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Routing & Switching · Advanced

IEEE 802.1ad (Q-in-Q)

A standard for stacking two VLAN tags (S-tag and C-tag) so providers can carry customer VLANs transparently.

Definition

The outer service tag uses TPID 0x88A8 and identifies the customer; the inner customer tag is preserved. Q-in-Q tunnelling lets a provider offer many customers overlapping VLAN IDs across its Metro Ethernet network. Layer 2 protocol tunneling can carry customer CDP, STP and VTP.

Example

switchport mode dot1q-tunnel / switchport access vlan 500

Exam tip

Q-in-Q adds 4 bytes per tag, so MTU must be increased in the provider network.

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