Cisco Catalyst 3550 series Software Configuration Manual page 325

Chapter 15 Configuring 802.1Q and Layer 2 Protocol Tunneling
Figure 15-2 Original (Normal), 802.1Q, and Double-Tagged Ethernet Packet Formats
Destination
address
DA
DA
DA
When the packet enters the trunk port of the SP egress switch, the metro tag is again stripped as the
switch processes the packet. However, the metro tag is not added when the packet is sent out the tunnel
port on the edge switch into the customer network. The packet is sent as a normal 802.1Q-tagged frame
to preserve the original VLAN numbers in the customer network.
All packets entering the SP network through a tunnel port on an edge switch are treated as untagged
packets, whether they are untagged or already tagged with 802.1Q headers. The packets are encapsulated
with the metro tag VLAN ID (set to the access VLAN of the tunnel port) when they are sent through the
SP network on an 802.1Q trunk port. The priority field on the metro tag is set to the interface class of
service (CoS) priority configured on the tunnel port. (The default is zero if none is configured.)
In Figure
entering the edge-switch tunnel ports with 802.1Q tags are double-tagged when they enter the
SP network, with the metro tag containing VLAN ID 30 or 40, appropriately, and the inner tag
containing the original customer VLAN number, for example, VLAN 100. Even if Customers A and B
both have VLAN 100 in their networks, the traffic remains segregated within the SP network because
the metro 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 SP
network.
78-11194-09
Source
address
Length/
EtherType
SA Len/Etype
SA Etype Tag
SA Etype Tag
15-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
Frame Check
Sequence
Original Ethernet frame
Data FCS
Len/Etype Data
Etype Tag Len/Etype
Catalyst 3550 Multilayer Switch Software Configuration Guide
Understanding 802.1Q Tunneling
802.1Q frame from
FCS
customer network
Data FCS
Double-tagged
frame in service
provider
infrastructure
15-3