Managing VLANs
VLANs
Cisco 220 Series Smart Switches Administration Guide Release 1.1.0.x
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VLANs address security and scalability issues. Traffic from a VLAN stays within
the VLAN, and terminates at devices in the VLAN. It also eases network
configuration by logically connecting devices without physically relocating those
devices.
If a frame is VLAN-tagged, a four-byte VLAN tag is added to each Ethernet frame.
The tag contains a VLAN ID between 1 and 4094, and a VLAN Priority Tag (VPT)
between 0 and 7.
When a frame enters a VLAN-aware device, it is classified as belonging to a VLAN,
based on the four-byte VLAN tag in the frame.
If there is no VLAN tag in the frame or the frame is priority-tagged only, the frame is
classified to the VLAN based on the Port VLAN Identifier (PVID) configured at the
ingress port where the frame is received.
The frame is discarded at the ingress port if Ingress Filtering is enabled and the
ingress port is not a member of the VLAN to which the packet belongs. A frame is
regarded as priority-tagged only if the VID in its VLAN tag is 0.
Frames belonging to a VLAN remain within the VLAN. This is achieved by sending
or forwarding a frame only to egress ports that are members of the target VLAN.
An egress port may be a tagged or untagged member of a VLAN.
The egress port:
•
Adds a VLAN tag to the frame if the egress port is a tagged member of the
target VLAN, and the original frame does not have a VLAN tag.
•
Removes the VLAN tag from the frame if the egress port is an untagged
member of the target VLAN, and the original frame has a VLAN tag.
VLAN Roles
VLANs function at Layer 2. All VLAN traffic (Unicast, Broadcast, and Multicast)
remains within its VLAN. Devices attached to different VLANs do not have direct
connectivity to each other over the Ethernet MAC layer.
Adjacent VLAN-aware devices exchange VLAN information with each other by
using Generic VLAN Registration Protocol (GVRP). As a result, VLAN information is
propagated through a bridged network. VLANs on a device can be created
statically or dynamically, based on the GVRP information exchanged by devices. A
VLAN can be static or dynamic (from GVRP), but not both. For more information
about GVRP, refer to the
section.
Some VLANs can have additional roles, including: