Understanding the network.pdf - Back to Home

The configuration layer uses the Generic Attribute Registration
Protocol (GARP) and Generic VLAN Registration Protocol (GVRP) to
provide a signaling mechanism to exchange VLAN membership
information between switches and other 802.1q-aware devices.
The distribution/resolution layer defines the 802.1q packet ingress
and egress rules, which handle packet VLAN ID assignment and
packet delivery. Under 802.1q, a packet's VLAN assignment is made
based on the VLAN Identifier (VID) of the inbound port on which the
packet is received. After the packet is associated with a VLAN, the
switch (or network of switches) delivers the packet only to the ports
associated with the packet's VLAN designation.
802.1q VLAN port membership is managed either through GVRP
messages sent over interswitch links or through manual configuration.
Regardless of the configuration method, it is essential that a single,
unique VID is used to identify each VLAN.
802.1q VLANs handle port identification and packet designation using
both implicit and explicit packet tagging. When a switch port is
designated for VLAN membership, part of the configuration is its
membership type assignment. The port can be assigned to be a
tagged or untagged VLAN member. Typically, ports used for
end-station terminations and interconnect links with
non-802.1q-aware devices are defined as untagged members. Ports
assigned to 802.1q-aware devices and 802.1q interswitch links are
defined as tagged members. Through the use of explicit packet
tagging, 802.1q maintains and distributes VLAN designation
information between interswitch links that might have membership to
multiple VLANs.
Under the 802.1 ingress rules, the packet is associated with the VLAN
designated to the inbound port. After the egress port has been
determined by the switch, the packet is either tagged or untagged,
depending on the egress port's VLAN membership assignment. If the
egress port is designated a tagged port, an explicit packet tag is
added to the frame, the CRC is recalculated, and the frame is
forwarded. If the egress port is an untagged member of the VLAN, no
tag is added. If a tag exists, it is removed, and CRC is recalculated
before it is forwarded out of the switch. Devices that are not
802.1q-aware do not understand the 802.1q tag, so they consider the
frame corrupt and discard it. For this reason, it is important that,
when you configure your 802.1q VLAN port assignments, you
correctly designate the port as a tagged or untagged member.