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W5-Ethernet-Manual.doc

Version 2.0-01

Watson 5 Ethernet

Operating Manual

Revision: 2006-04-27 3-15

3.2.6 Stacked VLANs

The Watson 5 Ethernet plugin also supports double-tagged VLANs (a.k.a stacked VLANs). Stacked

VLANs is a method to increase the number of available VIDs in a structured and hierarchical fash-

ion that is backward-compatible with single-tagged VLANs as long as the network supports packet

sizes of 1'526 bytes or more.

This method places an extra tag (often referred to as the S-TAG or Service Provider Tag) in front of

the first tag (known as the C-TAG or Customer Tag). The double tag format is shown in Figure

3-11.

DA SA TCI ML/TL/T DATA

PRE Preamble

SFD Start-of-frame Delimiter

DA Destination address

SA Source address

L/T 802.1 Qtag type (0x8100)

TCI Tag control information

PRI Priority

VID VLAN Identifier (1 .. 4094)

ML/T MAC client length/type

DATA MAC client data

PAD Padding bytes

CRC Cyclic redundancy check sequence

CRC-32SFDPRE PADTCIL/T

0x81

0x00

PRI[2:0] 0 VID[11:8]

VID[7:0]

S-TAG

0x81

0x00

PRI[2:0] 0 VID[11:8]

VID[7:0]

C-TAG

Figure 3-11: Stacked VLAN frame format

Each port can be configured for double tagging in incoming (ingress) and outgoing (egress) direc-

tion. Normally ingress and egress double tagging are enabled simultaneously.

A port with ingress double tagging enabled expects double tagged frames. The port will remove the

outer tag. If a frame contains only a single tag then this tag will be removed. Untagged frames are

not modified.

After removal of the outer tag the Ethernet bridge will process the packet as a single-tagged VLAN

packet, cf. 3.2.3. For packets with a single tag or no tag the default tag for this port will be used to

determine VLAN membership and possible egress ports.

A port with egress double tagging enabled will transmit all packets with an extra outer tag. When

the packet had no tag, it will be sent with a single tag. When the packet had a single tag, it will

egress double-tagged. The tag added is the default tag of the source port of the packet.

Note: Stacked VLAN functions are not available on the Watson 5 Ethernet tabletop modems. The table-

top modems support frame sizes of 1'526 bytes which means that they are transparent for double-

tagged Ethernet packets.

3.2.7 Class of Service (CoS)

The Watson 5 Ethernet plugin supports differentiated treatment of Ethernet packets according to

their class of service. The plugin supports four different classes named Q1 .. Q4 (Q4 is the high-

est priority queue, Q1 has lowest priority), classification based on layer 2 or layer 3 information and

ingress as well as egress rate limiting.

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