Cisco 6500 Series Software Configuration Manual page 183

Chapter 14 Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling
Figure 14-2 Untagged, 802.1Q-Tagged, and Double-Tagged Ethernet Frames
Destination
address
DA
DA
DA
When a tunnel port receives tagged customer traffic from an 802.1Q trunk port, it does not strip the
received 802.1Q tag from the frame header; instead, the tunnel port leaves the 802.1Q tag intact, adds a
2-byte Ethertype field (0x8100) followed by a 2-byte field containing the priority (CoS) and the VLAN.
The received customer traffic is then put into the VLAN to which the tunnel port is assigned. This
Ethertype 0x8100 traffic, with the received 802.1Q tag intact, is called tunnel traffic.
A VLAN carrying tunnel traffic is an 802.1Q tunnel. The tunnel ports in the VLAN are the tunnel's
ingress and egress points.
The tunnel ports do not have to be on the same network device. The tunnel can cross other network links
and other network devices before reaching the egress tunnel port. A tunnel can have as many tunnel ports
as required to support the customer devices that need to communicate through the tunnel.
An egress tunnel port strips the 2-byte Ethertype field (0x8100) and the 2-byte length field and transmits
the traffic with the 802.1Q tag still intact to an 802.1Q trunk port on a customer device. The 802.1Q trunk
port on the customer device strips the 802.1Q tag and puts the traffic into the appropriate customer
VLAN.
Note Tunnel traffic carries a second 802.1Q tag only when it is on a trunk link between service-provider
network devices, with the outer tag containing the service-provider-assigned VLAN ID and the inner tag
containing the customer-assigned VLAN IDs.
78-14099-04
Source
address
Length/
EtherType
SA Len/Etype Data
SA Etype Tag
SA Etype Tag
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Frame Check
Sequence
FCS Original Ethernet frame
Len/Etype Data
Etype Tag Len/Etype
Understanding How 802.1Q Tunneling Works
802.1Q frame from
FCS
customer network
Data FCS
Double-tagged
frame on trunk
links between
service provider
network devices
14-3