User's Manual

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Example 1
Figure 4 Typical configuration of Dot1Q tunnel
As shown in the figure above, port F0/1 of CE1 connects port F0/1 (or port G0/1) of PE1, PE1 connects
S8510 on port F0/2 (or port G0/2), PE2 connects S8510 on port F0/2 (or port G0/2), and port F0/1 (or
port G0/1) of PE2 connects port F0/1 of CE1.
Port G0/1 of PE is set to be the access port of VLAN 10 and on them Dot1Q Tunnel is enabled. However,
the ports of CE still need Trunk VLAN 200-300, enabling the link between CE and PE to be an
asymmetrical link. In this case, the public network only needs to distribute users a VLAN ID, 10. No
matter how many VLAN IDs of private network are planned in the user’s network, the newly distributed
VLAN ID of the public network will be mandatorily inserted into the tagged packets when these packets
enter the backbone network of ISP. These packets then pass through the backbone network through
the VLAN ID of the public network, reach the other side of the backbone network, that is, the PE devices,
get rid of the VLAN tag of the public network, resume the user’s packets and at last are transmitted to
the CE devices of the users. Therefore, the packets that are forwarded in the backbone network have
two layers of 802.1Q tag headers, one being the tag of the public network and the other being the tag
of the private network. The detailed flow of packet forwarding is shown as follows:
1) Because the egress port of CE1 is a Trunk port, all the packets that are transmitted by users to PE1
have carried the VLAN tag of the private network (ranging from 200 to 300). One of these packets
is shown in figure 5.
DA
(6B)
SA
(6B)
ETYPE(8100)
(2B)
VLAN TAG
(2B)
ETYPE
(2B)
DATA
(0~1500B)
FCS
(4B)
Figure 5 Structure of a packet from CE1
2) After the packets enter PE1, PE1, for the ingress port is the access port of Dot1Q tunnel, ignores
the VLAN tag of the private network but inserts the default VLAN 10’s tag into these packets, as
shown in figure 6.