Cisco Systems A9014CFD manual 1shows the tag structures of the double-tagged packets

Chapter 8 Configuring Ethernet Virtual Connections

Configuring Other Features on EFPs

Figure 8-1shows the tag structures of the double-tagged packets.

Figure 8-1 Original (Normal), 802.1Q, and Double-Tagged Ethernet Packet Formats

	Source
	address
Destination			Length/		Frame Check
address			EtherType			Sequence
							Original Ethernet frame
	DA	SA	Len/Etype	Data		FCS

DA

SA

Etype

Tag

Len/Etype

Data

FCS

IEE 802.1Q frame from customer network

DA

SA

Etype

Tag

Etype

Tag

Len/Etype

Data

FCS

74072

Double-tagged frame in service provider infrastructure

In Figure 8-2, Customer A is assigned VLAN 30, and Customer B is assigned VLAN 40. Packets entering the edge routers with 802.1Q tags are double-tagged when they enter the service-provider network, with the outer tag containing VLAN ID 30 or 40, appropriately, and the inner tag containing the original VLAN number, for example, VLAN 100. Even if both Customers A and B have VLAN 100 in their networks, the traffic remains segregated within the service-provider network because the outer tag is different. Each customer controls its own VLAN numbering space, which is independent of the VLAN numbering space used by other customers and the VLAN numbering space used by the service-provider network. At the outbound port, the original VLAN numbers on the customer's network are recovered.

	Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09		8-21