13-3
Catalyst 3750 MetroSwitch Software Configuration Guide
78-15870-01
Chapter 13 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Understanding 802.1Q Tunneling
When the double-tagged packet enters another trunk port in a service-provider core switch, the outer tag
is stripped as the packet is processed inside the switch. When the packet exits another trunk port on the
same core switch, the same metro tag is again added to the packet. Figure 13-2 shows the structure of
the double-tagged packet.
Figure 13-2 Normal, 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 packet is processed internally on the switch. However, the metro tag is not added when it
is sent out the tunnel port on the edge switch into the customer network, and the packet is sent as a normal
802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.
In Figure 13-1, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
entering the edge-switch tunnel ports 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. With 802.1Q tunneling, 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 number s 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 the traffic coming from a customer network is not tagged (native VLAN frames), these packets are
bridged or routed as if they were 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 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
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).
Double-tagged
frame in service
provider
infrastructure
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
74072
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