User's Manual

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15.3 Dot1q-tunnel Configuration
15.3.1 Introduction to Dot1q-tunnel
Dot1q-tunnel is also called Q-in-Q (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.
Figure 15-6 Dot1q-tunnel based Internetworking mode
As shown in above, 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-tuunel provides the ISP internet the ability of supporting many client VLANs by only
one VLAN of their selves. 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 1~4096 at users’ will).
SP networks
P
PE1
PE2
CE1
CE2
Trunk connection
Trunk connection
Unsymmetrical
connection
Unsymmetrical
connection
Customer
networks1
Customer
networks2
On the customer port
Trunk VLAN 200-300
This port on PE1 is enabled
QinQ and belong to VLAN3
On the customer port
Trunk VLAN 200-300
This port on PE1 is enabled
QinQ and belong to VLAN3