3Com Switch 4800G 24-Port Configuration Manual page 134

134 C 14: Q Q C
HAPTER IN
n
Implementations of
QinQ
Modification of TPID
Value of QinQ Frames
ONFIGURATION
Figure 39 Single-tagged frame structure vs. double-tagged Ethernet frame structure
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DA
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Advantages of QinQ:
Addresses the shortage of public VLAN ID resource.
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Enables customers to plan their own VLAN IDs, without running into conflicts
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with public network VLAN IDs.
Provides an easy-to-do Layer 2 VPN solution for small-sized MANs or intranets.
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The QinQ feature requires configurations only on the service provider network,
and not on the customer network.
There are two types of QinQ implementations: basic QinQ and selective QinQ.
1 Basic QinQ
Basic QinQ is a port-based feature, which is implemented through VLAN VPN.
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With the VLAN VPN feature enabled on a port, when a frame arrives at the
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port, the switch will tag it with the port's default VLAN tag, regardless of
whether the frame is tagged or untagged. If the received frame is already
tagged, this frame becomes a double-tagged frame; if it is an untagged frame,
it is tagged with the port's default VLAN tag.
2 Selective QinQ
Selective QinQ is a more flexible, VLAN-based implementation of QinQ. In
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addition to all the functions of basic QinQ, selective QinQ can tag the frame
with different outer VLAN tags based on different inner VLAN IDs.
A VLAN tag uses the tag protocol identifier (TPID) field to identify the protocol
type of the tag. The value of this field, as defined in IEEE 802.1Q, is 0x8100.
Figure 40 shows the 802.1Q-defined tag structure of an Ethernet frame.
4 bytes 2bytes
User
SA Etype
VLAN Tag
4 bytes 4 bytes
Nested User
SA
VLAN Tag VLAN Tag
Outer
Inner
VLAN
VLAN
tag
tag
46 to 1500 bytes 4 bytes
DATA FCS
Single-tagged frame
structure
2 bytes 46 to 1500 bytes
Etype DATA
Double-tagged frame structure
4bytes
FCS