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Dot1q-tunnel is also called QinQ (802.1Q-in-802.1Q), which is an expansion of
802.1Q. Its dominating idea is encapsulating the customer VLAN tag (CVLAN tag) to the
service provider VLAN tag (SPVLAN tag). Carrying the two VLAN tags the packet is
transmitted through the backbone network of the ISP internet, so to provide a simple
layer-2 tunnel for the users. It is simple and easy to manage, applicable only by static
configuration, and especially adaptive to small office network or small scale metropolitan
area network using layer-3 switch as backbone equipment.
Fig 5-4 Dot1q-tunnel based Internetworking mode
As shown in Fig 5-4, after being enabled on the user port, dot1q-tunnel assigns each
user an SPVLAN identification (SPVID). Here the identification of user is 3. Same SPVID
should be assigned for the same network user on different PEs. When packet reaches
PE1 from CE1, it carries the VLAN tag 200-300 of the user internal network. Since the
dot1q-tunnel function is enabled, the user port on PE1 will add on the packet another
VLAN tag, of which the ID is the SPVID assigned to the user. Afterwards, the packet will
only be transmitted in VLAN3 when traveling in the ISP internet network while carrying
two VLAN tags (the inner tag is added when entering PE1, and the outer is SPVID),
whereas the VLAN information of the user network is open to the provider network. When
the packet reaches PE2 and before being forwarded to CE2 from the client port on PE2,
the outer VLAN tag is removed, then the packet CE2 receives is absolutely identical to
the one sent by CE1. For the user, the role the operator network plays between PE1 and
PE2,is to provide a reliable layer-2 link.
The technology of Dot1q-tunnel provides the ISP internet the ability of supporting
many client VLANs by only one VLAN of themselves. Both the ISP internet and the
clients can configure their own VLAN independently.
It is obvious that, the dot1q-tunnel function has got following characteristics:
Applicable through simple static configuration, no complex configuration or
maintenance to be needed.
Operators will only have to assign one SPVID for each user, which increases
the number of concurrent supportable users; while the users has got the
ultimate freedom in selecting and managing the VLAN IDs (select within 1~4094
