Wo ist das der Unterschied?
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of an Ethernet Frame.
In modern implementations of Ethernet, the field within the Ethernet frame used to describe the EtherType also can be used to represent the size of the payload of the Ethernet Frame.
Double tagging
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-TAG (service tag) comes first,
- followed by the inner C-TAG (customer tag).
In such cases, 802.1ad specifies a TPID of 0x88a8 for service-provider outer S-TAG.