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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Understanding IEEE 802.1Q Tunneling
Figure 1-2 Original (Normal), IEEE 802.1Q, and Double-Tagged Ethernet Packet Formats
When the packet enters the trunk port of the service-provider egress switch, the outer tag is again
stripped as the switch internally 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 IEEE 802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.
In Figure 1-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
entering the edge switch tunnel ports with IEEE 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 tunnel port, the original VLAN numbers on the customer’s network are recovered. It is
possible to have multiple levels of tunneling and tagging, but the switch supports only one level in this
release.
If traffic coming from a customer network is not tagged (native VLAN frames), these packets are bridged
or routed as normal packets. All packets entering the service-provider network through a tunnel port on
an edge switch are treated as untagged packets, whether they are untagged or already tagged with IEEE
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 service-provider network on an IEEE 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.)
On Catalyst 3750-X switches, because 802.1Q tunneling is configured on a per-port basis, it does not
matter whether the switch is a standalone switch or a stack member. All configuration is done on the
stack master.
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 Ta g FCS
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