That way, the next-higher
layer of entities in each stack can peer with each other.
If one entity tries to ???peek??? at another entity??™s tag besides its own, e.g., if a bridge
looks at both tags, it violates the layering principle and thus stifles future progress. If
it cannot peek at the other tag, however, it cannot do its job. If it does peek at the other
tag, then no new layer of entities, and hence no new tag, can ever be inserted between
the two. Layer insertion, one of the principle justifications for layering, would be lost.
And of course, layer insertion is exactly how VLANs came about! Thus, peeking at two
VLAN tags would inhibit the ability of bridges to grow new features in the future.
The astute reader will observe that ???IGMP snooping??? violates layering principles. If
a tag is added to Ethernet frames being carried transparently through a bridged network,
one that the bridges do not peer with, then the bridges can no longer snoop on
IGMP frames. Introducing a useful layer-crossing feature has thus placed a restriction
on future development.
MAC Address Transparency The provider, of course, runs a Spanning Tree Protocol
algorithm to maintain a loop-free active topology in the provider network.
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