Moxa EDS-408A-1M2S-SC-T [56/109] Communication between vlans

Moxa Managed Ethernet Switches Featured Functions

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Managing a VLAN

A new or initialized Moxa switch contains a single VLAN—the Default VLAN. This VLAN has the following

definition:

• VLAN Name—Management VLAN

• 802.1Q VLAN ID—1 (if tagging is required)

All the ports are initially placed on this VLAN, and it is the only VLAN that allows you to access the management

software of the Moxa switch over the network.

Communication Between VLANs

If devices connected to a VLAN need to communicate to devices on a different VLAN, a router or Layer 3

switching device with connections to both VLANs needs to be installed. Communication between VLANs can

only take place if they are all connected to a routing or Layer 3 switching device.

VLANs: Tagged and Untagged Membership

The Moxa switch supports 802.1Q VLAN tagging, a system that allows traffic for multiple VLANs to be carried

on a single physical link (backbone, trunk). When setting up VLANs you need to understand when to use

untagged and tagged membership of VLANs. Simply put, if a port is on a single VLAN it can be an untagged

member, but if the port needs to be a member of multiple VLANs, tagged membership must be defined.

A typical host (e.g., clients) will be untagged members of one VLAN, defined as an Access Port in a Moxa

switch, while inter-switch connections will be tagged members of all VLANs, defined as a Trunk Port in a Moxa

switch.

The IEEE Std 802.1Q-1998 defines how VLANs operate within an open packet-switched network. An 802.1Q

compliant packet carries additional information that allows a switch to determine which VLAN the port belongs

to. If a frame is carrying the additional information, it is known as a tagged frame.

To carry multiple VLANs across a single physical link (backbone, trunk), each packet must be tagged with a

VLAN identifier so that the switches can identify which packets belong in which VLAN. To communicate between

VLANs, a router must be used.

The Moxa switch supports three types of VLAN port settings:

• Access Port: The port connects to a single device that is not tagged. The user must define the default port

PVID that assigns which VLAN the device belongs to. Once the ingress packet of this Access Port egresses

to another Trunk Port (the port needs all packets to carry tag information), the Moxa switch will insert this

PVID into this packet so the next 802.1Q VLAN switch can recognize it.

• Trunk Port: The port connects to a LAN that consists of untagged devices, tagged devices and/or switches

and hubs. In general, the traffic of the Trunk Port must have a Tag. Users can also assign a PVID to a Trunk

Port. The untagged packet on the Trunk Port will be assigned the port default PVID as its VID.

• Hybrid Port: The port is similar to a Trunk port, except users can explicitly assign tags to be removed from

egress packets.

The following section illustrates how to use these ports to set up different applications.