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4.2.3 Service VLAN
4.2.3.1 TPID setting
Tag protocol identifier (TPID).
With the IEEE standard
802.1ad
, double-tagging can be useful for
Internet service providers, allowing them to use VLANs internally while mixing traffic from clients that
are already VLAN-tagged. The outer (next to source MAC and representing ISP VLAN) S-VLAN Tag
(service VLAN tag) comes first, followed by the inner C-VLAN Tag (customer tag). In such cases,
802.1ad
specifies a TPID of 0x88a8 for service-provider outer S-VLAN Tag.
Figure: 4.2.6
The device determines whether a received frame carries a service provider VLAN tag (S-VLAN Tag)
or a customer VLAN tag (C-VLAN Tag) by checking the TPID value. After receiving a frame, the
device compares the configured TPID value with the value of the TPID field in the frame. If the two
match, the frame carries the corresponding VLAN tag.
Figure: 4.2.7
