Advanced VLANs

802.1pQ and VLANs are introduced on a separate web page.

802.1Q Tag Details

The IEEE 802.1Q standard defines the format of a 4 byte “tag” field. The presence of a tag is indicated by the Ethertype value of 0x8100. The remainder of the tag has 3 parts: a fixed tag protocol identifier (0x8100 in hex), a user priority value ranging from 0 to 7 (called an 802.1p value) a format identified and the Virtual LAN information (VLAN id).

Note: if the VLAN id is 0, the tag contains only user priority information (this allows the 802.1Q tag to be used when VLANs are not being used). The priority information can be used by a mannaged switch to select which queue is used to buffer any packets that can not be immediately processed.

The format of the header is:

Advanced use of VLANs

Virtual LANs, (VLANs) as defined by IEEE 802.1Q, are of two forms:

Note: A L2 Ethernet protocol can be used to announce creation or deletion of a VLAN throughout a L2 domain. In a managed network, a policy server may also be used to dynamically associate layer 2 ports with specific MAC addresses and VLANs. The IEEE has defined Multiple VLAN Registration Protocol (MVRP). This was formerly known as GARP VLAN Registration Protocol (GVRP). It is a layer 2 protocol that enables automatic configuration of VLAN information on switches. VTP is a CISCO-proprietary protocol that also automates VLAN configuration.

An IP router that receives frames from a VLAN interface, access or trunk, does not propagate the 802.1Q tag on the outgoing interface. That is, the output VID (if any) is assigned based on the routing decision and not on the VID value associated with a received frame. In this way, an IP router will normally change the VID associated with an IP packet as it performs L2 forwarding. There are no standards for layer 3 switching devices, and hence their treatment of VLANs is proprietary and can result in VIDs being propagated across a L3 switch. In a routed network, the VID can also be propagated over the router by tunnelling the Ethernet frames over IP (e.g. using L2TP, RFC3931).

Q-in-Q Tags

In metro-Ethernet, two VLAN Tags may be stacked (used one after another in a frame header) to increase this number of VIDs.

Provider Bridging

The IEEE has defined a standard that allows multiple levels of VLANs to be used, in a method known as provider bridging. This simply allows additional tag fields to be placed before an existing VLAN. At the destination, the additional tag is removed.

The standard is specified in 802.1ad, which identifies the tag value as 0x88a8 in the S-TAG and 0x8100 in the C-TAG.

In IEEE 802.1ad the CFI is replaced by a Drop Eligability Indicator (DEI), increasing the functionality of the PCP field.

Some MAC addresses have different mappings, e.g.

Note; Pre-standard implementations are sometimes referred to as "QinQ" and may use 0x8100 in both tags. This is not specified in 802.1ad. 802.1ad provider bridge uses the standard 0x88a8 in the S-TAG and 0x8100 in the C-TAG. At least on major manufacturer uses a default value of 0x9100 (Juniper), and this continues in practical use, although is being replaced by the 802.1ad specification.

Gorry Fairhurst - Date: 18/03/2012 EG3557