All of them, however, have
solutions??”see ???Using Spanning Tree Effectively??? and ???Spanning Tree Alternatives.???
Backbone Bridges??”MAC-in-MAC
The simplest way to introduce the concept of backbone bridges is simply to display the
format of a frame carried across a network of backbone bridges, as in Figure 13.9. This
figure shows the frame format as defined in Draft 3 of IEEE Project 802.1ah, the draft
standard for backbone bridges. (It should be emphasized that, as of this writing, this
standard has not been approved by IEEE, but this format is likely to be very close to
the one in the final version of the standard and is very close to what some vendors have
actually implemented.)
What has happened in the transition from the S-tagged frame (Figure 13.9c) to the
I-tagged frame (Figure 13.9e) is that
?– The S-tag has been removed from the (perhaps tagged) customer frame in
Figure 13.9a or b, so that the original customer??™s frame is restored.
?– A pair of MAC addresses, a new S-tag (called the B-tag for Backbone tag), and an
I-tag have been prepended to the customer??™s frame (Figure 13.9d and e).
?– The FRC has been recomputed.
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