26. Virtual LANs
ROX™ v2.2 User Guide
265
RuggedBackbone™ RX5000
26. Virtual LANs
ROX™ provides the following VLAN features:
• Support for up to 255 VLANs
• Configurable port-native VLAN.
• Port modes of operation tailored to edge devices (such as a PC or IED) and to network switch
interconnections.
• A default setting that ensures configuration-free connectivity in certain scenarios.
• Ability to force either tagged or untagged operation on the port-native VLAN.
• GARP VLAN Registration Protocol (GVRP).
26.1. VLAN Operation
26.1.1. VLANs and Tags
A virtual LAN or VLAN is a group of devices on one or more LAN segments that communicate as if
they were attached to the same physical LAN segment. VLANs are extremely flexible because they are
based on logical instead of physical connections.
When VLANs are introduced, all traffic in the network must belong to one or another VLAN. Traffic on
one VLAN cannot pass to another, except through an internetwork router or Layer 3 switch.
A VLAN tag is the identification information that is present in frames in order to support VLAN operation.
26.1.2. Tagged vs. Untagged Frames
Tagged frames are frames with 802.1Q (VLAN) tags that specify a valid VLAN identifier (VID). Untagged
frames are frames without tags or frames that carry 802.1p (prioritization) tags only having prioritization
information and a VID of 0. Frames with a VID=0 are also called priority-tagged frames.
When a switch receives a tagged frame, it extracts the VID and forwards the frame to other ports in
the same VLAN.
26.1.3. Native VLAN
Each port is assigned a native VLAN number, the Port VLAN ID (PVID). When an untagged frame
ingresses a port, it is associated with the port’s native VLAN.
By default, when the switch transmits a frame on the native VLAN, it sends the frame untagged. The
switch can be configured to transmit frames on the native VLAN tagged.
26.1.4. Edge and Trunk Port Types
Each port can be configured as an Edge or Trunk port:
Edge Port
An edge port attaches to a single end device, such as a PC or IED. An edge port carries traffic on
a single pre-configured VLAN: the native VLAN.
Trunk Port
Trunk ports are part of the network and carry traffic for all VLANs between switches. Trunk ports
are automatically members of all VLANs configured in the switch.
The switch can “pass through” traffic, forwarding frames received on one trunk port out of another trunk
port. The trunk ports must be members of all VLANs that the “pass through” traffic is part of, even if
none of those VLANs are used on edge ports.