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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
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Chapter 8 Configuring Ethernet Virtual Connections
Configuring Other Features on EFPs
Figure 8-1 shows the tag structures of the double-tagged packets.
Figure 8-1 Original (Normal), 802.1Q, and Double-Tagged Ethernet Packet Formats
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.
Double-tagged
frame in service
provider
infrastructure
IEE 802.1Q frame from
customer network
Original Ethernet frame
Destination
address
Length/
EtherType
Frame Check
Sequence
Source
address
SADA Len/Etype Data FCS
SADA Len/Etype DataEtype Tag FCS
SADA Len/Etype DataEtype Tag Etype Tag FCS
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