3Com Switch 4800G 24-Port - Page 134

134 CHAPTER 14: QINQ CONFIGURATION
Figure 39 Single-tagged frame structure vs. double-tagged Ethernet frame structure
Advantages of QinQ:
■ Addresses the shortage of public VLAN ID resource.
■ Enables customers to plan their own VLAN IDs, without running into conflicts
with public network VLAN IDs.
■ Provides an easy-to-do Layer 2 VPN solution for small-sized MANs or intranets.
n
The QinQ feature requires configurations only on the service provider network,
and not on the customer network.
Implementations of
QinQ
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.
■ With the VLAN VPN feature enabled on a port, when a frame arrives at the
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
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.
Modification of TPID
Value of QinQ Frames
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.
Etype
DA SA
User
VLAN Tag
Etype DATA FCS
DA SA
Nested
VLAN Tag
DATA FCS
User
VLAN Tag
6bytes 6bytes
Single-tagged frame
structure
Double-tagged frame structure
4bytes
6bytes 6bytes 4bytes 4bytes
4bytes
4bytes
2bytes
2bytes
46 to 1500 bytes
46 to 1500 bytes
Outer
VLAN
tag
Inner
VLAN
tag