The two providers cannot be expected to manage their VLANs in the same
manner so the same EVC may use two different VLAN IDs in the two different provider
networks. Across the network-network interface (NNI), therefore, some translation of
the VLAN IDs is necessary.
IEEE Std 802.1ad-2005 provides for translation from one VLAN ID to another in
both the input and output directions on each bridge port. When such translation occurs,
if the providers are using any of the bridge control protocols that have internal
Ethernet Bridging 389
references to VLAN IDs, those internal references are translated by the bridges in the
same manner as the data frames. Any VLAN tags carried inside the outermost VLAN
tags, e.g., C-tags inside S-tags, are unaffected by this transformation, as proper layering
dictates.
Note that translating in one direction only would be adequate for point-to-point links.
However, as soon as three bridges or more are connected to a single medium, e.g., to
an EVC offered by another provider, translation in both directions is necessary in the
general case. On a shared medium, there must be a ???wire??? VLAN ID for each EVC, and
each bridge must translate in both directions between the ???wire??? VLAN ID and the ID
used in that bridge??™s network.
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