Protocol layering is the most fundamental
principle in networking, and its use is precisely why you do not need to upgrade every
piece of network equipment in the world every time one carrier or another adds
a new feature to its network. Layering means that a new LAN can be substituted for
Ethernet, as long as it looks to the upper layers like Ethernet. VLANs were added to
bridges without disrupting the operations of stations because the same service was offered
to the stations as was offered before VLANs were introduced.
When protocols are properly layered, protocol ???entities??? in different systems communicate
with each other as peers, utilizing the services provided by lower layers and
offering some service to the higher layers. They can be stacked ad infinitum. Although
this sounds trivial, it has a direct effect on tags added to frames. If one entity adds a tag
(e.g., a C-tag) as a frame leaves a port, then its peer entities, and only its peer entities,
Ethernet Bridging 387
have any business inspecting the contents of that tag, and if the frame is passed up
the stack of entities at the receiving end, the tag is removed.
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