User`s manual

VLAN Tagging

Static port-based VLANs were originally the only way to segment a network without using routing. But, these

port-based VLANs could only be implemented on a single switch or switches cabled together. Routing was

required to transfer traffic between unconnected switches.

As an alternative to routing, some vendors created proprietary schemes for sharing VLAN information across

switches. These methods would, of course, only operate on that vendor's equipment and were not an acceptable

way to implement VLANs.

802.1Q

The 802.1Q standard was designed to change all that. It standardized VLANs and has eliminated the need for

proprietary solutions. With the adoption of this standard, traffic can be confined to VLANs that exist on multiple

switches from different vendors.

802.1Q tag header

This interoperability and traffic containment across different switches is the result of a switch's ability to use and

recognize the 802.1Q Tag Header, called VLAN tagging.

Switches that implement 802.1Q tagging add this tag

header to the frame directly after the destination and source MAC addresses as shown here:

This tag header indicates:

that the packet has a tag

whether the packet should have priority over others and2.

which VLAN it belongs to so the Switch can forward or filter it correctly3.

802.1Q, 802.1p and the 802.1D Bridging Standard

Closely allied with the 802.1Q VLAN tagging, the 802.1p standard defined ways to prioritize traffic using the

802.1Q tag. Although many still refer to 802.1Q and 802.1p, they are now officially incorporated into the 802.1D

Bridging Standard.

802.1p uses tagging to create up to eight different traffic class priorities. For details on how the Switch

implements 802.1p, see Quality of Service.

General configuration steps

Incoming

Determine how you want a port to handle incoming packets:

set port acceptable types:

whether the port should admit all packets or only admit tagged packets

❍

enable ingress filtering:

drop the packet if this port is not a member of the VLAN that is identified in the incoming packet

❍

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Outgoing

Enable tagging on a port to have the Switch add VLAN or priority information as the packet is forwarded

over the Switch port

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A complete description of these standards and how they relate is beyond the scope of this manual. For an