Tagging modes

Overview

Sharing transport channels between multiple users requires the identification of MAC frames. Tagging is the process of attaching an identifier, a “tag”, to a MAC frame in order to identify the user to which the frame pertains.

These tagging modes are supported:

• Transparent tagging (“VPN tagging”)

• IEEE 802.1Q/IEEE 802.1ad VLAN tagging

  • VLAN tagging compliant with IEEE 802.1Q-1998 (“IEEE 802.1Q VLAN tagging”)

  • VLAN tagging compliant with IEEE 802.1ad (“IEEE 802.1ad VLAN tagging”, “Provider bridge tagging mode”)

The different tagging modes are explained later-on in this section.

Important! Note that it is not possible to use different tagging modes at the same time on the same TransLAN^® card.

However, within the transparent tagging mode there can be virtual switches in the repeater mode, LAN interconnect mode, or LAN-VPN mode (with or without IEEE 802.1p QoS) at the same time on the same physical switch.

Transparent tagging

Transparent tagging (or “VPN tagging”) is a double tagging mode used to identify end-user frames in the LAN-VPN mode of operation.

To enable bandwidth sharing, a customer identification (CID) is associated with every LAN port. This CID is inserted into incoming Ethernet frames, in an extra tag. MAC address filtering and learning is done independently for every CID.

Ethernet frames that are already tagged become double tagged. Already present end-user VLAN tags remain unused in the transparent tagging mode, i.e. every VLAN tag is transmitted transparently through the SDH network.

Outgoing frames are only transmitted on LAN ports which have the respective CID associated. The extra tag is removed before the Ethernet frames are forwarded to an external LAN.

Note that in the VPN tagging mode the term “LAN group” is synonymously used to the term “virtual switch”.

Configuration rules and guidelines

Please observe these configuration rules and guidelines:

• The port role of the LAN and WAN ports is fixed in the operational modes that make use of the VPN tagging mode (see above):

  • LAN ports are always customer role ports.

  • WAN ports are always network role ports.

• On LAN ports the CID needs to be provisioned manually.

• The CID provisioned on each LAN port must be unique within a shared WAN to create a fully independent VPN.

The VPN provisioning on the WAN ports is done automatically by means of the proprietary spanning tree with VPN registration protocol (STVRP).

Important! Changing the tagging mode from transparent tagging to IEEE 802.1Q/IEEE 802.1ad VLAN tagging or vice versa is traffic affecting! Furthermore, most objects provisioned in one mode will be deleted or reset to default - except the LAN group/virtual switch infrastructure - when switching to the other mode.

IEEE 802.1Q VLAN tagging

IEEE 802.1Q VLAN tagging is used to identify end-user frames in the STP virtual switch mode compliant with IEEE 802.1Q.

These are the IEEE 802.1Q VLAN tagging rules:

• On end-user LAN interfaces:

  • At each network ingress port, untagged user frames are tagged with a default identifier, the port VLAN identifier (PVID) which is removed from the frame at the network egress port.

    Already tagged frames are forwarded if their VLAN identifier is in the port's static or dynamic list of VLAN IDs, i.e. if the port belongs to the configured port member set for that VLAN ID. The static VLAN ID list is configurable. The dynamic VLAN ID list is automatically generated by making use of the GARP VLAN Registration Protocol (GVRP).

  • At each network egress port, the port VLAN identifier (PVID) is removed from previously untagged frames that were tagged with the PVID at the ingress port. VLAN tagged frames are forwarded if the port belongs to the configured port member set for the respective VLAN ID.

• On trunking LAN interfaces, all tagged frames are forwarded in both directions. Untagged frames are discarded (dropped).

• The end-customer VLAN tag sets have to be disjunct.

IEEE 802.1ad VLAN tagging

The IEEE 802.1ad VLAN tagging mode (“provider bridge tagging mode”) is a double tagging mode with provisionable Ethertype (TPID), used to identify end-user frames in the STP virtual switch mode compliant with IEEE 802.1ad (“provider bridge mode”).

At each customer role port, a provider bridge tag carrying a customer identifier (CID) is inserted into each Ethernet frame in the ingress direction, and removed from the frame in the reverse direction. Frames that are already tagged become double tagged. The IEEE 802.1ad VLAN tagging mechanism is transparent to the end-customer. VPNs on transit nodes (no customer LAN port) are automatically instantiated by means of the standard GVRP protocol which optionally can be disabled.

The value of the Ethertype (TPID) can be flexibly chosen. However, some values are reserved for specific purposes, for example:

• 0x0800 for IP

• 0x0806 for ARP

• 0x8847 for MPLS

• 0x8100 is not selectable because this is the default value for the STP virtual switch mode compliant with IEEE 802.1Q.

Configuration rules and guidelines

Please observe these configuration rules and guidelines:

• The provider bridge mode can be configured by selecting the IEEE 802.1Q/IEEE 802.1ad tagging mode in combination with provisioning an Ethertype in the range 0x0601 … 0xFFFF, but unequal to 0x8100. Provisioning the value 0x8100 for the Ethertype results in the selection of the STP virtual switch mode compliant with IEEE 802.1Q.

  The recommended value for the Ethertype in the provider bridge tagging mode is 0x9100. Please also observe the reserved values as given above.

• The customer identification (CID) can be configured in the range [0 … 4094].

Important! Changing the tagging mode is traffic affecting!

Furthermore, most objects provisioned in one mode will be deleted or reset to default - except the LAN group/virtual switch infrastructure - when switching to a different mode.

Tagged MAC frame

The following figure illustrates the structure of the MAC frame in different tagging modes as well as the structure of the respective tags.

Legend:

TPID	Tag protocol identifier (“Ethertype”) - indicates the presence of a VLAN tag (or CID tag, respectively). Furthermore, it indicates that the length/type field can be found at a different position in the frame (moved by 4 bytes).
UP (3 bits)	User priority - “0” (low priority) … “7” (high priority).
CFI (1 bit)	Canonical Format Identifier - indicates the presence or absence of routing information.
ID (12 bits)	Identification - customer identification which can be configured in the range [0 … 4094].

Concerning their structure there is no difference between a VLAN tag (C-tag) and a CID tag (S-tag). A distinction between both types of tags can be made by means of the value in the TPID field, the “Ethertype”. In the IEEE 802.1ad VLAN tagging mode (provider bridge tagging mode), the Ethertype can be provisioned per virtual switch.

The value of the Ethertype depends on the mode of operation:

• In the transparent tagging modes (VPN tagging modes), the value of the Ethertype is 0xFFFF, and cannot be changed.

• In the IEEE 802.1Q VLAN tagging mode, the value of the Ethertype is 0x8100, and cannot be changed.

• In the IEEE 802.1ad VLAN tagging mode (provider bridge tagging mode), the value of the Ethertype can be flexibly chosen in the range 0x0601 … 0xFFFF, but unequal to 0x8100. The recommended value for the Ethertype in the provider bridge tagging mode is 0x9100.

Comparison of different tagging schemes

The next figure summarizes the possible tagging schemes:

• No tagging

• Single tagging (IEEE 802.1Q VLAN tagging)

• Double tagging (VPN tagging, IEEE 802.1ad VLAN tagging)

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