Layer 2

Q-in-Q (802.1ad)

Build or decode a frame with 802.1ad double tagging: the provider S-TAG (0x88a8) and the customer C-TAG (0x8100), each with PCP, DEI and VLAN ID in binary, plus the 8-byte overhead.

Mode

S-TAG Provider tag TPID 0x88A8

S-VID (service VLAN) 100

Usable: 1–4094. Assigned by the operator to separate customers.

S-PCP

S-DEI

C-TAG Customer tag TPID 0x8100

C-VID (customer VLAN) 200

Usable: 1–4094. The original VLAN of the customer's network.

C-PCP

C-DEI

redzilla.cl — qinq

S-TCI —

C-TCI —

Double tag —

S-VID · service VLAN

—

C-VID · customer VLAN

—

Q-in-Q overhead

8 bytes

2 tags × 4 bytes

Binary breakdown of each TCI (16 bits)

PCP (3 bits) DEI (1 bit) VID (12 bits)

802.1ad double-tagged frame structure

Order after the source MAC, two 4-byte tags are stacked: first the S-TAG (outer) and then the C-TAG (inner), before the EtherType.

S-TAG from the provider, TPID 0x88A8 (802.1ad). Its VID (S-VID) identifies the service or the customer inside the operator's network.

C-TAG from the customer, TPID 0x8100 (802.1Q). Its VID (C-VID) is the original VLAN the customer carries untouched.

TCI PCP = (TCI >> 13) & 0x7 · DEI = (TCI >> 12) & 0x1 · VID = TCI & 0x0FFF.

Overhead 8 bytes total. The trunk MTU must absorb them: with a 1500-byte payload, the double-tagged frame reaches 1526 bytes on the wire (jumbo / baby-giant frames are often used).

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