THOMSON R7.4, Configuration Manual

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E-DOC-CTC-20080229-0001 v1.0
7
2| VLAN Overview
Stacked VLAN frame format
VLAN stacking means that a frame contains two VLAN tags. An example of a stacked VLAN frame format is
shown in the following illustration:
An S-tag can be distinguished from a C-tag as follows:
The value of the TPID field of an S-tag is set to 0x88a8.
An S-tag contains a DEI (Drop Eligible Indicator) field instead of a CFI field.
Destination
MAC Address
Source
MAC Address
Length/
Type
TPID TCI FCS
6 6 2 4 2 2
Pad Data/LLC
0 .. 1500 46 .. 0 bytes
Min. 46 bytes - Max. 1500 bytes
Min. 72 bytes - Max. 1526 bytes
TPID TCI
2 2
Customer
VLAN tag
= C-tag
Service Provider
VLAN tag
= S-tag
Priority DEI VID
3 1 12 bits