They could even be in different rooms or different buildings
to maximize the resiliency of the network. The two connections from paired virtual
bridge E to bridge A are aggregated by IEEE Std 802.3ad link aggregation into a single
virtual link. Similarly, the two links between the paired virtual bridge E to bridge B
are aggregated. Bridge A sees one connection to bridge E via an aggregated link, as
does bridge B. If either of the bridges fail, or if any of the four links fail, connectivity
remains unchanged.
Figure 13.14 shows a second layer of bridges in the access layer??”paired virtual
bridges, P through Z. Each of these bridges has a single aggregated link, consisting of
408 Chapter 13
four physical links each, to one of the paired virtual bridges in the distribution layer.
Only those access bridges connected to paired virtual bridge E are shown. The number
of access bridges, and hence UNI ports, could clearly be quite large.
The important thing to notice about Figure 13.14 is that, as far as the control protocols
are concerned, there are no redundant links, and hence no possible loops, present
in this network, except in the core cloud (A??“D).
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