Installation And User Manual

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Switch Management and Operating Concepts
27
The NovaScale Blade 1GB Intel® Ethernet Switch Module implements IEEE 802.1Q VLANs,
which require tagging. This enables them to span the entire network (provided that all switches on
the network are IEEE 802.1Q-compliant).
VLANs enable a network to be segmented to reduce the size of broadcast domains. All packets
entering a VLAN will be forwarded (over IEEE 802.1Q enabled switches) only to the stations that
are members of that VLAN. This includes broadcast packets, multicast packets and unicast packets
from unknown sources.
VLANs can also provide a level of security to your network. IEEE 802.1Q VLANs will deliver
packets only between stations that are members of the VLAN.
Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q
VLANs enables VLANs to work with legacy switches that do not recognize VLAN tags in packet
headers (tag-unaware devices). The tagging feature enables VLANs to span multiple 802.1Q-
compliant switches through a single physical connection and enables the Spanning Tree Protocol to
be enabled on all ports and work normally.
The IEEE 802.1Q standard restricts the forwarding of untagged packets to the VLAN of which the
receiving port is a member.
The main characteristics of IEEE 802.1Q are as follows:
•
Assigns packets to VLANs by filtering
• Assumes the presence of a single global spanning tree
• Uses an explicit tagging scheme with one-level tagging
IEEE 802.1Q VLAN packet forwarding
The switch module makes packet-forwarding decisions based on the following types of rules:
Ingress rules Rules relevant to the classification of received packets belonging to a VLAN.
Forwarding rules between ports
The switch module decides whether to filter or forward the packet.
Egress rules The switch module determines whether the packet must be sent tagged or untagged.
The following illustration shows the 802.1Q VLAN packet-forwarding decision-making process of
the switch module. For more information about packet forwarding, see “Packet forwarding” on page
24. For more information about port VLAN IDs (PVIDs), see “Port VLAN ID” on page 29. For
more information about tagging and untagging, see “Tagging and untagging” on page 30. For more
information about port states, see “IEEE 802.1D STP port states” on page 261 and “IEEE 802.1w
STP port states” on page 262.